Abstract
We generalize the common notion of descending and ascending central series. The descending approach determines a naturally graded Lie ring and the ascending version determines a graded module for this ring. We also link derivations of these rings to the automorphisms of a group. This process uncovers new structure in 4/5 of the approximately 11.8 million groups of size at most 1000 and beyond that point pertains to at least a positive logarithmic proportion of all finite groups.
1 Introduction
Since early work by Magnus, it was known that the lower central series could be used to make a correspondence with free groups and graded Lie rings. This approach became a mainstay of nilpotent groups in part owing to Graham Higmanβs promotion in lectures such as βLie ring methods in the theory of finite nilpotent groupsβ, which he delivered to the International Congress of Mathematics in 1958 [7]. Already Lazard and Malβcev had exploited this and even more subtle connections; see [9, Chapter 3] for an excellent modern survey. Higmanβs point was to popularize connections to Lie rings ββ¦βless precise, but much more generally applicableβ [7, p.β307]. The spirit of the present article is similar.
This article points out a few ingredients of group/graded Lie ring correspondences that perhaps could have been known at the time but seem largely overlooked in the literature since.
First we observe that the upper central series has an appropriate dual role, leading to Lie module upon which the associated graded Lie ring acts.
Second we show that in general one can refine gradings, and the upper and lower series of a group.
We call descending generalization a filter and the ascending generalization a layering.
The result of such refinement is to further refine inductive studies of groups.
As it stands, the common concept of nilpotence degree clusters most finite group theory into a single family of groups of nilpotence class 2.
Fragmenting this huge family of groups by whatever means helps when studying these groups in detail, for example in enumeration and isomorphism studies [2, 12, 14, 13, 17].
With such applications in mind, we point out how to lift our filters and layerings of a group πΊ to
For simplicity, we first state our results for the traditional
Now consider the usual lower central series where
This gives
Given any group πΊ, a commutative ring π , and an π -module π,
is a natural graded Lie module of the natural Lie ring
For torsion free groups, the use of
For finite groups, the upper central series factors
of the lower central series factors
We will make both these results far more general below.
Our further connection between central factors and Lie algebras relates to the structure of automorphism groups.
In general, we expect no relationship to exist between the central series of a group and its automorphism group (consider
Let πΊ and π΄ be groups and
Both
Theorem 1 and especially its corollary appear classical to us, though we have not discovered them in the literature so far. So we give a synopsis. The mechanics came from a largely unrelated setting. We imitate Whitneyβs cap-product from algebraic topology; cf. [6, Section 3.3]. The cap-product is a mixture of homology and cohomology groups. Likewise, commutation products involving the upper central series factors mix the lower and upper central series factors as follows:
(Here and throughout, we use β£ to distinguish multilinear maps from linear and other functions.) To make a module, we βshuffleβ this product. For example, we create
When the
2 Generalized ascending and descending central factors
We aim for a generalization of Theorems 1 and 2 that introduces Lie products for groups that are graded by monoids other than the positive integers. The purpose is to find new structure.
By permitting acyclic gradings, we can first profit from all the known Lie methods with standard gradings, and then add recursive tools to refine the rings and modules by increasing the grading parameter.
Throughout, we fix
We liberally assume the usual commutator calculus such as in [16, Chapter 5].
Following [17, Section 3], a filter
Thus the burden of explaining the structure of commutation is shared between the operation and the ordering on the monoid. For instance, it is possible to give a total order, i.e. a series of subgroups, but use indices that come from a non-cyclic monoid so that the commutation between the series terms can be far more intricate than by in usual central series.
To every filter
Every filter determines the following π-graded Lie ring [17, Theorem 3.1]:
Here,
Actually, each
In a dual manner, a layering
Form the boundary
These definitions determine abelian groups as follows (proved in Section 3):
To state the main result, we need a version with coefficients. For that, we fix an π -module π and set
While filters give natural ring structures, layerings produce natural modules for these rings. A ring is always better understood with a module.
For every filter
then
Observe that, in the case where
The next important thrust is to push the effects of a filter into the automorphism group and thus linearize various properties of automorphisms that once required more difficult inspection.
Fix groups πΊ and π΄, and a group homomorphism
For groups πΊ and π΄, a group homomorphism
3 The layers L t β’ ( Ο ; Q R ) and the action by L t β’ ( Ο ; R )
Since filters were covered in detail along with examples in [17], we can forgo most of that and focus on the new aspects which concern the interplay with layerings.
We begin by confirming the claims that
Fix a layering
Thus
In particular,
Unlike the case of filters, there is no immediate commutation product to impose on
Straining our metaphors, if (2.2) holds, we say that a filter
If a filter
is well-defined and biadditive.
Proof
Under the assumptions, π also sifts
Now we prove biadditivity of β.
For all
As
Also, for
So
There is of course a similar map
Suppose we have an arbitrary bimap (biadditive map)
Such shuffling of products was introduced for nonassociative division rings (semifields) by Knuth and reformulated in a basis free way by Liebler [10, 11]. Though Knuth called these βtransposesβ, we use the less overloaded term of βshuffleβ as suggested by Uriya First [5]. Here, we use them simply to configure the objects above into familiar vocabulary of Lie modules.
Now let
where, for
This new product is π
-bilinear and further permits us to take the direct sum over all
For all
Proof
From the HallβWitt identity, it follows that, for all
Fix
4 Proof of Theorem 3
To complete the proof of Theorem 3, we must now consider Proposition 3.4 in the presence of the appropriate KnuthβLiebler shuffle.
Fix
Hence, we have
So in
5 Proof of Theorem 4
Now we prove our second claim, Theorem 4, which claims that, for an π΄-group πΊ with an π΄-invariant filter
as follows:
It further follows that
Fix
(using also the fact that π is π΄-invariant).
So
Now suppose
So
Finally, suppose that
Next we show that, for each
so that
modulo
Last we show that
So
6 Examples
We close with some examples. In particular, we demonstrate how filters and layerings arise naturally within arbitrary groups and are not confined solely to nilpotent contexts.
Consider first the case when
Using an acyclic monoid, we get at the structure of π» and πΎ independently.
For example, use
It follows that π is a filter sifting the layering π.
More generally, given groups β and each
We call this a product filter and respectively a product layering.
6.1 Classical and semi-classical filters
Commutation products that are graded by cyclic semigroups we call classical.
Suppose that πΊ is an arbitrary finite or pro-finite group.
In particular, such groups have Sylow π-subgroups and also π-cores
The filters so described depend essentially on words, the commutator, π-th powers, etc. and consist of verbal and marginal subgroups. While they need not have cyclic gradings, they are all characterized by passing through the Fitting subgroup (or begin at the nilpotent residual in the case of layerings) and are then built from classical filters of the primary components of a residually nilpotent group. All such filters we refer to as semi-classical, and they serve as the natural starting point for later refinements which we explore next.
6.2 Some non-semi-classical filters
Now we turn our attention to non-semi-classical filters and explain more carefully what we are reporting in Figure 1. We know of five easily computed non-classical commutation products, three introduced in [17] and two here.
In [17, Section 4], three rings were introduced as tools to create filers (and now also layerings). There are two further rings more compatible with decomposing the composition products of layerings so we now include those as well.
Suppose we start with a known filter π sifting a layering π.
We therefore have various bimaps
First we have a Lie ring of derivations
Next we have three unital associative rings of left, mid, and right scalars.
(Here, we use
Here, π means to take the center of the ring.
Under very mild assumptions on β, the conditions on
In [17],
6.3 Data from small groups
It had been shown in [17, Theorem 4.9] that, using the ring
In [13], Maglione created a series of algorithms for the computer algebra system Magma which permit the physical construction of the
While certainly in some cases the newly recovered characteristic subgroups will have been knowable by other means, we hasten to mention that most of these groups (97β%) are nilpotent of class 2 and have no other standard characteristic subgroups other than the center and commutator. So we are locating structure that is genuinely new. Table 2 shows a breakdown by ring types for groups of prime power order.
π | ||||
---|---|---|---|---|
π | 4 | 5 | 6 | 7 |
2 | 57β% | 75β% | 80β% | 97β% |
3 | 60β% | 70β% | 86β% | 93β% |
5 | 60β% | 68β% | 88β% | 86β% |
7 | 60β% | 67β% | 88β% | 81β% |
541 | 60β% | 51β% | 98β% | β |
π | ||||||
---|---|---|---|---|---|---|
Total % | 57β% | 75β% | 80β% | 97β% | 97β% | 79β% |
% by | 43β% | 51β% | 63β% | 81β% | 68β% | 37β% |
% by | 57β% | 75β% | 76β% | 92β% | 95β% | 79β% |
% by | 57β% | 57β% | 60β% | 65β% | 31β% | 5.4β% |
7 Closing remarks
We have emphasized groups, but the ideas generalize to nonassociative rings and also to loops by first using the work of [15] to obtain an initial associated graded ring. From there, the source for new gradings will likely benefit from the rings described in Section 6.2. There is enormous symmetry within these rings, and often a discovery by one ring can be found by another, but we have examples within the small group data above which show each is needed.
We close with some speculations as to why the proportions seen in Figure 1 are so high.
First, if a group πΊ has a non-semi-classical refinement then so does any central extension of πΊ.
This explains some of the proportions for π-groups of class at least 3.
For π-groups of class 2, we are less certain.
We can take very large random polycyclic presentations of π-groups of class 2, and our methods regularly find nothing of interest.
On the other hand, in another model of randomness, such as taking quotients or subgroups of upper triangular matrices, we find almost all such examples admit non-semi-classical Lie product.
So the likelihood of encountering examples with interesting structure depends greatly on the origin of the groups of interest.
Finally, for non-nilpotent groups, what seems a likely explanation (and occurs in random examples) is that they have relatively large Fitting subgroups, and thus the proportions for nilpotent groups have a strong influence.
But this is not the whole picture as for example 99β% of the
Acknowledgements
The author wishes to thank the referee for suggested improvements, J. Maglione for discussions of the mathematics, and L. Bartholdi who pointed out Magnusβ priority to Higman in the history of associated graded Lie rings of groups.
Communicated by: Evgenii I. Khukhro
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