On the additivity of projective presentations of maximal rank

Abstract

We study projective presentations of finite-dimensional modules over finite-dimensional algebras. We discuss if projective presentations of maximal rank behave additively. More precisely, we ask if the direct sum of copies of a projective presentation of maximal rank is again of maximal rank. The modules which have a projective presentation of maximal rank are exactly the $\tau$-regular modules. This class of modules can be seen as a generalization of modules of projective dimension at most one, and of $\tau$-rigid modules. The $\tau$-regular modules form open subsets of module varieties. Their closures are therefore unions of irreducible components, which are called generically $\tau$-regular. We discuss when a $\tau$-regular module or a generically $\tau$-regular component can be reduced to a module or component of projective dimension at most one, and we show that this is closely related to the question on the additivity of maximal rank presentations.