  In this paper we shall describe some correlation function computations in
perturbative heterotic strings that, for example, in certain circumstances can
lend themselves to a heterotic generalization of quantum cohomology
calculations. Ordinary quantum chiral rings reflect worldsheet instanton
corrections to correlation functions involving products of Dolbeault cohomology
groups on the target space. The heterotic generalization described here
involves computing worldsheet instanton corrections to correlation functions
defined by products of elements of sheaf cohomology groups. One must not only
compactify moduli spaces of rational curves, but also extend a sheaf
(determined by the gauge bundle) over the compactification, and linear sigma
models provide natural mechanisms for doing both. Euler classes of obstruction
bundles generalize to this language in an interesting way.
