Semi-Infinite Induction and Wakimoto Modules

The purpose of this paper is to suggest the construction and study properties of semi-infinite induction, which relates to semi-infinite cohomology the same way induction relates to homology and coinduction to cohomology. We prove a version of the Shapiro Lemma, relating the semi-infinite cohomology of a module with that of the semi-infinitely induced module. A practical outcome of our construction is a simple construction of Wakimoto modules, highest-weight modules used in double-sided BGG resolutions of irreducible modules.