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Chern–Simons boundary variation and BF equations
Pure mathematics; no physical premises or empirical claim.
Statement
Assume the Axiom of Countable Choice . Let be a compact oriented smooth three-manifold with boundary, a finite-dimensional real Lie algebra with invariant nondegenerate symmetric pairing, and a smooth connection in a fixed trivialization. For an arbitrary smooth variation , the Chern–Simons first variation is
where has the outward-normal-first orientation. Thus the boundary term is part of the variation; the bulk Euler equation is when variations are compactly supported in (or when boundary data make the boundary variation vanish). For the BF action has first variation
so interior-compact variations give the equations and . No boundary condition is imposed by these bulk equations.
For fixed, the boundary potential current of is . On a compact oriented smooth surface (with smooth boundary allowed), put and identify each tangent space with . For define , and let . The restriction to the flat-connection solution space, wherever it is smooth, is the possibly degenerate presymplectic form. Explicitly, on tangent vectors it is the constant closed form
At a flat connection (), tangent variations to the flat-connection equations satisfy . For an infinitesimal gauge parameter , the gauge direction is and
Consequently gauge parameters vanishing on generate null directions on the linearized flat locus. If is field-independent, the displayed boundary variation is for . For , nonzero boundary parameters need not be null and can carry such charges when the boundary conditions permit them; no charge algebra or integrability assertion for field-dependent parameters is made. If , the displayed , and vanish identically, and every gauge direction on the linearized flat locus is null.
Statement references
Facts & Assumptions
Given: A compact oriented smooth three-manifold with boundary, a finite-dimensional real Lie algebra with the invariant nondegenerate pairing in the action definition, smooth fields and variations , and the stated assumption.
The Axiom of Countable Choice (): says every countable family of nonempty sets has a choice function.
Chern–Simons and BF action data: , , , and .
The general Stokes theorem: under , for an oriented smooth -manifold with boundary and a compactly supported smooth -form , with outward-normal-first boundary orientation. Here compactness of makes all smooth forms used below compactly supported.
Proof
Differentiate the Chern–Simons integrand along . The quadratic term contributes . For the cubic term, the graded bracket and invariant pairing give the cyclic identities and for degree-one . Thus the three differentiated terms in are equal, and their average is . The full unintegrated variation is .
Differentiate at ; bilinearity of the bracket gives . Thus the BF variation before integration by parts is .
The graded Leibniz rule gives . Since a two-form commutes with a one-form under wedge, . Substitute this into step 1.1, combine , and apply [F2] to obtain the stated Chern–Simons formula, including its boundary sign.
For variations compactly supported in , the boundary term in step 2.1 is zero. If stationarity holds for every such , then for every local test variation. If were nonzero at some point, nondegeneracy of the pairing would select an algebra element pairing nontrivially with one component; continuity preserves its sign on a smaller chart, and a nonnegative bump supported there would make the integral nonzero. Hence on .
For constant tangent vectors on the affine space , differentiating the potential current from step 2.1 gives and . The field-space exterior derivative is their difference; antisymmetry of the wedge gives . Its coefficients are constant, so its field-space exterior derivative is zero.
Invariance of the pairing makes obey the graded Leibniz rule on paired forms. Therefore . As in step 2.1, the two-form commutes under wedge with . Apply [F2] to obtain the stated BF formula. For variations supported in the interior the boundary integral vanishes. Independent compactly supported and then test and separately; the local nondegenerate-pairing bump argument in step 3.1 shows each must vanish.
At a flat , , so is tangent to the flat locus. For a tangent , . Invariance of the pairing yields . Integrating and applying [F2] on gives , which proves the gauge-contraction formula. If , this contraction vanishes for every tangent . If is fixed under variation, direct differentiation of gives exactly the same contraction.
For , nonzero boundary parameters can in fact give nonzero contractions. Take with pairing multiplication, the unit disk with its standard orientation, , , and . Then , , and . Polar coordinates give , so when . This establishes the stated possibility without asserting that every nonzero boundary parameter has a nonzero charge. If , the formulas for , and all have the common factor and vanish; hence every gauge direction is null in this case.