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Independent dominating sets in graphs of girth five

Part of: Extremal combinatorics Graph theory

Published online by Cambridge University Press:  15 October 2020

Ararat Harutyunyan ,
Paul Horn  and
Jacques Verstraete
Ararat Harutyunyan*
Affiliation:
LAMSADE, CNRS, Université Paris-Dauphine, PSL Research University, 75016 Paris, France
Paul Horn
Affiliation:
Department of Mathematics, University of Denver, CO 80210, USA
Jacques Verstraete
Affiliation:
Department of Mathematics, University of California at San Diego, 9500 Gilman Drive, La Jolla, CA 92093, USA
*
*Corresponding author. Email: [email protected]
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Abstract

Let $\gamma(G)$ and $${\gamma _ \circ }(G)$$ denote the sizes of a smallest dominating set and smallest independent dominating set in a graph G, respectively. One of the first results in probabilistic combinatorics is that if G is an n-vertex graph of minimum degree at least d, then

$$\begin{equation}\gamma(G) \leq \frac{n}{d}(\log d + 1).\end{equation}$$

In this paper the main result is that if G is any n-vertex d-regular graph of girth at least five, then

$$\begin{equation}\gamma_(G) \leq \frac{n}{d}(\log d + c)\end{equation}$$
for some constant c independent of d. This result is sharp in the sense that as $d \rightarrow \infty$ , almost all d-regular n-vertex graphs G of girth at least five have
$$\begin{equation}\gamma_(G) \sim \frac{n}{d}\log d.\end{equation}$$

Furthermore, if G is a disjoint union of ${n}/{(2d)}$ complete bipartite graphs $K_{d,d}$ , then ${\gamma_\circ}(G) = \frac{n}{2}$ . We also prove that there are n-vertex graphs G of minimum degree d and whose maximum degree grows not much faster than d log d such that ${\gamma_\circ}(G) \sim {n}/{2}$ as $d \rightarrow \infty$ . Therefore both the girth and regularity conditions are required for the main result.

MSC classification

Primary: 05C35: Extremal problems 05C69: Dominating sets, independent sets, cliques 05C80: Random graphs 05D40: Probabilistic methods
Type
Paper
Information
Combinatorics, Probability and Computing , Volume 30 , Issue 3 , May 2021 , pp. 344 - 359
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Copyright
© The Author(s), 2020. Published by Cambridge University Press

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Footnotes

†

Research supported by an Alfred P. Sloan Research Fellowship and NSF grant DMS 0800704.

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