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Erdös-Ko-Rado from intersecting shadows

Gyula O.H. Katona, Ákos Kisvölcsey (2012)

Discussiones Mathematicae Graph Theory

A set system is called t-intersecting if every two members meet each other in at least t elements. Katona determined the minimum ratio of the shadow and the size of such families and showed that the Erdős-Ko-Rado theorem immediately follows from this result. The aim of this note is to reproduce the proof to obtain a slight improvement in the Kneser graph. We also give a brief overview of corresponding results.

Euler's idoneal numbers and an inequality concerning minimal graphs with a prescribed number of spanning trees

Jernej Azarija, Riste Škrekovski (2013)

Mathematica Bohemica

Let α ( n ) be the least number k for which there exists a simple graph with k vertices having precisely n 3 spanning trees. Similarly, define β ( n ) as the least number k for which there exists a simple graph with k edges having precisely n 3 spanning trees. As an n -cycle has exactly n spanning trees, it follows that α ( n ) , β ( n ) n . In this paper, we show that α ( n ) 1 3 ( n + 4 ) and β ( n ) 1 3 ( n + 7 ) if and only if n { 3 , 4 , 5 , 6 , 7 , 9 , 10 , 13 , 18 , 22 } , which is a subset of Euler’s idoneal numbers. Moreover, if n ¬ 2 ( mod 3 ) and n 25 we show that α ( n ) 1 4 ( n + 9 ) and β ( n ) 1 4 ( n + 13 ) . This improves some previously estabilished bounds.

Extending the MAX Algorithm for Maximum Independent Set

Ngoc C. Lê, Christoph Brause, Ingo Schiermeyer (2015)

Discussiones Mathematicae Graph Theory

The maximum independent set problem is an NP-hard problem. In this paper, we consider Algorithm MAX, which is a polynomial time algorithm for finding a maximal independent set in a graph G. We present a set of forbidden induced subgraphs such that Algorithm MAX always results in finding a maximum independent set of G. We also describe two modifications of Algorithm MAX and sets of forbidden induced subgraphs for the new algorithms.

Extremal bipartite graphs with a unique k-factor

Arne Hoffmann, Elżbieta Sidorowicz, Lutz Volkmann (2006)

Discussiones Mathematicae Graph Theory

Given integers p > k > 0, we consider the following problem of extremal graph theory: How many edges can a bipartite graph of order 2p have, if it contains a unique k-factor? We show that a labeling of the vertices in each part exists, such that at each vertex the indices of its neighbours in the factor are either all greater or all smaller than those of its neighbours in the graph without the factor. This enables us to prove that every bipartite graph with a unique k-factor and maximal size...

Extremal Matching Energy of Complements of Trees

Tingzeng Wu, Weigen Yan, Heping Zhang (2016)

Discussiones Mathematicae Graph Theory

Gutman and Wagner proposed the concept of the matching energy which is defined as the sum of the absolute values of the zeros of the matching polynomial of a graph. And they pointed out that the chemical applications of matching energy go back to the 1970s. Let T be a tree with n vertices. In this paper, we characterize the trees whose complements have the maximal, second-maximal and minimal matching energy. Furthermore, we determine the trees with edge-independence number p whose complements have...

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