Configurations in abelian categories. I. Basic properties and moduli stacks

Abstract

This is the first in a series of papers math.AG/0503029, math.AG/0410267, math.AG/0410268 on &#34;configurations&#34; in an abelian category A. Given a finite partially ordered set (I,<), an (I,<)-configuration (σ,ι,π) is a finite collection of objects σ(J) and morphisms ι(J,K) or π(J,K) : σ(J) --> σ(K) satisfying some axioms, where J,K are subsets of I. Configurations describe how an object X in A decomposes into subobjects, and are especially useful for studying stability conditions on A. This paper defines and motivates the idea of configurations, and explains some natural operations upon them -- subconfigurations, quotient configurations, refinements, improvements and substitution. Then we study moduli spaces of (I,<)-configurations in A, using the theory of Artin stacks. We prove well-behaved moduli stacks exist when A is an abelian category of coherent sheaves or vector bundles on a projective K-scheme P, or of representations of a quiver Q. We define many natural 1-morphisms between the moduli stacks, some of which are representable or of finite type. The sequels will apply these results to construct and study infinite-dimensional algebras associated to a quiver Q, and to define systems of invariants of a projective K-scheme P that &#34;count&#34; (semi)stable coherent sheaves and satisfy interesting identities.