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Classical Master Equation and Nilpotence of the BV Differential
Statement
Let be the graded-commutative canonical field/antifield algebra of the preceding definition: , , , and . Its canonical antibracket is odd, has ghost number , and satisfies the shifted skew and Jacobi identities stated there. Assume the center of contains no nonzero element of ghost number one. In the finite-dimensional coordinate algebra this follows because a central element has zero derivative with respect to every field and antifield coordinate and is therefore a numerical constant; for a formal local-functional domain it is an explicit hypothesis. For any even of ghost number zero, define . Then on if and only if the classical master equation holds.
In the local-field case of the preceding definition, let be nonempty and open, let be a finite-dimensional real Lie algebra, and let be the stated parity-preserving representation on the graded matter space. Take the minimal fields and their antifields, with and for every admitted smooth . Assume without equations of motion, where , and that for every field configuration implies . Let be a local functional with well-defined variational derivatives, invariant under these transformations. Choose a functional domain and support or boundary conditions for which the antibracket, integrations by parts, and gauge-variation identity are well-defined and boundary terms vanish. For , , and , form the minimal action in the order specified in the definition. Then this minimal action satisfies the classical master equation .
Statement references
Facts & Assumptions
Given: The canonical BV algebra, its odd antibracket, and an even ghost-number-zero element .
The chosen domain has no nonzero central elements of ghost number one; this follows from canonical finite-dimensional coordinates and is separately assumed for any formal local-functional extension. Classical BV Antibracket and Yang–Mills Master-Action Data
Classical BV Antibracket and Yang–Mills Master-Action Data: the antibracket has ghost number , satisfies shifted graded skew-symmetry and shifted Jacobi, and the BV differential convention is .
Classical BV Antibracket and Yang–Mills Master-Action Data: in the minimal local-field setup, the gauge action is irreducible and off-shell closed, is invariant, and the functional domain admits the stated integrations by parts with vanishing boundary terms.
Classical BV Antibracket and Yang–Mills Master-Action Data: the candidate action is with in the displayed order.
Canonical Antibracket Generates the Declared BRST Field Variations: the antibracket action of on each minimal field and ghost coordinate is .
Nilpotence of the Finite-Jet Non-Abelian BRST Differential: the specified squares to zero on the minimal fields, ghost, and every finite-jet generator.
Proof
For homogeneous of parity , the shifted Jacobi identity with entries gives . Shifted skew-symmetry gives . Combining these identities yields .
Write . Since is antifield-independent, . The displayed antifield signs and left/right derivative convention give , with acting only on here. Gauge invariance says the first variation of vanishes for every admitted even parameter ; its coefficientwise Noether identity therefore remains zero after replacing by the formal ghost , so . The boundary hypothesis justifies the variational derivatives and integrations by parts in this identity. Both and are even, so shifted skew-symmetry gives .
If , step 1.1 gives for every . This is the forward implication.
Conversely, if for every , step 1.1 implies for every , so is central. Its ghost number is by [F1], and [A1] therefore forces . This proves the reverse implication.
By [F3], agrees with on each minimal field and ghost, and therefore on every finite-jet expression in them. Applying the identity of step 1.1 with in place of gives by [F4]. The canonical bracket formula gives . Since is linear in antifields, these variational derivatives determine it modulo boundary terms and yield . It vanishes by [F4].
Expanding , the pure term and both cross terms vanish by step 1.2, while the pure term vanishes by step 2.3. Thus the defined minimal action satisfies the classical master equation under the stated gauge-invariance, off-shell-closure, and domain hypotheses.