Weak exceptional sequences

Abstract

We introduce weak exceptional sequence of modules which can be viewed as another modification of the standard case, different from the works of Igusa-Todorov [IT17] and Buan-Marsh [BM18]. For hereditary algebras it is equivalent to standard exceptional sequences. One important new feature is: if the global dimension of an algebra is greater than one, then the size of a full sequence can exceed the rank of the algebra. We use both cyclic and linear Nakayama algebras to test combinatorial aspects of this new sequence. For some particular classes, we give closed form formulas which returns the number of a full weak exceptional sequences, and compare them with the number of exceptional sequences of types and linear radical square zero Nakayama algebras [Sen19-2].