On the Picard group: torsion and the kernel

Abstract

For a homomorphism f: A --> B of commutative rings, let D(A,B) denote Ker[Pic(A) --> Pic(B)]. Let k be a field and assume that A is a f.g. k-algebra. We prove a number of finiteness results for D(A,B). Here are four of them. 1: Suppose B is a f.g. and faithfully flat A-algebra which is geometrically integral over k. If k is perfect, we find that D(A,B) is f.g. (In positive characteristic, we need resolution of singularities to prove this.) For an arbitrary field k of positive characteristic p, we find that modulo p-power torsion, D(A,B) is f.g. 2: Suppose B = A tensor k^sep. We find that D(A,B) is finite. 3: Suppose B = A tensor L, where L is a finite, purely inseparable extension. We give examples to show that D(A,B) may be infinite. 4: Assuming resolution of singularities, we show that if K/k is any algebraic extension, there is a finite extension E/k contained in K/k such that D(A tensor E,A tensor K) is trivial. The remaining results are absolute finiteness results for Pic(A). 5: For every n prime to char(k), Pic(A) has only finitely many elements of order n. 6: Structure theorems are given for Pic(A), in the case where k is absolutely f.g. All of these results are proved in a more general form, valid for schemes. Hard copy is available from the authors.