In mathematical physics, a Berezin integral, named after Felix Berezin, (or Grassmann integral, after Hermann Grassmann) is a way to define integration of elements of the exterior algebra (Hermann Grassmann 1844). It is called integral because it is used in physics as a sum over histories for fermions, an extension of the path integral.
Integration on an exterior algebra
Let be the exterior algebra of polynomials in anticommuting elements over the field of complex numbers. (The ordering of the generators is fixed and defines the orientation of the exterior algebra.) The Berezin integral on is the linear functional with the following properties:
for any where means the left or the right partial derivative. These properties define the integral uniquely. The formula
expresses the Fubini law. On the right-hand side, the interior integral of a monomial is set to be where ; the integral of vanishes. The integral with respect to is calculated in the similar way and so on.
Change of Grassmann variables
Let be odd polynomials in some antisymmetric variables . The Jacobian is the matrix
where the left and the right derivatives coincide and are even polynomials. The formula for the coordinate change reads
Consider now the algebra of functions of real commuting variables and of anticommuting variables (which is called the free superalgebra of dimension ). This means that an element is a function of the argument that varies in an open set with values in the algebra Suppose that this function is continuousand vanishes in the complement of a compact set The Berezin integral is the number
Change of even and odd variables
Let a coordinate transformation be given by , where are even and are odd polynomials of depending on even variables The Jacobian matrix of this transformation has the block form:
where each even derivative commutes with all elements of the algebra ; the odd derivatives commute with even elements and anticommute with odd elements. The entries of the diagonal blocks and are even and the entries of the offdiagonal blocks are odd functions, where mean right derivatives. The Berezinian (or the superdeterminant) of the matrix is the even function
defined when the function is invertible in Suppose that the real functions define a smooth invertible map of open sets in and the linear part of the map is invertible for each The general transformation law for the Berezin integral reads
where is the sign of the orientation of the map The superposition is defined in the obvious way, if the functions do not depend on In the general case, we write where are even nilpotent elements of and set
where the Taylor series is finite.
The mathematical theory of the integral with commuting and anticommuting variables was invented and developed by Felix Berezin. Some important earlier insights were made by David John Candlin. Other authors contributed to these developments, including the physicists Khalatnikov (although his paper contains mistakes), Matthews and Salam, and Martin.
- ^ I.M. Khalatnikov (1954), "Predstavlenie funkzij Grina v kvantovoj elektrodinamike v forme kontinualjnyh integralov" (Russian). JETP, 28, 635.
- ^ P.T. Matthews, A. Salam (1955), "Propagators of quantized field". Nuovo Cimento 2, 120.
- ^ J.L. Martin (1959), "The Feynman principle for a Fermi System". Proc. Roy. Soc. A 251, 543.