Generalized primitivity of labeled digraphs

We study reachability properties of digraphs whose edges are labeled by elements of a semigroup. We introduce a notion of primitivity for such digraphs, which allows us to unify a variety of recent results on the combinatorial structure of semigroups of nonnegative square matrices. We make use of Stallings foldings, a key procedure in combinatorial group theory, to design an almost-linear time algorithm for checking primitivity in an important case of allowable digraphs improving the previously known algorithm. We also present computational complexity results related to computation of the exponent.