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On Vertex-Disjoint Triangles in Tripartite Graphs and Multigraphs
Graphs and Combinatorics ( IF 0.7 ) Pub Date : 2020-06-06 , DOI: 10.1007/s00373-020-02188-3
Qingsong Zou , Jiawang Li , Zizheng Ji

Let G be a tripartite graph with tripartition \((V_{1},V_{2},V_{3})\), where \(\mid V_{1}\mid =\mid V_{2}\mid =\mid V_{3}\mid =k>0\). It is proved that if \(d(x)+d(y)\ge 3k\) for every pair of nonadjacent vertices \(x\in V_{i}, y\in V_{j}\) with \(i\ne j(i,j\in \{1,2,3\})\), then G contains k vertex-disjoint triangles. As a corollary, if \(d(x)\ge \frac{3}{2}k\) for each vertex \(x\in V(G)\), then G contains k vertex-disjoint triangles. Based on the above results, vertex-disjoint triangles in multigraphs are studied. Let M be a standard tripartite multigraph with tripartition \((V_{1},V_{2},V_{3})\), where \(\mid V_{1} \mid =\mid V_{2}\mid =\mid V_{3} \mid =k>0\). If \(\delta (M)\ge 3k-1\) for even k and \(\delta (M)\ge 3k\) for odd k, then M contains k vertex-disjoint 4-triangles \(\varDelta _{4}\)(a triangle with at least four edges). Furthermore, examples are given showing that the degree conditions of all our three results are best possible.



中文翻译:

三部图和多重图中的顶点不相交三角形

G为具有三重\((V_ {1},V_ {2},V_ {3})\)的三方图,其中\(\ mid V_ {1} \ mid = \ mid V_ {2} \ mid = \ mid V_ {3} \ mid = k> 0 \)。它证明了当\(d(X)+ d(Y)\ GE 3K \)对于每对不相邻顶点的\(X \在V_ {I},Y \在V_ {Ĵ} \)\(ⅰ \ ne j(i,j \ in \ {1,2,3 \})\),则G包含k个顶点不相交的三角形。作为推论,如果对于每个顶点\(d(x)\ ge \ frac {3} {2} k \)\(x \ in V(G)\),则G包含k个顶点不相交的三角形。基于以上结果,研究了多图中顶点不相交的三角形。让M是具有三方\((V_ {1},V_ {2},V_ {3})\)的标准三方多重图,其中\(\ mid V_ {1} \ mid = \ mid V_ {2} \ mid = \ mid V_ {3} \ mid = k> 0 \)。如果\(\增量(M)\ GE 3K-1 \)为偶数ķ\(\增量(M)\ GE 3K \)对于奇数ķ,然后中号包含ķ顶点不相交4-三角形\(\ varDelta _ {4} \)(至少四个边的三角形)。此外,给出的示例表明,我们所有三个结果的程度条件都是最可能的。

更新日期:2020-06-06
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