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Let G be a finite, simple, and undirected graph and let S be a set of vertices of G. If no vertex of G that does not belong to S has two neighbors in S, then S is class="mathmlsrc">class="formulatext stixSupport mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S1571065316301986&_mathId=si1.gif&_user=111111111&_pii=S1571065316301986&_rdoc=1&_issn=15710653&md5=96ff73e5ebf9901b87e8c36fdd1b0606" title="Click to view the MathML source">P3class="mathContainer hidden">class="mathCode">-convex. The class="mathmlsrc">class="formulatext stixSupport mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S1571065316301986&_mathId=si1.gif&_user=111111111&_pii=S1571065316301986&_rdoc=1&_issn=15710653&md5=96ff73e5ebf9901b87e8c36fdd1b0606" title="Click to view the MathML source">P3class="mathContainer hidden">class="mathCode">-convex hull class="mathmlsrc">class="formulatext stixSupport mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S1571065316301986&_mathId=si2.gif&_user=111111111&_pii=S1571065316301986&_rdoc=1&_issn=15710653&md5=0626efe9e513daebf4ae76cd593959de" title="Click to view the MathML source">H(S)class="mathContainer hidden">class="mathCode"> of S is the smallest class="mathmlsrc">class="formulatext stixSupport mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S1571065316301986&_mathId=si1.gif&_user=111111111&_pii=S1571065316301986&_rdoc=1&_issn=15710653&md5=96ff73e5ebf9901b87e8c36fdd1b0606" title="Click to view the MathML source">P3class="mathContainer hidden">class="mathCode">-convex set containing S . If class="mathmlsrc">class="formulatext stixSupport mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S1571065316301986&_mathId=si3.gif&_user=111111111&_pii=S1571065316301986&_rdoc=1&_issn=15710653&md5=c1caba0167de713dc9cf3b4fcd9ec028" title="Click to view the MathML source">H(S)=V(G)class="mathContainer hidden">class="mathCode"> we say that S is a class="mathmlsrc">class="formulatext stixSupport mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S1571065316301986&_mathId=si1.gif&_user=111111111&_pii=S1571065316301986&_rdoc=1&_issn=15710653&md5=96ff73e5ebf9901b87e8c36fdd1b0606" title="Click to view the MathML source">P3class="mathContainer hidden">class="mathCode">-hull set of G . The cardinality class="mathmlsrc">class="formulatext stixSupport mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S1571065316301986&_mathId=si4.gif&_user=111111111&_pii=S1571065316301986&_rdoc=1&_issn=15710653&md5=b1d9495b31aaf036533c41fdb3c4bcf1" title="Click to view the MathML source">h(G)class="mathContainer hidden">class="mathCode"> of a minimum class="mathmlsrc">class="formulatext stixSupport mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S1571065316301986&_mathId=si1.gif&_user=111111111&_pii=S1571065316301986&_rdoc=1&_issn=15710653&md5=96ff73e5ebf9901b87e8c36fdd1b0606" title="Click to view the MathML source">P3class="mathContainer hidden">class="mathCode">-hull set in G is called the class="mathmlsrc">class="formulatext stixSupport mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S1571065316301986&_mathId=si1.gif&_user=111111111&_pii=S1571065316301986&_rdoc=1&_issn=15710653&md5=96ff73e5ebf9901b87e8c36fdd1b0606" title="Click to view the MathML source">P3class="mathContainer hidden">class="mathCode">-hull number of G. In this paper w extend the result of Centeno et al. [Theoretical Computer Science 412 (2011), 3693–3700] showing that, given a graph G and an integer k , deciding whether class="mathmlsrc">class="formulatext stixSupport mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S1571065316301986&_mathId=si5.gif&_user=111111111&_pii=S1571065316301986&_rdoc=1&_issn=15710653&md5=078ef96d68cee8b57a3902d9ac9c0179" title="Click to view the MathML source">h(G)≤kclass="mathContainer hidden">class="mathCode"> remains NP-complete for the Cartesian product of graphs.