Gerstenhaber algebra

In mathematics and theoretical physics, a Gerstenhaber algebra (sometimes called an antibracket algebra or braid algebra) is an algebraic structure discovered by Murray Gerstenhaber (1963) that combines the structures of a supercommutative ring and a graded Lie superalgebra. It is used in the Batalin–Vilkovisky formalism.

Definition

A Gerstenhaber algebra is a graded commutative algebra with a Lie bracket of degree -1 satisfying the Poisson identity. Everything is understood to satisfy the usual superalgebra sign conventions. More precisely, the algebra has two products, one written as ordinary multiplication and one written as [,], and a Z-grading called degree (in theoretical physics sometimes called ghost number). The degree of an element a is denoted by |a|. These satisfy the identities

• |ab| = |a| + |b| (The product has degree 0)
• |[a,b]| = |a| + |b| - 1 (The Lie bracket has degree -1)
• (ab)c = a(bc) (The product is associative)
• ab = (−1)^|a||b|ba (The product is (super) commutative)
• [a,bc] = [a,b]c + (−1)^(|a|-1)|b|b[a,c] (Poisson identity)
• [a,b] = −(−1)^{(|a|-1)(|b|-1)} [b,a] (Antisymmetry of Lie bracket)
• [a,[b,c]] = [[a,b],c] + (−1)^{(|a|-1)(|b|-1)}[b,[a,c]] (The Jacobi identity for the Lie bracket)

Gerstenhaber algebras differ from Poisson superalgebras in that the Lie bracket has degree -1 rather than degree 0. The Jacobi identity may also be expressed in a symmetrical form

$(-1)^{(|a|-1)(|c|-1)}[a,[b,c]]+(-1)^{(|b|-1)(|a|-1)}[b,[c,a]]+(-1)^{(|c|-1)(|b|-1)}[c,[a,b]] = 0.\,$

Examples

• Gerstenhaber showed that the Hochschild cohomology H^*(A,A) of an algebra A is a Gerstenhaber algebra.
• A Batalin–Vilkovisky algebra has an underlying Gerstenhaber algebra if one forgets its second order Δ operator.
• The exterior algebra of a Lie algebra is a Gerstenhaber algebra.
• The differential forms on a Poisson manifold form a Gerstenhaber algebra.
• The multivector fields on a manifold form a Gerstenhaber algebra using the Schouten–Nijenhuis bracket