Maximally extended sl(2|2) as a quantum double

We derive the universal R-matrix of the quantum-deformed enveloping algebra of centrally extended sl(2|2) using Drinfeld's quantum double construction. We are led to enlarging the algebra by additional generators corresponding to an sl(2) automorphism. For this maximally extended algebra we construct a consistent Hopf algebra structure where the extensions exhibit several uncommon features. We determine the corresponding universal R-matrix containing some non-standard functions. Curiously, this Hopf algebra has one extra deformation parameter for which the R-matrix does not factorize into products of exponentials.


Introduction
Integrable models are usually characterized by an invertible finite-dimensional solution R : C n ⊗ C n → C n ⊗ C n of the so-called Yang-Baxter equation (1.1) For instance, the R-matrix corresponds to the scattering matrix in integrable field theories. In the language of the algebraic Bethe Ansatz, the R-matrix describes the symmetry algebra that underlies the integrable model. It also parameterizes the Hamiltonian. Alternatively, knowing the full symmetry algebra of the model usually allows one to derive the R-matrix. The intimate relation between algebra and R-matrices is made manifest in quasitriangular Hopf algebras. These Hopf algebras contain an operator R, called the universal R-matrix, which is an invertible operator that intertwines the Hopf algebra structure and its opposite counterpart. One can show that the universal R-matrix satisfies the Yang-Baxter equation (1.2) In particular, R-matrices whose symmetry algebra is a quasi-triangular Hopf algebra can then be obtained by evaluating the universal R-matrix in the corresponding representation, R = (ρ ⊗ ρ )(R). For a large class of solutions of the former Yang-Baxter equation, the associated quasi-triangular Hopf algebra is known and can be formulated very explicitly. Prominent examples are q-deformed, quantum affine and Yangian algebras based on simple Lie algebras and superalgebras. However, there exist several peculiar R-matrices for which the question of the underlying algebra remains obscure. In particular, despite many efforts, the algebraic structure that governs the R-matrix of the one-dimensional Hubbard model and of the AdS/CFT integrable system is only known to some extent. In this paper we take some first steps towards understanding the universal structure of the R-matrix of these integrable systems.
The R-matrix that underlies the integrability of the Hubbard model [1], see also [2], was found by Shastry [3] without knowledge of its algebraic origins. Only parts of the underlying algebra were known. For instance, it is well known that the Hubbard model exhibits two sl (2) algebras, that are associated with spin and charge. These algebras can even be extended to a full Yangian symmetry [4], but these symmetries are not sufficient to determine the R-matrix.
Later, the algebraic structure of the Hubbard model was elucidated by using input from a rather different area of theoretical physics. It turned out that the Hubbard model has a remarkable relation to string and gauge theory via the AdS/CFT correspondence. The prime example of the gauge/string correspondence -the duality between N = 4 SYM and superstrings on AdS 5 × S 5 -proved to be an integrable system. Moreover, the R-matrix that describes this system was found to consist of two copies of the Hubbard model R-matrix [5,6].
From string and gauge theory considerations it then became clear that Shastry's Rmatrix actually exhibits supersymmetry. More precisely, there is an unusual Lie superalgebra underlying [7] the AdS/CFT R-matrix and hence also the Hubbard model R-matrix exhibits this symmetry algebra. This Lie algebra is centrally extended psl(2|2) and the symmetry algebra of the R-matrix is given by a novel type of Yangian algebra [8] corresponding to this Lie superalgebra. The next question that arises is whether a universal R-matrix exists from which the Hubbard model R-matrix can be derived. Answering this question is important for our understanding of the Hubbard and AdS/CFT integrable models. In particular, it would indicate whether the Hopf algebra is quasi-triangular. Moreover, the unusual nature of the algebra might lead to some new algebraic structures that arise in the construction of the universal R-matrix. A positive answer to the question of the existence of the R-matrix would potentially have important implications. For example, it should provide a proof of the BES conjecture [9]. Moreover, the universal R-matrix can be used to compute correlation functions [10], which will help solving the Hubbard model and the AdS/CFT integrable model.
Hints of a universal algebraic structure can be found at the classical level [11,12]. At the classical level, the R-matrix reduces to a classical r-matrix that satisfies the classical Yang-Baxter equation. In fact, it was shown that the universal classical r-matrix put forward in [12] indeed correctly describes the classical limit of the scattering matrices appearing in the AdS 5 ×S 5 superstring [13,14]. Remarkably, it was even found to describe the R-matrix to quadratic order [14]. Furthermore, the R-matrices from these models already contain universal sub-structures [15]. Nevertheless, despite all these indications a universal R-matrix has never been found.
In the theory of quantum algebras there is a standard way to generate universal Rmatrices, which goes under the name of a quantum double [16], see also [17]. The idea behind it is to construct from a given Hopf algebra H and its 'dual' Hopf algebra H * , a quasi-triangular Hopf algebra -the quantum double DH -whose R-matrix is simply given by the sum over a pair of dual bases R = i e i ⊗ e * i . For a given R-matrix, whose symmetry algebra is known, one can then endeavor to write this symmetry algebra as a quantum double or embed it into one. In other words, we would like to construct the smallest Hopf algebra that can be written as a double that contains the symmetry algebra of the Hubbard model.
In the present paper we will consider, as a starting point, the centrally extended psl(2|2) Lie superalgebra, which is finite-dimensional, rather than the corresponding infinite-dimensional Yangian algebra. Not only is this a logical first step to take, but the R-matrix of the Hubbard model in the fundamental representation is actually fixed by the finite-dimensional algebra [7]. In other words, we might already gain insight into the structure of the Hubbard model R-matrix by restricting to this case.
However, in order to get a non-trivial quantum algebra to which we can apply the quantum double construction, we need to q-deform the algebra. In [18] this algebra, denoted by U q (psl(2|2) C 3 ), was defined by considering the quantum deformation of the universal enveloping algebra of psl(2|2) C 3 . Analogously to the presence of Yangian symmetry for the undeformed case, the symmetry algebra can be enlarged by an affine extension [19]. Also in the deformed case, the classical limit exhibits universal structures [20]. The undeformed model can be recovered by taking the rational limit q → 1.
In this paper we successfully construct the smallest double algebra that contains U q (psl(2|2) C 3 ). To this end we need to introduce three additional boost operators that are dual to the central extensions. They form an sl(2) algebra. We find that the total algebra, which we shall call the maximal extension of (quantum-deformed) psl(2|2), U q,κ sl(2) psl(2|2) C 3 (1.3) is of a novel type. It depends on an additional parameter κ and has some unusual features. For instance, its commutation relations depend (polynomially) not only on q but also on := log q. We will derive all algebra and coalgebra relations that define this Hopf algebra. Moreover, we explicitly work out its universal R-matrix. This paper is organized as follows. In Sec. 2 we will discuss some general features of quantum deformed algebras as well as the construction of the quantum double. After this we work out the example of sl(3) in detail. We then turn to the algebra of interest: the quantum-deformed maximally extended sl(2|2) algebra. First we summarize the algebra relations in Sec. 3. We then carry out the construction of a quantum double in Sec. 4, which leads to a natural extension of the algebra. Subsequently in Sec. 5 we derive the universal R-matrix for this extended algebra. Finally, we perform the classical limit in Sec. 6. The details of our computation are presented in the appendix.

Hopf algebras as a quantum double
In this section, we briefly introduce the notion of a quantum double and quantum enveloping algebras. We will work with superalgebras and to this end we introduce the corresponding Z 2 grading. The degree of a generator a is denoted by |a| := 0, a is even, 1, a is odd. (2.1) We furthermore use the graded tensor product and all commutators are to be understood in the graded sense, i.e.

The quantum double
In the following, we will develop the general framework underlying the construction of a quantum double.
Hopf algebras. A Hopf algebra is a unital associative algebra (H, µ, 1) together with linear maps ∆, and S, called coproduct, counit and antipode, that satisfy for all a ∈ H where the symbol ∼ = in (2.6) denotes the canonical isomorphisms between H and C ⊗ H and H ⊗ C. Furthermore algebra and coalgebra need to satisfy the compatibility relations for any a, b ∈ H ∆(ab) = ∆(a)∆(b), (ab) = (a) (b). (2.8) It is often convenient to write the coproduct using the Sweedler notation Here (a (1),k , a (2),k ) form a collection of pairs of elements describing the coproduct of a ∈ H. Usually we shall drop the sum and use the abbreviated middle form. Within a Hopf algebra it is useful to define two bilinear compositions called the left and right adjoint actions (2) . (2.10) These actions provide generalizations of conjugation and the commutator in the q-deformed case, as will be seen later. Note that 1 a = a 1 = (a)1 is the Hopf algebra relation (2.7) and that the action obeys the composition rule a (b c) = (ab) c.
Provided that the antipode of a Hopf algebra H is invertible, one can define another Hopf algebra H cop with the opposite coproduct ∆ cop := τ • ∆ and antipode S cop := S −1 .
Quasi-triangular Hopf algebras. Integrable systems are closely related to quasitriangular Hopf algebras. These algebras constitute a special class of Hopf algebras for which the coproduct and opposite coproduct are related by a similarity transformation.
More precisely, a quasi-triangular Hopf algebra (H, R) is a Hopf algebra H together with an invertible element R ∈ H ⊗ H, called the universal R-matrix. It relates the coproduct and the opposite coproduct for any a ∈ H in the following way ∆ cop (a)R = R∆(a), (2.11) and furthermore has to satisfy the so-called fusion relations If we write R = R (1) ⊗ R (2) , then R ij is the element of H ⊗ H ⊗ H with R (1) in the i-th factor of the tensor product, R (2) in the j-th factor, and 1 in the remaining factor.
The above axioms directly imply the Yang-Baxter equation which is the key relation in the theory of integrable systems. The universal R-matrix describes the scattering in an integrable model from an algebraic point of view.
Dual Hopf algebra. We call a Hopf algebra H * the dual 1 of a Hopf algebra H, if there exists a non-degenerate bilinear pairing ·, · : H * ⊗ H → C, satisfying for all f, g ∈ H * and a, b ∈ H and Given a basis {e i } i∈I of a Hopf algebra H with e 0 = 1 and (e i ) = 0, for all i = 0, we formally construct the dual Hopf algebra H * and the pairing by defining the dual basis {e * i } i∈I ∈ H * such that e * i , e j = δ ij . The dual Hopf structure is then found from the pairing relations (2.14). The product of two elements f, g ∈ H * can be expanded in the dual basis as 16) and the coproduct of an element f ∈ H * can be expanded in the dual tensor basis as Notice that the (co)algebra structure of H * is completely fixed in terms of the (co)algebra structure of H. Specifically, the coalgebra structure on H determines the algebra structure on H * and vice versa. It already follows from the requirement (e i ) = δ i0 that the dual of the unit 1 * is also the unit of the dual. We will omit the star on the unit for convenience 1 * = 1.
Quantum double. For any given Hopf algebra H we can construct its quantum double DH, which is a Hopf algebra with a quasi-triangular structure. It is generated by H and H * cop as Hopf sub-algebras 2 and can be built on H ⊗ H * cop as a vector space. We need to specify the algebra relations that deal with elements from both H and H * cop . These so-called cross-relations are defined by Since the coproduct on the dual was transposed, the pairing is now a skew pairing, i.e. the pairing relations (2.14) and (2.15) are replaced by One of the virtues of the quantum double is that there is an explicit formula for the universal R-matrix R = i∈I e i ⊗ e * i ∈ DH ⊗ DH, (2.20) where {e i } i∈I ⊂ H and {e * i } i∈I ⊂ H * cop are dual bases. The cross-relations (2.18) can in fact be found by the condition that the R-matrix of this form has to satisfy (2.11).

Quantum enveloping algebra
In this paper we will consider q-deformed universal enveloping algebras U q (g) of a Lie (super)algebra g. The quantum enveloping algebra U q (g) is the unital associative algebra over the ring of formal power series C[[ ]], where q = e , freely generated by 1 and the generators of g satisfying q-deformed commutation relations, which we will define specifically later. For simple Lie algebras g, the quantum enveloping algebra U q (g) is quasi-triangular. The R-matrix can be obtained by writing U q (g) as the quantum double of the positive Borel sub-algebra U q (b + ). The positive and negative Borel sub-algebras b ± of a Lie algebra g are defined in terms of the positive and negative root space g ± and the Cartan sub-algebra h as In order to relate the quantum double In this way we obtain two copies of the Cartan algebra h. Taking this fact into account, we can write We will go through this procedure for g = sl(3) as a guideline and in order to illustrate the calculations. Then we will focus on the actual algebra of interest psl(2|2) C 3 . For this algebra (2.22) does not hold true and we will need to extend it in a consistent way such that the extended algebra satisfies (2.22) and consequently can be written as a double.

U q (sl(3)) as a quantum double
Next we will apply the techniques discussed above to describe the dual structure of U q (sl(3)). We refer to [21] for additional details. While this example is considerably simpler than extended psl(2|2), it still exhibits most features that we will encounter later on.
Algebra. We begin by specifying the algebra structure of U q (sl (3)). The algebra is most conveniently defined in terms of Chevalley-Serre generators. These are the positive and negative simple-root vectors E i and F i as well as the Cartan generators H i , i = 1, 2. The commutation relations among these are given by where the Cartan matrix is In addition, the simple-root vectors need to satisfy the Serre relations (i = j) Coalgebra. The coproduct of the simple-root generators is defined as The expressions for the counit and antipode can be easily derived from the coproduct via their defining properties (2.6) and (2.7).
Basis of the Borel sub-algebra. In order to deal with the cubic Serre relations (2.26) and to define a Poincaré-Birkhoff-Witt basis, it is convenient to define additional generators, corresponding to non-simple roots. To that end we observe that the Serre relations can be expressed in terms of the adjoint actions (2.10) as It is therefore natural to define non-simple-root vectors (2.33) The Serre relations are then expressed as 3 We can now define a convenient Poincaré-Birkhoff-Witt basis for the positive Borel subalgebra U q (b + ) spanned by the basis With the definition of this basis we made a particular choice regarding the ordering of the generators and the definition of the non-simple-root vector E 12 . A priori, it would have also been possible to define for instance E 21 = E 2 E 1 as the non-simple-root vector. The basis (2.36) is however chosen such that later calculations, especially of the R matrix, are rather simple. What this exactly means and how the ordering of the simple-root vectors is connected to the definition of non-simple-root vectors will be discussed in a later chapter.
Dual of the Borel sub-algebra. Let us now consider the dual of the positive Borel sub-algebra U q (b + ) * as defined in (2.14). We will explicitly calculate the Hopf structure of the dual generators. The dual Hopf algebra U q (b + ) * is, by definition, spanned by the dual basis The product of two dual generators expressed in this basis is given by (2.16). From that we can find the algebra relations on the dual. For example let us calculate the product Remember that we omit the star at the dual unit 1 * = 1 for convenience. Let us notice at this point that the positive Borel sub-algebra of U q (sl(3)) is self-dual.
This can be seen directly from the identifications (2.46) U q (sl(3)) as a quantum double. We can now proceed to construct the quantum To that end we need to calculate the crossrelations (2.18). Explicitly we find The identification Note that each Borel half contains a copy of the Cartan sub-algebra. To distinguish them in the double U q (b + ) ⊗ U q (b − ) we denote the Cartan generators of U q (b − ) by H i . We can, however, identify the two copies by quotienting out the ideal generated by H i − H i and thereby recover the Hopf algebra U q (sl (3)). Thus we find that we can write U q (sl(3)) as the quantum double The R-matrix. Having written U q (sl(3)) as a quantum double, it is now straightforward to find the underlying universal R-matrix from formula (2.20). It requires fixing a basis of the positive Borel sub-algebra which we already did in (2.36). Its dual basis (2.37) can be expressed in terms of the generators of the negative Borel sub-algebra as

Presentations and deformations
In this paper we will construct a novel quantum algebra by making a general ansatz for the algebra relations and requiring consistency in order to constrain the parameters of the ansatz. The parameters can be of different types with different implications for the structure of the algebra. In particular one should distinguish between two classes of parameters.
One class of coefficients is related to the presentation of the algebra, i.e. how to write the algebra in terms of symbols that form a generating set of the algebra. Changing the labeling does not actually change the algebra, hence these presentation parameters have no significance, yet some work is needed to identify their nature.
The second class is formed by the remaining coefficients that are actual parameters of the algebra. There are some standard deformations which can be applied to general quantum groups. The parameters that are associated with these deformations are under good control. However, the parameters that do not have such an explanation are the most interesting ones because they signal the presence of non-standard deformations. One may view the quantum parameter q = e to be among them because there is no deformation procedure (along the lines discussed below) to derive it. Nevertheless we will basically not consider this parameter and restrict our attention to the novel parameters appearing in the construction of our particular algebra.
Let us therefore discuss some standard manipulations of quantum algebras that will be needed later.
Change of basis. A quantum algebra is usually presented in terms of a set of symbols, e.g. X i , and relations among them. We can redefine the symbols X i = f (X i , k ) as functions of the original symbols and potentially some parameters k . The algebra relations will take a different form and the presentation parameters may change. Yet they will still represent the same algebra. Of particular interest are transformations that set the presentation parameters to special values. This makes most sense if there is a canonical choice to reduce the complexity or to make the resulting expressions more symmetric.
In q-deformed quantum algebras, the Cartan sub-algebra of the underlying Lie algebra plays a central role. While it is undeformed, it largely determines the deformations of the remaining algebra. Therefore, transformations of the basis should preserve the weights (charges under the Cartan elements) in order not to obscure the algebra relations.
Similarity transformations. Similarity transformations form a special class of basis changes. For an invertible element G all basis elements are transformed according to (2.56) Clearly this change of basis preserves the form of all algebra relations. The form of the coalgebra relations usually changes unless the element G is group-like. The latter case will not affect the presentation parameters because no relations are changed. Even though one might ignore such similarity transformations right away, they are relevant when counting parameters of the algebra relations vs. similarity transformations. A standard similarity transformation uses a Cartan element H X i = e αmHm X i e −αmHm . (2.57) Since the conjugation element is group-like, this transformation has no effect on any of the Hopf algebra relations. By performing the commutators one can see that the similarity transformation amounts to a rescaling of all generators with the exponent given by the weight Symmetric twist. One can also perform a similarity transformation with a quadratic combination of the Cartan elements Here γ mn is a symmetric matrix of coefficients, and the similarity transformation amounts to multiplying the generators by exponents of the Cartan elements. The conjugation element is not group-like, and effectively only the form of the coproduct changes. Therefore, instead of transforming the generators, one can also take the different but equivalent point of view to only redefine the coproduct by the following twist ∆ (X) = e γmnHm⊗Hn ∆(X)e −γmnHm⊗Hn . (2.60) More explicitly, the conjugation of the coproduct acts by inserting various factors of exponentiated Cartan elements A noteworthy special case of the symmetric twist is the transformation on the simpleroot generators It shifts the position of the exponential factors in the coproduct (2.28,2.29) Anti-symmetric twist. A standard deformation of the quantum algebra is given by the Reshetikhin twist of the coproduct [23] ∆ (X) = e βmnHm⊗Hn ∆(X)e −βmnHm⊗Hn , where in contradistinction to (2.60) β mn is an anti-symmetric matrix. As above in (2.61), the twist effectively inserts exponential Cartan elements into the coproduct. In general this twist cannot be compensated by a basis transformation and will therefore lead to a different Hopf algebra. If the Hopf algebra was quasi-triangular then the twisted Hopf algebra is so as well and it has R = e βmnHn⊗Hm Re −βmnHm⊗Hn , (2.66) as its R-matrix.

Maximally extended U q (psl(2|2))
In the following we will state the Hopf algebra structures of the maximal extension of U q (psl(2|2)) which is one of the central results of this paper. This section is meant to provide an overview and summary of the structures and relationships of the algebra. All derivations and proofs will be postponed to the following sections. First, we will give an overview of the algebra and its generators, then we shall summarize the previously known relations of the central extension of U q (psl(2|2)), and finally state the results of the maximal extension of U q (psl(2|2)). The labeling of the corresponding Borel sub-algebras [p][s]b ± will follow the same scheme. The signs +/− indicate to which of the Borel sub-algebras b + /b − the generators belong; ± represents Cartan generators which belong to both. The algebras will be defined in terms of the Chevalley-Serre generators. The simple algebra psg = psl(2|2) has three pairs of positive and negative simple-root generators E i , F i as well as the three Cartan generators H i (which are subject to one constraint). Of these generators E 2 and F 2 are odd, while the other generators are even. The central extension sg = psl(2|2) C 3 is obtained from psg = psl(2|2) by relaxing three constraints. The resulting three additional generators are central, and they are denoted by C, P, K. Dual to the central extension is the extension by an sl(2) outer automorphism algebra pg := sl(2) psl(2|2). We will denote the sl(2) automorphism generators by H A , L, M . The maximal extension 5 g = sl(2) psl(2|2) C 3 finally combines both extensions into one algebra, where now the sl(2) automorphisms also act non-trivially on the C 3 part. Please refer to Fig. 1 and Fig. 2 for an overview of the generators and their weights.

Overview of the algebras
In order to identify the additional generators unambiguously, the extensions C, P, K spanning C 3 will be called momentum generators 6 while the extensions H A , L, M spanning sl(2) will be called boost generators. These terms follow from the fact that the maximally extended algebra sl(2) psl(2|2) C 3 can be viewed as a peculiar supersymmetric Poincaré algebra in three dimensions. In this case C 3 serves as the ideal of momentum generators whereas sl (2) is the sub-algebra of Lorentz rotations; the simple algebra psl(2|2) contains 8 supercharges along with two further internal sl(2) symmetry algebras.
Finally, let us mention two relevant relationships for the elements of g. The invariant quadratic form of g induces a dual pairing of the (qualitative) form: This paring is needed to relate the double of the Borel sub-algebra to the full algebra. Note in particular that the boosts are dual to the momenta. The other relationship is the algebra automorphism which interchanges the Borel sub-algebras: The combination of the two above relationships relates each Borel sub-algebra to its dual (as a bi-algebra).

Hopf structure of the centrally extended algebra
We start by reviewing the q-deformed universal enveloping algebra U q (sg) as provided in [18].
Algebra. The commutation relations of the simple-root generators take the standard form expressed in terms of the symmetric Cartan matrix and the vector of signs Note that the Cartan matrix has non-maximal rank 2. Correspondingly there is a central element within the Cartan sub-algebra, given by 7 In addition, the simple-root generators satisfy the Serre relations which can also be expressed much more compactly using the adjoint action (2.10) as It is straightforward to show that there are two further central elements P and K Setting them to zero reduces the algebra to U q (sl(2|2)), in which case the relations P = K = 0 serve as the quartic Serre relations common to Lie superalgebras. Furthermore setting C = 0 leads to the simple algebra U q (psl(2|2)) = U q (psg). For completeness, let us state the centrality relations Coalgebra. We define the q-deformed coproduct as The coproduct of the central elements C, P, K follows from their definitions (3.8), (3.14) and (3.15) via the compatibility condition and takes the form The counit and the antipode follow from the coproduct by their defining property (2.6) and (2.7) and are given by Non-simple generators. For later usage we introduce non-simple-root generators as polynomials in the simple roots. The positive non-simple-root generators read (3.27) The corresponding negative ones read (3.31) Note that the central elements P and K, which have been introduced above, are naturally among the non-simple-root generators. The Serre relations (3.13) are now expressed as Other algebraic relations of the non-simple-root generators follow from their definitions, and we shall not write them out. Due to the special role of P and K, we shall nevertheless provide many of their relations.

Hopf structure of the maximally extended algebra
In the following we present the maximally extended algebra U q,κ (g). This is understood as the smallest quantum algebra which has the form of a quantum double DU q,κ (b + ) and which contains centrally extended U q (sg) as a proper sub-algebra. It turns out there exists a one-parameter family U q,κ (g) of such algebras labeled by the parameter κ. The algebraic relations we present here have two parameters κ, ω (apart from the conventional quantum parameter q = e ). The first is a true parameter of the Hopf algebra, while the second parameter ω is merely a parameter of the presentation.
We will first simply state the defining relations of said Hopf algebra. Compared to the above algebra, it suffices to specify the relations involving any of the boost generators H A , L, M . In the subsequent section we will provide its construction.
Algebra. The algebra has one additional Cartan generator H A . It is therefore convenient to extend the Cartan matrix a of psl(2|2) (3.7) by one row and one column and define a new matrixã as follows (3.34) We added the new elements at the top and on the left of the Cartan matrix a so that the indices run now through (i, j = A, 1, 2, 3). 8 The extended Cartan matrixã now has full rank 4. There is some freedom to choose the top-left element a AA , and we will parametrize this freedom by the variable The commutation relations of the Cartan generators and the simple-root generators can now be written in terms of the extended Cartan matrix The centrally extended algebra sg is contained as a sub-algebra in the bigger algebra. Thus the commutation relations (3.5), the Serre relations (3.9) and the centrality relations (3.16) carry over to the maximally extended algebra. Note, however, that the momentum generators C, P, K are no longer central in the maximally extended algebra. For instance, The algebra relations involving the positive boost L read whereas those for the negative boost M take the analogous form Finally, the cross-relation for the boosts reads It is convenient to note the algebra relations between the boost and momentum extensions Coalgebra. The coproduct of the simple-root vectors (3.17) is unchanged, and also the boost element H A of the Cartan sub-algebra follows the standard trivial form. For the boosts L and M we find the following expressions (3.57) The antipode reads (3.59)

Special features
Finally, we collect and discuss various salient and unusual features that our algebra exhibits.
Combinations of generators. First, let us comment on the appearance of exponential functions: conventional q-deformed algebras can be formulated in terms of exponentiated Cartan generators K i := q H i and the quantum parameter q := e without the need to resort to log K i or log q (merely the R-matrix requires these in one factor). In this sense, the new Cartan generator H A appears in a non-standard form because it is never exponentiated in the algebra or coalgebra relations. If H A was replaced by its exponent K A := q H A , many relations would have to be formulated in terms of log K A . Conversely, the Cartan generator C can almost always be exponentiated, except for a few terms which vanish upon setting the presentation parameter ω = −κ. In other words, this is an artifact of our presentation rather than a feature of the algebra itself. On a related note, some plain factors of = log q appear in the Hopf algebra relations, for instance in [H 2 , L]. However, this factor cancels neatly for exponentiated Cartan generators, e.g.
Another unusual feature is the appearance of non-trivial products of generators in both the algebra and coalgebra structures, see e.g. (3.38,3.56).
Undeformed automorphisms. While the q-deformation for the psl(2|2) generators and the momenta C, P, K is rather standard, the deformation for the boosts H A , L, M is faint. For instance, when dropping all other generators, the boosts obey the algebra U(sl(2)) rather than U q (sl (2)). Moreover, we can even remove the appearance of the momenta C, P, K by a redefinition (with b + c = ±1) (3.62) Their algebra then reads which is undeformed U(sl (2)) up to the term proportional to κ. This feature is related to the absence of exponentials of the type q H A noted above. It can be attributed to the coefficients d i governing the norm of simple roots in non-simply laced Lie algebras. While the coefficients d i , i = 1, 2, 3 for the simple algebra psl(2|2) all equal ±1, the coefficient d A for the boost generators is (in some sense) infinitesimally small. Therefore the corresponding exponential is approximated well by the constant and linear term, and consequently the algebra of the boosts is undeformed.
Symmetry of the presentation. Whereas the centrally extended algebra is symmetric w.r.t. the interchange of simple-root generators 1 ↔ 3, the automorphisms appear to break this discrete symmetry, (3.38). However, the breaking is due to our choice of basis. There is an equivalent presentation which makes [L, E 1 ] rather than [L, E 3 ] non-trivial; there is also a presentation which makes the 1 ↔ 3 symmetry manifest (see [24]), but this choice will not be convenient to calculate the universal R-matrix. Note that the asymmetry 1 ↔ 3 also shows up in the q-deformed secret symmetry [25].
Momentum invariant. Note that there is a quadratic invariant X involving the momentum generators C, P, K, whose form was already observed in the shortening condition for representations in [18] Since C, P, K are central in U q (sg), it suffices to check that [M, X] = [L, X] = 0 to ensure centrality in U q,κ (g). The latter follows from the algebra relations presented in Sec. 3.3.
Deformation parameters. A final note is that our algebra has two non-trivial deformation parameters and κ (besides the standard ones discussed in Sec. 2.4). The additional parameter ω has no significance for the Hopf algebra because it merely deforms the presentation. In particular, it can be absorbed completely by a redefinition It is nevertheless instructive to keep it in the presentation rather than fixing it to a specific value. The existence of the parameter κ can be attributed to the unconstrained element a AA in the extended Cartan matrix. A curious fact is that the parameter κ can be removed from all algebra relations as well (but not from the coalgebra relations) by a redefinition Here f (C, X) is a function of C and the momentum invariant X in (3.64), and it should obey the differential equation This equation can be solved by a deformation function (with Y : We refrain from implementing this transformation because it would mess up the coalgebra.

Extending the algebra
We aim to express the centrally extended algebra U q (psl(2|2) C 3 ) as a quantum double. The procedure will be analogous to the example of sl(3) discussed in Sec. 2.3. However, the presence of the momentum ideal C 3 will turn out to cause the addition of an sl(2) sub-algebra to our algebra. In the end we find that the centrally extended algebra can be embedded in the larger algebra U q (sl(2) psl(2|2) C 3 ) presented above and that the latter takes the form of a quantum double. In this section we will derive the aforementioned Hopf algebra by enlarging U q (sg) with additional generators L, H A , M such that this enlarged algebra can be identified with the quantum double of its Borel sub-algebra. This is done in several steps: First, we shall construct the dual of the positive Borel sub-algebra U q (sb + ). This is isomorphic to the algebra U q (pb + ) which contains some of the relations of the additional generators. Some of their relations follow from the fact that they are dual to the momentum generators C, P, K, respectively, see (3.3). The dual of the established central extension U q (sg) thus contains relations involving the boosts (but not the momenta). This statement can be made exact at the level of Borel sub-algebras U q (sb + ) * ∼ = U q (pb + ). (4.1) Next, we enlarge the positive Borel sub-algebra U q (sb + ) by an additional Cartan generator H A and by a new generator L to U q,β (b + ). We will do this in a very general way which leaves us with four freely adjustable parameters β i , i = A, 1, 2, 3. Finally, we construct the quantum double of U q,β (b + ), and find that the matching of the Cartan sub-algebras of the dual U q,β (b + ) * with the negative sub-algebra U q,β (b − ) imposes further restrictions on the β i . The resulting double is nevertheless not unique, but it forms a oneparameter family of consistent Hopf algebras U q,κ (g) which contain the centrally extended algebra U q (sg).

Dual of the centrally extended Borel sub-algebra
In the following we will construct the relations of the dual Hopf algebra of the positve Borel sub-algebra U q (sb + ) of the centrally extended algebra as defined in Sec. 3.2. As a result of this construction, we will observe explicitly that U q (sb + ) is not dual to the negative Borel sub-algebra Consequently we cannot write the centrally extended algebra U q (sg) as the quantum double of its positive Borel sub-algebra DU q (sb + ) which motivates the introduction of the boosts in the next chapter.
Dual algebra. The calculation of the commutation relations on the dual algebra is completely analogous to the calculation in the sl(3) case. We first need to fix a basis of U q (sb + ). We choose the PBW basis with following ordering E n 2 2 E n 12 12 P n P E n 32 32 E n 132 and define the dual vector space U q (sb + ) * as the span of the dual basis (E n 2 2 E n 12 12 P n P E n 32 32 E n 132 The product of two dual generators expressed in the dual basis is found as prescribed by (2.16). For instance, the commutator [H * i , E * j ] follows from the basis expansions In this way all commutators and Serre relations are calculated. We find the commutators 9 (4.14) Dual coproduct. The coproduct of dual generators can be expressed in the dual basis using (2.17). As an example let us consider the coproduct ∆P * . To find all contributions to that coproduct, we need to consider all pairs of basis elements (x, y) such that their product xy expressed in the basis (4.3) contains a contribution of P . This happens in the following cases Performing the analogous consideration for all dual generators we find the dual coproducts, noting that (H n i i ) * = (H * i ) n i /n!
For details on the calculation of the coproduct ∆P * see also App. A.
Dual Hopf algebra structure. The above dual Hopf algebra relations show explicitly that U q (sb + ) is neither dual to itself nor to U q (sb − ). This fact can be noticed in several of the algebra relations: First of all (4.7) shows that there is no element in the dual Cartan sub-algebra that is central. Therefore it is impossible to identify the dual Cartan sub-algebra with the Cartan sub-algebra of U q (sb ± ). Furthermore, the dual quartic Serre relation (4.11) has no generator on the left hand side, and is not related to P * . So again, we cannot make an identification with U q (sb ± ) because there is no element in the dual we could identify with K. Finally, P * is a non-central element (4.8) which has no analogue in U q (sb ± ). Alternatively, this fact follows from the dual coproduct: To identify E j or F j with E * j one would also need to identify H j with −1 3 i=1 a ij H * i to make the exponent in the coproduct match. This is, however, not possible since the Cartan matrix a ij is degenerate. Furthermore, the unusual form of the coproduct of P * makes it clear that we cannot identify it with any element in U q (sb ± ).
All in all we find that, unlike in the sl(3) example, we cannot identify the dual of the positive Borel sub-algebra with the negative Borel sub-algebra U q (sb + ) * cop U q (sb − ). This is solely due to the presence of the central elements which fail to be central upon dualization.
We will eventually fix this issue by the introduction of three additional boost generators H A , L, M to the algebra, such that the Borel sub-algebras of that extended algebra satisfy the duality relation The idea is that the dual generators of the boosts H * A , L * shall be identified with the (almost) central generators C, K while the the duals of the (almost) central generators C * , P * shall be identified with the boosts H A , M . In total, the situation is depicted in Tab. 1.
Furthermore the introduction of the boost generators shall be such that it keeps centrally extended U q (psl(2|2) C 3 ) unchanged as a Hopf sub-algebra of the enlarged Hopf algebra.

Extending the positive Borel sub-algebra
We extend the Borel sub-algebra U q (sb + ) to U q (b + ) by adding the two boost generators H A andL such that its quantum double DU q (b + ) contains a sub-algebra that can be identified with U q (sg). This requirement will fix the Hopf structure of the boost generators. We denoteL with a tilde to leave the plain L for a redefined version of it later. We will make a general ansatz with a couple of free parameters which we subsequently constrain to ensure a consistent Hopf algebra structure.
PBW basis. We have to include the new generators in our basis and define a PBW basis for the positive Borel sub-algebra U q (b + ) with the following ordering of generators This ordering will turn out to be a convenient choice for calculating the R-matrix. The reason for that will be explained later in Sec. 5. Given this basis we define the dual space as the span of the dual basis Extending the Cartan sub-algebra. First we focus on the Cartan sub-algebra and the additional boost generator H A . We have seen before that the ranks of the Cartan matrix and the dual Cartan matrix did not match. Now let us make the Cartan subalgebra self-dual by adding an additional generator H A . For it to be part of the Cartan sub-algebra we require For the commutators with the simple-root vectors we extend the Cartan matrix by a fourth row The new entriesã Aj have to be such that the rank of the extended Cartan matrix is equal to the rank of the extended dual Cartan matrix which will turn out to be 3. Thus we requireã A1 + 2ã A2 +ã A3 = 0. By a redefinition of H A we can always setã Ai = δ 2i . So without loss of generality, we have the extended Cartan matrix Now, let us repeat the calculation of the dual of the extended Cartan sub-algebra to see whether the Cartan sub-algebra has become self-dual. Actually, hardly anything changes compared to the non-extended case in Sec. 4

.1. The new dual generator H *
A commutes with all simple-root generators since H A does not appear in the coproduct of any of the simple-root generators E i The commutator (4.7) remains unchanged We observe that now we have a central element in both the Cartan (C) and the dual Cartan sub-algebra (H * A ) so that its possible to identify them with each other.
Before we continue with the introduction of the additional boost generatorsL we note that the coproduct of the simple-root generators (4.17) now also has an exponential in H * A appearing in the right tensor factor. This is due to (4.23) and our choice of PBW basis (4.20) Introducing the positive boost. Next we introduce the additional boost generator L to our Borel sub-algebra. We denote it with a tilde to leave the plain L for a redefined version of it later. The strategy that we are following is this: The Hopf structure is completely fixed once we know the coproduct of all generators and their duals. However, we do not immediately know what the coproduct of the new elementL should be. Also the coproduct of P * calculated above might get additional terms through the introduction of L * . So in order to find the coproduct ofL and P * we first consider the commutators. Only commutators that produce a single factor ofL * or P i.e. [·, ·] =L * + . . . or [·, ·] = P + . . . can give rise to contributions of the coproduct ofL and P * , respectively. Therefore, we first focus on commutators of such form and try to determine them. The requirement to leave the sg sub-algebra unchanged indeed fixes them so far that we only need to use 8 parameters to make the most general ansatz. Subsequently, we can calculate the coproduct of all generators and their dual generators. This fixes also all other commutators. Finally the parameters are constrained by the requirement of compatibility between coproduct and commutators.
In the case of non-trivial P we have shown above that the dual quartic Serre relation remains trivial, [E * 12 , E * 32 ] = 0. In order to accommodate for the momentum extension in the dual, we modify this relation by the dual generatorL * in analogy to the definition of the momentum P = [E 12 , E 32 ] in (3.27) Here, we fixed the prefactor corresponding to a rescaling ofL * for later convenience. From this new relation and the coproduct of the E * i in (4.28), the coproduct ofL * follows straight-forwardly ∆L * =L * ⊗ e −2H * A + 1 ⊗L * . However, we have some freedom to modify the commutators of the Cartan sub-algebra with P * given in (4.8) along with [H * A , P * ] = 0 by the introduction ofL * as follows The four new parameters β A,1,2,3 parametrize our ignorance. One could also allow for additional product terms such asL * H * n j . They, however, will not affect the calculation of the coproduct, and, eventually, consistency of the Hopf structure will rule them out.
Similarly, we can now construct some of the algebra relations ofL which will be needed for the coproduct ∆P * . From the dual coproduct ∆L * in (4.30) the algebra relations of the Cartan sub-algebra withL are determined to some extent by dualization (2.17). In analogy to (4.32) we can extend the resulting relations by the introduction of P H A ,L = 2L + α A P, H i ,L = α i P, i = 1, 2, 3.

(4.33)
This adds four more free parameters α A,1,2,3 to our algebra. At this stage, the remaining coproducts ∆P * and ∆L * are fixed from the algebra relations. Note that we do not yet know the relation [L, E j ] and [P * , E * j ], but due to the weights of the involved generators we know that they cannot contain a term proportional to the basis elements P andL * . By dualization (2.17) of the above algebra relations we obtain where the latter equation extends the relation (4.18). This completes the structure of the coalgebras. The remaining algebra relation follow by dualizing once more Let us also derive two noteworthy commutators It remains to be seen if our ansatz (in terms of 8 parameters α i , β i ) gives indeed a consistent Hopf algebra, i.e. we need to check the compatibility of product and coproduct. In particular, compatibility of the commutators [L, E] and [P * , E * ] with the coproduct induce relations between the parameters α and β. By considering the termsL ⊗ E, E ⊗L and P * ⊗E * , E * ⊗P * we find that the coproduct is only compatible with the commutators if Thus, we find that the extended Hopf structure is consistent if and only if (4.40) is satisfied. This provides three constraints leaving us with five free parameters. This concludes the construction of the Hopf relations of the added boost generators in terms of 8 parameters α i , β j . Before we continue to construct the quantum double of the enlarged Borel subalgebra let us try to understand the parameters of our ansatz.
Presentations and deformations. We have derived a quantum algebra for b + along with its dual in terms of 8 additional parameters α A,1,2,3 , β A,1,2,3 subject to 3 constraints. Let us investigate the deformations of the algebra and of its presentation along the lines of Sec. 2.4 in order to understand the roles of these parameters better. We can perform similarity transformations by conjugating with exponentiated Cartan elements. As usual these merely rescale the generators by numerical factors, and change neither the algebra relations nor their presentation. One noteworthy similarity transformation is by e C : It leaves all elements unchanged, but transformsL according to (4.41) A shift ofL by P is thus inconsequential. Similarity transformations by exponentiated quadratic combinations of the Cartan elements lead to symmetric twists of the coalgebra. Most of these modify the presentation of the centrally extended sub-algebra, and thus we do not want to consider them here. There remains one admissible symmetric twist by e C 2 /2 which merely transformsL according to This similarity transformation changes the coefficient of C ⊗ P in the coproduct ∆L and it introduces an additional term P ⊗ C.
The anti-symmetric twists also modify the centrally extended algebra structures, hence it remains to consider redefinitions of the generators. In particular, we will focus on the redefinitions ofL in order to preserve the centrally extended algebra manifestly: We can rescaleL (without rescaling P at the same time). This transformation amounts to changing the normalization chosen in (4.29). We can also shiftL by E 2 E 132 , E 2 E 12 E 3 , E 2 E 32 E 1 , E 12 E 32 or H A P . This changes the presentations of the Hopf algebra relations substantially. These transformations can be used to lift the distinguished role of E 3 in (4.36) and instead let E 1 take this role. Similarly, one can find a more democratic prescription where E 1 and E 3 are on equal footing. These presentations, however, will not be convenient for our discussion of the R-matrix, and we shall not consider them here.
Finally, we can shift the boost H A by the momentum C. This transformation can be seen to introduce a term P ⊗ C in ∆L and at the same time shift the commutator [H A ,L] by P . This transformation combines nicely with the above similarity transformation such that the additional contributions C ⊗ P in the coproduct ∆L cancel. The relevant combination readsL Thus only the set of established parameters changes by Two Hopf algebras related by this change of parameters are actually identical. This fact can be used to fix one of the parameters β i or α A to any convenient value, e.g. α A = 0. 10 Altogether this implies that the above Borel sub-algebra algebra U q,β (b + ) can be specified in terms of the four parameters β A,1,2,3 along with the quantum parameter q. The parameter α A merely serves as a deformation of the presentation and can be fixed at convenience.
Duality relationship. Finally, let us discuss the structure of the dual algebra. In our construction we imposed a similar set of relations on the algebra and its dual. Therefore it is likely that the algebra is structurally self-dual. Indeed by identifying the generators with the dual generators as follows 11 where λ 2 = 1, λ 12 = −q, λ 32 = −q −1 and λ 132 = 1, the dual Hopf algebra has the same structure as the original one. Even though the Hopf algebra structure is the same, the parameters β A,1,2,3 and α A,1,2,3 change between the algebra and its dual according to 12 (4.54) Thus the formal duality statement is Only for the special choice of parameters 10 This is possible unless β A = −1 (q − q −1 ), a special case which will not be of further interest to us. 11 Note that the combination of terms appearing in the identification ofL is reminiscent of the conjugation of P * by e H * A H * 2 / . However, e −H * , i.e. the two expressions are unrelated. 12 One can observe that the undetermined parameter ζ in the above identification translates between the particular choices of α A and α A in each of the algebras.
the Hopf algebra becomes self-dual. Here, the choice ζ = −2ω − κ of the undetermined elementã AA of the Cartan matrix (3.34) ensures that the duality transformation maps between equal presentations of the algebra. This algebra has one degree of freedom κ whereas ω merely describes a degree of freedom of its presentation.

Doubling the extended sub-algebra
In this section we compute the quantum double corresponding to the extended positive sub-algebra. We find that identifying the dual with the negative sub-algebra puts additional restrictions on our parameters. From now on we use the dual with the opposite coproduct U q (b + ) * cop as required by the quantum double construction.
Cross-relations. Let us first calculate the cross-relations defined by (2.18). The commutation relations between the generators and their duals of the original sub-algebra are The commutators between the Cartan sub-algebra and the new generators are given by Identification. We have constructed the quantum double of the enlarged Borel subalgebra DU q,β (b + ) = U q,β (b + ) ⊗ U q,β (b + ) * cop . Instead of using the dual generators we would rather like to express the double with the generators of the negative Borel subalgebra. Indeed, for the generators of the negative Borel half of sg the identification with respective dual generators is straight-forwardly found by comparing the commutators and the coproduct of U q (b + ) * cop and U q (b − ) So far we have not yet defined the negative boost generator M . Therefore we define it essentially as the dual generator P * . On the level of the algebra this means that we define the negative Borel half of the maximally extended algebra g via However, we have a certain freedom in doing so and we use this freedom to choose a symmetric version between both Borel halves.
To that end we also redefine the boost The Cartan generators H j of the negative Borel sub-algebra are identified as This identification explains why it was useful to introduce the A-column in the extended Cartan matrix (3.34). Here, the new parameterã AA = ζ represents the freedom to add the momentum generator C to H A .
Reduction. This concludes the identification U q,β (b + ) * cop ∼ = U q,β (b − ), and we can thus write the quantum double as DU q,β (b + ) = U q,β (b + ) ⊗ U q,β (b − ). The quantum double, however, contains two copies of the Cartan generators, so that we would like to identify them by quotienting out the respective ideal as we have seen in the sl (3) case. This identification of the two copies of Cartan generators provides another constraint on the parameters α i , β i . Namely for the commutators (4.62) and (4.33) to be consistent using the identification (4.75), we require This provides an additional four relations on our a priori 8 + 1 parameters α i , β i and ζ.
Together with the three constraints (4.40) we are left with two degrees of freedom. We express the family of solutions in terms of two free parameters κ, ω This set of parameters is exactly the same set of parameters (4.56) that is required for a self-dual Borel sub-algebra. Therefore, self-duality is naturally required by the identification of the quantum double with U q,κ (g). This concludes our derivation of the algebra relations for the maximally extended algebra g presented in Sec. 3.3. At this point it makes sense to discuss the remaining parameters. The requirement that the Hopf structure satisfies the compatibility relation between coproduct and product together with the requirement that we can identify the two copies of the Cartan subalgebra in the quantum double fixes all but two parameters of our ansatz α i , β i and ζ. Furthermore, the redefinition in (4.44) reduces in terms of the generators (4.73) and (4.74) to and acts on the remaining parameters ω, κ as This shows that the resulting Hopf algebra has merely one degree of freedom κ whereas ω serves as a parameter of the presentation. We can thus set ω to any desired value such as ω = 0 or ω = −κ.
In conclusion, we have found a one-parameter family of Hopf algebras that contain q-deformed centrally extended sl(2|2) as a Hopf sub-algebra.

R-matrix
Having constructed the quantum double of our extended algebra, we are left with the construction of the corresponding R-matrix. It follows from the general formula for the universal R-matrix of a quantum double The above sum runs over a complete basis {e i } i ⊂ U q (b + ) and its dual basis.

Basis
In order to get a compact expression for the R-matrix it is important to make a good choice for the basis. Therefore let us first briefly explain what we consider a good basis and whether such a basis exists for our algebra g.
General considerations. Since we are dealing with a universal enveloping algebra a convenient basis will be of PBW type e n 1 1 e n 2 2 · · · e n l l in terms of some generators {e i } 1≤i≤l . In addition it should also satisfy that its dual basis can be expressed as PBW type basis of the dual generators {e * i } 1≤i≤l . In other words we would like that the pairing relation factorizes such that e * n 1 1 · · · e * n l l = (−1) l i=1 l j=i+1 n i n j |e i ||e j | e * n 1 1 , e n 1 1 · · · e * n l l , e n l l (e n 1 1 · · · e n l l ) * .
The benefit is that then also the R-matrix factorizes which provides an easier expression R = ∞ n 1 ,··· ,n l (e n 1 1 · · · e n l l ) ⊗ (e n 1 1 · · · e n l l ) * = ∞ n 1 e n 1 1 ⊗ e * n 1 1 e * n 1 1 , e n 1 1 · · · ∞ n l e n l l ⊗ e * n l l e * n l l e n 1 l .

(5.3)
A sufficient condition for the paring to factorize is the following: Given the unit 1 and l generators e i , i = 1, . . . , l with (e i ) = 0 for all i. Define for 1 ≤ i ≤ j ≤ l the sets Our algebra. Let us now assume that B 1l is a PBW basis of U q (g). Furthermore assume that the Hopf structure of the generators e i satisfies the following conditions regarding the linear spans B ij : • The product respects the ordering of the basis e i e j ∈ B min(i,j) max(i,j) . (5.5) • The coproduct respects the ordering of the basis If these conditions are met then the pairing factorizes as given by (5.2). A proof of this statement is given in App. B. For the quantum double of the enlarged algebra constructed above we can only find such a basis if ω = κ = ζ = 0. In that case our basis choice (4.20) satisfies the conditions above. To see this, consider first the commutators They tell us that to satisfy the condition (5.5) we have to put P between C and L and between H A and L in the ordering of the basis; the latter, however, only if ω = 0. Now, consider the following part of the coproduct ofL The last two terms tell us that in order to satisfy condition (5.6) we have to choose the ordering H A LP and P LC; the latter of course only if ω + κ = 0. It is now immediate to see that we can only find an ordering of generators satisfying conditions (5.5) and (5.6) if ω = κ = 0. In that case our choice of PBW basis (4.20) satisfies these conditions.

Computation
We will first calculate the universal R-matrix for the special case ω = κ = 0. Later we will extend the calculation to the general case; this will take considerably more effort, and it will not lead to the factorized form (5.3).
= δ n,m [n; q −ã ii ]!, i = 1, 3, From what we have learned above, the R-matrix factorizes in our choice of basis into powers of each generator Altogether the R-matrix of the quantum double DU q,0 (b + ) is 13 R-matrix for κ = 0. For κ = 0 we cannot find a PBW basis that satisfies the conditions (5.5) and (5.6). One can see this for instance from the coproduct ofL (4.34): with the C ⊗ P term appearing we would need to choose an ordering PLC which is in violation with the orderingLP C demanded by the commutator [L, C] ∝ P .
In particular, unlike the κ = 0 case, the universal R-matrix does not factorize as nicely. The complication arises from the pairings L * n P * m ,L k P l that are no longer proportional 13 The exponents for the odd terms terminate after the first term, e.g. exp(E 2 ⊗ E * 2 ) = 1 ⊗ 1 + E 2 ⊗ E * 2 . The q-exponentials were defined in (2.55).
to δ n,k δ m,l . One can convince oneself that the introduction of κ will only affect these pairings; the part of the R-matrix involving other generators will stay the same. In the following we set w.l.o.g. ω = 0 since it can be reintroduced by a simple redefinition of generators at the end.
The pairing of arbitrary monomials is calculated by reducing it to pairings of single generators using multiple times (2.14). The details of this rather lengthy calculation are found in App. C. At the end (combining Lemma C.3, Lemma C.4 and Lemma C.5) we obtain the following expression for the relevant pairing: Here θ A denotes the characteristic function 20) and the sequence f n is generated by the function Now, to get a nice expression for the R-matrix, the next step is to express the dual basis -in particular (L n P m ) * -in terms of the PBW basis of dual generators, i.e. L * k P * l . The pairing provides the coefficients t a nm = L * a−n P * n ,L a−m P m = θ n≥m (a − m)! n! q − q −1 n−m f n−m We used the fact that only monomials with the same total number of generators contribute, as can be seen from (5.19). Therefore the basis transformation is a direct sum of basis transformations of finite-dimensional subspaces labelled by a ≥ 0. However, we are actually interested in the inverse transformation: L a−n P n * = a m=0t a nmL * a−m P * m . (5.24) As shown in Lemma C.6 the inverset a mk is given bỹ wheref n is generated bỹ We have now found all ingredients for the R-matrix. The parts of it that do not contain the generatorL or P are just the same as in the κ = 0 case. The term involving L and P is: Finally, the R-matrix for κ = 0, ω = 0 is given by The generalization to ω = 0 is straight-forward. We will not need it here, and we shall do it after transforming to the basis for U q,κ (g) introduced in Sec. 3.

Chevalley-Serre form
Instead of the dual generators we would like to express the R-matrix in terms of the Chevalley-Serre generators of the negative Borel sub-algebra. In the identification of the fermionic generators (4.70) some factors of e H * A appear. Surprisingly, these are exactly the factors appearing if we commute the exp[H A ⊗ H * A ] term from the left to the right of the R-matrix So eventually in terms of the generators of the negative Borel sub-algebra and the redefined L we have Now this expression contains two mixed exponentials each with an unwanted term P H A ⊗ K. Interestingly, the unwanted terms come with the opposite sign. Therefore it makes sense to combine these two exponents. Using Lemma C.7 and its inverse with appropriately chosen X and Y , and Z = 1 2 H A ⊗ 1 we arrive at where we defined the function The conjugation of the term P ⊗ K with P H A ⊗ K yields The R-matrix now takes the compact form We have verified the κ-dependence explicitly by means of the quasi-cocommutativity relation (2.11). Note that some commutations of exponents induce a derivative of g 2 which cancels against a contribution from g 1 using the relation g 2 = −g 1 + (1 + x) −1 .
R-matrix for ω = 0. With the redefinition of H A in (4.80) we can reintroduce a nontrivial ω from the case ω = 0: This is achieved by simply replacing H A → H A + ωC in (5.36) leading to the fully complete universal R-matrix Note that the combination −(κ + 2ω) is just the variable elementã AA = ζ of the extended Cartan matrix. As such the terms on the latter line are precisely the quadratic combination of the Cartan sub-algebra specified by the inverse extended Cartan matrix.

Classical limit
Let us finally consider the classical limit → 0. Our algebra U q,κ (g) admits a well-defined → 0 limit and we will consider the leading and the sub-leading order. To leading order, the algebra should simply reduce to the Lie superalgebra g. In particular, we will see that the Lie superalgebra g does not depend on κ. The effects of the quantum deformation and κ are seen in the next-to-leading order. At this order, our algebra U q,κ (g) reduces to a Lie bialgebra with an interesting cobracket and classical r-matrix.
Lie algebra. First, let us consider the commutation relations when → 0. The boost generators should form a standard sl(2) sub-algebra whose commutation relations were already specified in [6].
Taking  where, of course, E 132 and F 213 are understood as the classical limit of (3.26) and its analogue in the negative Borel sub-algebra. It is easy to see that these relations agree with [6] in the case κ = ω = 0. The parameters κ and ω only appear in then the commutation relations of the boost generators (6.1). They can be completely absorbed by the redefinition, cf. Thus, we see that the algebra relations can be made κ independent and the algebra simply reduces to g.
Lie bialgebra. To study the effects of the quantization, we associate a quasi-triangular Lie bialgebra to our one-parameter family of Hopf algebras. We will now work to first order in and introduce the cobracket δ and classical r-matrix r ∆J − ∆ cop J =: 2 δ(J) + O( 2 ), (6.9) R =: 1 + 2 r + O( 2 ). (6.10) The cobrackets of the boost operators then directly follow from their coproducts Notice that the redefinition (6.7) will only eliminate ω but not κ. In particular, one can make either the cobracket or the commutation relations independent of κ.
Similarly, one can derive the classical r-matrix directly from (5.18) Moreover, the r-matrix generates the cobracket via the so-called coboundary condition which is easily checked through direct computation.

Conclusions and discussion
In this paper we considered Drinfeld's quantum double construction for q-deformed centrally extended psl(2|2). We find that the dual elements corresponding to the central extensions are not central in the dual algebra. We are therefore led to the introduction of a new set of boost generators that form an sl(2) algebra to serve as the duals of the central extensions. By adjoining these generators to centrally extended psl(2|2) we form a novel algebra which we call maximally extended psl(2|2). This algebra is defined as the smallest Hopf algebra that contains centrally extended psl(2|2) as a sub-algebra and that can be written as a double. These requirements lead to the algebra U q,κ sl(2) psl(2|2) C 3 , This novel algebra displays a number of exciting features that are not present for standard quantum algebras, cf. Sec. 3.4. For example, we observe the appearance of plain factors of = log q and parts of the extended algebra are actually not q-deformed. Nevertheless, maximally extended psl(2|2) can be written as a quantum double and thus it has a universal R-matrix (5.36). It turns out that, just like the maximally extended algebra, the R-matrix also displays some peculiar features. In particular, it has a nontrivial functional form involving a dilogarithm function. Curiously, the R-matrix does not factorize into products of exponentials. We have also computed the corresponding classical r-matrix, which yields a novel solution of the classical Yang-Baxter equation.
This is a first rigorous derivation of a universal R-matrix which is related to centrally extended psl(2|2). Yet the R-matrix (5.36) is not the universal R-matrix that describes the one-dimensional Hubbard model or the AdS/CFT integrable system. Nevertheless, it should provide a first important step in the construction of the universal R-matrix of these models. In particular, for the Hubbard and AdS/CFT integrable models, the central extensions are identified with one braiding generator that deforms the coproduct. Moreover, these models also admit Yangian or quantum affine extensions, which we have not considered in the current paper.
Finally, the representation theory of this algebra is unexplored. It is not clear what kind of representations it admits. For instance, a (minimal) finite-dimensional representation could be applied in the construction of transfer matrices and the algebraic Bethe ansatz. However, such a representation could not be unitary due to the structure of the algebra. For purposes of physics, it would therefore be equally important to work out some unitarizable infinite-dimensional representation. Last but not least, it would be useful to find some physical model that exhibits the maximally extended algebra as a symmetry and for which the R-matrix would certainly play an important role.
The derivation of the coproduct for P * in (4.18) is as follows The coproducts of powers of simple-root vectors are given by where the q-binomial is defined via the q-numbers

B Orthogonality condition
Consider a Hopf algebra with unit 1 and l generators e i , i = 1, · · · , l with (e i ) = 0 for all i. For any pair of integers 1 ≤ i ≤ j ≤ l, define the sets i+1 · · · e n j j n k ∈ N 0 , i ≤ k ≤ j , where we understand e 0 k = 1 as the unit. Let us assume that B 1l is a PBW basis of U q (g). Moreover, assume that the Hopf structure of the generators e i satisfies the following conditions regarding the linear spans B ij • the product respects the ordering of the basis e i e j ∈ B min(i,j) max(i,j) , (B.2) • the coproduct respects the ordering of the basis We can then prove the following result that was used to compute the R-matrix (cf. equation ( a ∈ B ir , b ∈ B js ⇒ ab ∈ B min(i,j) max(r,s) , (B.5) • the coproduct respects the ordering of the basis Proof. Consider two elements a = e n i i · · · e nr r ∈ B ir and b = e n j j · · · e ns s ∈ B js . For r < j the concatenation of both words is already in the correct order of the PBW basis and we immediately have ab ∈ B is = B min(i,j) max(r,s) . For j ≤ r however we need to commute the generators e j up to e min(r,s) at the beginning of the second word through the generators e max(i,j) up to e r at the end of the first word e n i i · · · e n max(i,j) max(i,j) · · · e nr r e n j j · · · e n min(r,s) min(r,s) · · · e ns s .
Due to (B.2) the commutators satisfy [e u , e v ] ∈ B uv . So whatever is created by reordering the generators in the product can at most lie in B min(i,j) max(r,s) . The statement for the coproduct follows from the fact that the coproduct is an algebra homomorphism ∆(e n i i · · · e n j j ) = ∆(e i ) n i · · · ∆(e j ) n j . (B.8) Since for each generator the first tensor factor lies in B il also their product lies therein due to (B.5). Equally since the second tensor factor of the coproduct of each generator lies in B 1j also their product lies therein. By linearity of the (co)product the lemma follows. Proof. We prove this by induction over i. For i = 1 the result follows from the previous lemma. Now assume that for some i, 1 ≤ i < l the statement (B.17) holds. For i + 1 we have from (2.14) i+1 , e n 1 1 · · · e n l l = e * m 1 i+1 , ∆(e n 1 1 · · · e n l l ) .

(B.18)
Due to the induction assumption the first tensor factor only evaluates non-trivially on e k 1 1 · · · e k i i for some k i . According to (B.6) no such term can appear in the first tensor factor of the coproduct ∆(e n i+1 i+1 · · · e n l l ) ∈ B i+1l ⊗ B 1l , therefore only the unit is permitted in the first tensor factor of that part of the coproduct. Lemma B.2 above tells us there is only one such term 1 ⊗ e n i+1 i+1 · · · e n l l . Now considering the second tensor factor we know that it only evaluates non-trivially on e k i+1 i+1 for some k i+1 . Due to (B.6) we know that ∆(e n 1 1 · · · e n i i ) ∈ B 1l ⊗ B 1i cannot have such a term in the second tensor factor and therefore must have the unit there. Again there is only one such term e n 1 1 · · · e n i i ⊗ 1. We have now an analogous situation to the proof of the previous lemma. The only contributing terms are ∆ e n 1 1 · · · e n i i · ∆ e n i+1 i+1 · · · e n l l ↓ ↓ e n 1 1 · · · e n i i ⊗ 1 · 1 ⊗ e n i+1 i+1 · · · e n l l .

C Details of the R-matrix calculation
We start by calculating the pairing step by step. To facilitate the calculation we will set w.l.o.g. ω = 0.
L * n ,L k P l = n a=0 n a L * n−a ,L k L * a , P l δ a,0 δ l,0 = δ l,0 L * n ,L k = δ l,0 δ n,k n!. L * n P * m ,L k P l = L * n ⊗ P * m , ∆(L k P l ) = n! (L n ) * ⊗ P * m , ∆(L k P l ) . (C.11) To be non-zero we need exactlyL n in the left tensor factor of ∆(L k P l ) = (∆L) k (∆P ) l . SinceL is never produced by any commutatorL n can only come directly from the product of n termsL ⊗ 1, stemming from n factors ∆L, multiplied by terms that have the identity in the left factor, i.e. only k − n terms q −2C ⊗L from ∆L, and l terms q −2C ⊗ P from ∆P . In particular n ≤ k, otherwise we get zero. There are k n choices to pick n terms 1 ⊗L from the k coproducts ∆L. Thus L * n ⊗ P * m , ∆L k P l = n! k n P * m ,L k−n P l . (C.12) Similarly, for P * m ,L k P l = ∆P * m ,L k ⊗ P l = l! ∆P * m ,L k ⊗ P * l * (C. 13) to be non-zero we need exactly l terms 1 ⊗ P * and m − l terms P * ⊗ e −2H * A . There are m l choices to pick these terms form ∆P * m . In particular for l > m the pairing will evaluate to zero. P * m ,L k P l = l! m l P * m−l ,L k . (C.14) To complete the calculation of the pairing we are left with the calculation of P * m ,L n : Lemma C.4. P * n ,L m = δ n,m n! n! q − q −1 n f n , (C. 15) where f n is given by the recursion relation f 0 = 1, nf n = (n − 1)f n−1 − κ 4 n−2 a=0 f a n − a , n ≥ 1. (C. 16) Proof. To evaluate the pairing we split it into P * n ,L m = P * ⊗ P * n−1 , ∆L m . (C.17) For a non-trivial evaluation we need to consider the parts in ∆L m = (∆L) m that have exactly a single P in the left tensor factor. The terms in ∆L that can give rise to such a term are ∆L =L ⊗ 1 + q −2C ⊗L − 1 2 (q − q −1 )P ⊗ H A + 1 2 κ(q − q −1 )Cq −2C ⊗ P + · · · . (C.18) Now P ⊗ · can arise by products of these in one of three cases: 1. There is one term − 1 2 (q − q −1 )P ⊗ H A . Then it cannot be multiplied by any terms L ⊗ 1 or Cq −2C ⊗ P , because they would lead to higher products P n or P C on which the pairing P * , · would evaluate to zero. Therefore non-zero contributions have to come from where denotes equality up to terms on which the pairing evaluates to zero.
2. There is no − q−q −1 2 P ⊗ H A term but one term β 2 Cq −2C ⊗ P . Then there needs to be exactly one termL ⊗ 1 on the right of it to produce a P in the left tensor factor. A quick induction shows that P * n ,L m ∝ δ n.m . Define f n through P * n ,L n = n! n! q − q −1 n f n , (C. 23) and the recursion (C.22) leads to (C. 16).
Lemma C. 5. The sequence f n is generated by the function Proof. Using the recursion relation we get the differential equation which is solved by (C.24) for f 0 = 1.
For each a ≥ 0 and 0 ≤ n, m ≤ a we can write the transformation as