Tag Archives: homology

Homological interpretation of the supertrace

Let \mathcal{V} = V_0 \oplus V_1 be a \mathbb{Z}_2-graded vector space, where both V_0 and V_1 are finite-dimensional. Suppose that \phi is a degree-preserving endomorphism of \mathcal{V} (so it consists of endomorphisms \phi_i for both V_i). Then the supertrace of \phi is defined as the difference of the two traces:

\mathrm{str}\phi = \mathrm{tr}\phi_0 - \mathrm{tr}\phi_1.

Obviously, the definition of the supertrace is very simple, but it begs for a justification. For example, why is this better than just adding the two traces? I’ve seen two facts offered in defense of the supertrace (see the Wikipedia article): 1) The supertrace is invariant when applied to endomorphisms of modules over commutative superalgebras, and 2) The supertrace of a supercommutator is 0.

I feel like neither of these justifications is particularly satisfying; the first requires a much more complicated setup than what I described above, and the second passes the burden of proof onto the notion of supercommutator. So I’d like to offer another way of thinking about the supertrace, which should be reasonably convincing to anyone who knows a little bit of homological algebra. I feel like this is one of those ideas that the practitioners in the field all know, but nobody bothers to write down.

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