In mathematical physics, a Grassmann integral, or, more correctly, Berezin integral, is a way to define integration for functions of Grassmann variables. It is not an integral in the Lebesgue sense; it is called integration because it has analogous properties and since it is used in physics as a sum over histories for fermions, an extension of the path integral. The technique was invented by the Russian mathematician Felix Berezin and developed in his textbook. Some earlier insights were made by the physicist David John Candlin in 1956.
Contents
Definition
The Berezin integral is defined to be a linear functional
where we define
so that :
These properties define the integral uniquely.
This is the most general function, because every homogeneous function of one Grassmann variable is either constant or linear.
Multiple variables
Integration over multiple variables is defined by Fubini's theorem:
Note that the sign of the result depends on the order of integration.
Suppose now we want to do a substitution:
where as usual (ξj) implies dependence on all ξj. Moreover the function θi has to be an odd function, i.e. contains an odd number of ξj in each summand. The Jacobian is the usual matrix
the substitution formula now reads as
Substitution formula
Consider now a mixture of even and odd variables, i.e. xa and θi. Again we assume a coordinate transformation as
The change of the integral will depend on the Jacobian
This matrix consists of four blocks:
A and D are even functions due to the derivation properties, B and C are odd functions. A matrix of this block structure is called even matrix.
The transformation factor itself depends on the oriented Berezinian of the Jacobian. This is defined as:
For further details see the article about the Berezinian.
The complete formula now reads as:
Gaussian integrals over Grassmann variables
The following formulas for Gaussian integrals are used often in the path integral formulation of quantum field theory:
with
with
From the above formulas, other useful formulas follow:
with