Geometric characterization of flat modules
  Let $R$ be a commutative ring. Roughly speaking, we prove that an $R$-module $M$ is flat iff it is a direct limit of $R$-module affine algebraic varieties, and $M$ is a flat Mittag-Leffler module iff it is the union of its $R$-submodule affine algebraic varieties. 
Carlos Sancho, Fernando Sancho, Pedro Sancho
math.AG math.AC