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Euclidean Yang–Mills self-dual action bound
Statement
Assume full Axiom of Choice. Let be a nonempty, connected, compact, oriented, finite-dimensional Hausdorff second-countable smooth four-manifold without boundary, with Riemannian metric . Let be a compact non-Abelian matrix Lie group in its specified global form, let be a smooth right principal -bundle, and let be a supplied smooth principal connection. Let be a positive-definite, symmetric, -invariant bilinear form on . For any local section , set and . Invariance of makes the local four-forms and restrictions of global forms. Define . For a real scalar , define the Euclidean Yang–Mills functional by
where the Hodge operator is characterized by and acts componentwise on -valued forms. Then
Equality holds exactly in these cases: if , then ; if , then ; if , then . Every self-dual or anti-self-dual supplied connection satisfies the source-free Euclidean Yang–Mills equation , where . The statement asserts neither existence nor uniqueness of a connection in any charge sector.
Statement references
Facts & Assumptions
Given: Full AC, the nonempty compact oriented Riemannian four-manifold and bundle of the instanton-number definition, a supplied connection, invariant positive-definite , and a real .
Full AC means every family of nonempty sets has a choice function. The Axiom of Choice
The global charge form is , and ; the Hodge star and the self-dual/ASD decomposition are those fixed by the orientation and metric. Trace-normalized instanton number and self-dual sectors
The bilinear form is positive definite. Trace-normalized instanton number and self-dual sectors
The Hodge star on two-forms is an orthogonal involution whose and eigenspaces are orthogonal for the inner product induced by . Hodge decomposition of two-forms in four dimensions
The local curvature satisfies . Principal curvature, Cartan structure equation, and Bianchi identity
Proof
Write with and . Let and . Orthogonality of the two Hodge eigenspaces [F3] makes the cross terms vanish; positivity of [F2] gives . Consequently , while . By [F1], and .
For nonnegative , , so the claimed bound follows after division by . If , then and equality is equivalent to . Its integrand is the continuous nonnegative squared norm of ; if were nonzero at a point, it would be bounded away from zero on a neighborhood of positive Riemannian volume, contradicting . Thus . If , equality in is equivalent to ; the same continuity and positivity argument gives . If , equality forces , so nonnegativity gives and both components vanish, hence . Conversely each of the three stated conditions makes its corresponding norm vanish and gives equality.
By [F4], . For a self-dual connection, ; for an anti-self-dual connection, . This is the stated source-free Euclidean Yang–Mills equation. No boundary term or variational argument is used.