文摘
A kernel of a digraph is a set of vertices which is both independent and absorbent. Let \(D\) be a digraph such that every proper induced subdigraph of \(D\) has a kernel; \(D\) is said to be kernel perfect digraph (KP-digraph) if the digraph \(D\) has a kernel and critical kernel imperfect digraph (CKI-digraph) if the digraph \(D\) does not have a kernel. We characterize the CKI-digraphs with a partition into an independent set and a semicomplete digraph. The generalized sum \(G(F_u)\) of a family of mutually disjoint digraphs \(\{F_u\}_{u\in V(G)}\) over a graph \(G\) is a digraph defined as follows: Consider \(\cup _{u\in V(G)}F_u\), and for each \(x\in V(F_v)\) and \(y\in V(F_w)\) with \(\{v,w\}\in E(G)\) choose exactly one of the two arcs \((x,y)\) or \((y,x)\). We characterize the asymmetric CKI-digraphs which are generalized sums over an edge or a cycle. Furthermore, we give sufficient conditions on \(G\) and the family \(\{F_u\}_{u\in V(G)}\), such that the generalized sum \(G(F_u)\) has a kernel or is a is KP-digraph. Keywords Digraphs Kernel CKI-digraphs Operations in digraphs