Finite presheaves and $A$-finite generation of unstable algebras mod nilpotents

Abstract

Inspired by the work of Henn, Lannes and Schwartz on unstable algebras over the Steenrod algebra modulo nilpotents, a characterization of unstable algebras that are $A$-finitely generated up to nilpotents is given in terms of the associated presheaf, by introducing the notion of a finite presheaf. In particular, this gives the natural characterization of the (co)analytic presheaves that are important in the theory of Henn, Lannes and Schwartz. However, finite presheaves remain imperfectly understood, as illustrated by examples. One important class of examples is shown to be provided by unstable algebras of finite transcendence degree (under a necessary weak finiteness condition). For unstable Hopf algebras, it is shown that the situation is much better: the associated presheaf is finite if and only if its growth function is polynomial. This leads to a description of unstable Hopf algebras modulo nilpotents in the spirit of Henn, Lannes and Schwartz.