Coherent functors, with application to torsion in the Picard group

Abstract

Let A be a commutative noetherian ring. Call a functor < > --> < > coherent if it can be built up (via iterated finite limits) from functors of the form B \mapsto M tensor_A B, where M is a f.g. A-module. When such a functor F in fact takes its values in < >, we show that there are only finitely many prime numbers p such that _p F(A) is infinite, and that none of these primes are invertible in A. This (and related statements) yield information about torsion in Pic(A). For example, if A is of finite type over Z, we prove that the torsion in Pic(A) is supported at a finite set of primes, and if _p Pic(A) is infinite, then the prime p is not invertible in A. These results use the (already known) fact that if such an A is normal, then Pic(A) is finitely generated. We obtain a parallel result for a reduced scheme X of finite type over Z. We show that the groups which can occur as the Picard group of a scheme of finite type over a finite field all have the form (finitely generated) + sum_{n=1}^infty F, where F is a finite p-group. Hard copy is available from the author. E-mail to jaffe@cpthree.unl.edu.