ISSN 1234-3099 (print version)

ISSN 2083-5892 (electronic version)

Discussiones Mathematicae Graph Theory

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Discussiones Mathematicae Graph Theory


Discussiones Mathematicae Graph Theory  18(1) (1998)   85-89
DOI: 10.7151/dmgt.1065


Hortensia Galeana-Sánchez and V. Neumann-Lara

Instituto de Matemáticas, UNAM
Ciudad Universitaria, Circuito Exterior
04510  México, D.F., México


A kernel of a digraph D is a subset N ⊆ V(D) which is both independent and absorbing. When every induced subdigraph of D has a kernel, the digraph D is said to be kernel-perfect. We say that D is a critical kernel-imperfect digraph if D does not have a kernel but every proper induced subdigraph of D does have at least one. Although many classes of critical kernel-imperfect-digraphs have been constructed, all of them are digraphs such that the block-cutpoint tree of its asymmetrical part is a path. The aim of the paper is to construct critical kernel-imperfect digraphs of a special structure, a general method is developed which permits to build critical kernel-imperfect-digraphs whose asymmetrical part has a prescribed block-cutpoint tree. Specially, any directed cactus (an asymmetrical digraph all of whose blocks are directed cycles) whose blocks are directed cycles of length at least 5 is the asymmetrical part of some critical kernel-imperfect-digraph.

Keywords: digraphs, kernel, kernel-perfect, critical kernel-imperfect, block-cutpoint tree.

1991 Mathematics Subject Classification: 05C20.


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Received 22 April 1997
Revised 3 November 1997