Abstract
The zero set of a momentum mapping is shown to have a singularity at each point with symmetry. The zero set is diffeomorphic to the product of a manifold and the zero set of a homogeneous quadratic function. The proof uses the Kuranishi theory of deformations. Among the applications, it is shown that the set of all solutions of the Yang-Mills equations on a Lorentz manifold has a singularity at any solution with symmetry, in the sense of a pure gauge symmetry. Similarly, the set of solutions of Einstein's equations has a singularity at any solution that has spacelike Killing fields, provided the spacetime has a compact Cauchy surface.
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Abbreviations
- (P,ω):
-
symplectic manifold
- \(T_{x_0 } P\) :
-
tangent space toP atx 0εP
- G,g:
-
Lie group, Lie algebra
- g·x=Φ g (x):
-
action ofG onP
- ξ P :
-
infinitesimal generator of the action onP corresponding toξεg
- J:P→g*:
-
momentum mapping
- \(dJ\left( {x_0 } \right):T_{x_0 } P \to \mathfrak{g}^ * \) :
-
differential ofJ atx 0
- \(\mathbb{J}\) :
-
complex structure onP
- \(S_{x_0 } \) :
-
slice for theG-action atx 0
- \(I_{x_0 } \) :
-
isotropy group ofx 0; {g εG|gx 0=x 0}
- \(\mathcal{L}_{x_0 } ,\mathfrak{s}_{x_0 } \) :
-
identity component of\(I_{x_0 } \), its Lie algebra [Eq. (10)]
- «,»:
-
(weak) metric onP
- 〈,〉:
-
pairing between g* and g
- \((,)_{x_0 } \) :
-
inner product of g* depending onx 0εP
- \(dJ\left( {x_0 } \right)^ * :\mathfrak{g} \to T_{x_0 } P\) :
-
adjoint ofdJ(x 0) relative to 〈,〉 and «,»
- \(dJ\left( {x_0 } \right)^\dag :\mathfrak{g}^ * \to T_{x_0 } P\) :
-
adjoint ofdJ(x 0) relative to (,) and «,»
- \(T_{x_0 } P = Range\left[ {\mathbb{J} \circ dJ\left( {x_0 } \right)^ * } \right] \oplus Range\left[ {dJ\left( {x_0 } \right)^ * } \right] \oplus \left[ {\ker \left( {dJ\left( {x_0 } \right) \circ \mathbb{J}} \right) \cap \ker dJ\left( {x_0 } \right)} \right]\) :
-
Moncrief's decomposition
- ℙ:g*→RangeddJ(x 0):
-
orthogonal projection
- \(\mathcal{C} = J^{ - 1} \left( 0 \right)\) :
-
zero set ofJ (or constraint set)
- \(\mathcal{C}_\mathbb{P} = \left( {\mathbb{P}J} \right)^{ - 1} \left( 0 \right)\) :
-
zero set of ℙ∘J
- \(N_{x_0 } \) :
-
x's with the same orbit type asx 0
- \(\mathcal{N}_{x_0 } \) :
-
\(N_{x_0 } \cap S_{x_0 } \) (Lemma 1)
- \(\mathfrak{g}_{x_0 }^ * \) :
-
elements in g* with same symmetry asx 0 (Lemma 5)
- f :
-
\(f = \left( {Id - \mathbb{P}} \right) \circ J:\mathcal{C}_\mathbb{P} \cap S_{x_0 } \to \ker dJ\left( {x_0 } \right)^\dag \) (Lemma 8)
- Δ=dJ(x 0)∘dJ(x 0)† :
-
“elliptic” operator associated withJ
- G=Δ−1∘ℙ:
-
Greens' function for Δ
- F(x)=x+dJ(x 0)†∘G∘Q(h),h=x−x 0 :
-
Kuranishi map (Lemma 9)
- \(C_{x_0 } \) :
-
homogeneous cone associated withd 2 J(x 0) (Theorem 3)
- :
-
orthogonal projection onto kerdJ(x 0) (Theorem 3)
- \(\mathfrak{h}\), ℋ:
-
a Lie subalgebra of\(\mathfrak{s}_{x_0 } \), its Lie group
- :
-
points with symmetry (at least) ℋ (Theorem 4)
- :
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Communicated by A. Jaffe
This work was partially supported by the National Science Foundation. The second author was supported by a Killam Visiting fellowship at the University of Calgary during the completion of the paper
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Arms, J.M., Marsden, J.E. & Moncrief, V. Symmetry and bifurcations of momentum mappings. Commun.Math. Phys. 78, 455–478 (1981). https://doi.org/10.1007/BF02046759
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DOI: https://doi.org/10.1007/BF02046759