Kernel perfect and critical kernel imperfect digraphs structure

A kernel N of a digraph D is an independent set of vertices of D such that for every w ∈ V (D)−N there exists an arc from w to N . If every induced subdigraph of D has a kernel, D is said to be a kernel perfect digraph. Minimal non-kernel perfect digraph are called critical kernel imperfect digraph. If F is a set of arcs of D, a semikernel modulo F , S of D is an independent set of vertices of D such that for every z ∈ V (D)−S for which there exists an Sz−arc of D − F , there also exists an zS−arc in D. In this talk some structural results concerning critical kernel imperfect and sufficient conditions for a digraph to be a critical kernel imperfect digraph are presented.