English

Distance Laplacian eigenvalues of graphs and chromatic and independence number

Combinatorics 2022-02-15 v1

Abstract

For a connected graph GG of order nn, let Diag(Tr)Diag(Tr) be the diagonal matrix of vertex transmissions and D(G)D(G) be the distance matrix of GG. The distance Laplacian matrix of GG is defined as DL(G)=Diag(Tr)D(G)D^L(G)=Diag(Tr)-D(G) and the eigenvalues of DL(G)D^{L}(G) are called the distance Laplacian eigenvalues of GG. Let 1L(G)2L(G)nL(G)\partial_{1}^{L}(G)\geq \partial_{2}^{L}(G)\geq \dots \geq \partial_{n}^{L}(G) be the distance Laplacian eigenvalues of GG. Given an interval II, let mDL(G)Im_{D^{L} (G)} I (or simply mDLIm_{D^{L} } I) be the number of distance Laplacian eigenvalues of GG which lie in the interval II. For a prescribed interval II, we determine mDLIm_{D^{L} }I in terms of independence number α(G)\alpha(G), chromatic number χ\chi, number of pendant vertices and diameter dd of the graph GG. In particular, we prove that mDL(G)[n,n+2)χ1m_{D^{L}(G) }[n,n+2)\leq \chi-1, ~mDL(G)[n,n+α(G))nα(G)m_{D^{L}(G) }[n,n+\alpha(G))\leq n-\alpha(G) and we show that the inequalities are sharp. We also show that mDL(G)(n,n+nχ)nnχCG+1m_{D^{L} (G )}\bigg( n,n+\left\lceil\frac{n}{\chi}\right\rceil\bigg)\leq n- \left\lceil\frac{n}{\chi}\right\rceil-C_{\overline{G}}+1 , where CGC_{\overline{G}} is the number of components in G\overline{G}, and discuss some cases where the bound is best possible. In addition, we prove that mDL(G)[n,n+p)npm_{D^{L} (G )}[n,n+p)\leq n-p, where p1p\geq 1 is the number of pendant vertices. Also, we characterize graphs of diameter d2d\leq 2 which satisfy mDL(G)(2n1,2n)=α(G)1=n21m_{D^{L}(G) } (2n-1,2n )= \alpha(G)-1=\frac{n}{2}-1. At the end, we propose some problems of interest.

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Cite

@article{arxiv.2202.05987,
  title  = {Distance Laplacian eigenvalues of graphs and chromatic and independence number},
  author = {S. Pirzada and Saleem Khan},
  journal= {arXiv preprint arXiv:2202.05987},
  year   = {2022}
}

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17 pages