On the Sum and Product of Covering Numbers of Graphs and their Line Graphs

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1 Communications in Mathematics Applications Vol. 6, No. 1, pp. 9 16, 015 ISSN (online); (print) Published by RGN Publications On the Sum Product of Covering Numbers of Graphs their Line Graphs Research Article Susanth C* 1 Sunny Joseph Kalayathankal 1 Department of Mathematics, Research & Development Centre, Bharathiar University, Coimbatore 6106, Tamilnadu, India Department of Mathematics, Kuriakose Elias College, Mannanam, Kottayam , Kerala, India *Corresponding author: susanth_c@yahoo.com Abstract. The bounds on the sum product of chromatic numbers of a graph its complement are known as Nordhaus-Gaddum inequalities. In this paper, we study the bounds on the sum product of the covering numbers of graphs their line graphs. We also provide a new characterization of the certain graph classes. Keywords. Covering number; Independence number; Matching number; Line graph. MSC. 05C69; 05C70 Received: March 19, 015 Accepted: August 8, 015 Copyright 015 Susanth C Sunny Joseph Kalayathankal. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, reproduction in any medium, provided the original work is properly cited. 1. Introduction For all terms definitions, not defined specifically in this paper, we refer to [10]. Unless mentioned otherwise, all graphs considered here are simple, finite have no isolated vertices. Many problems in extremal graph theory seek the extreme values of graph parameters on families of graphs. The classic paper of Nordhaus Gaddum [6] study the extreme values of the sum (or product) of a parameter on a graph its complement, following solving these problems for the chromatic number on n-vertex graphs. In this paper, we study such problems for some graphs their associated graphs.

2 10 On the Sum Product of Covering Numbers of Graphs... : Susanth C S.J. Kalayathankal Definition 1.1 ([5]). A Walk, W = v 0 e 1 v 1 e v... v k 1 e k v k, in a graph G is a finite sequence whose terms are alternately vertices edges such that, for 1 i k, the edge e i has ends v i 1 v i. Definition 1. ([5]). If the vertices v 0, v 1,..., v k of a walk W are distinct then W is called a Path. A path with n vertices will be denoted by P n. P n has length n 1. Definition 1.3 ([1]). The Covering number of a graph G is the size of a minimum vertex cover in a graph G, known as the vertex cover number of G, denoted by β(g). Definition 1. ([]). Two vertices that are not adjacent in a graph G are said to be independent. A set S of vertices is independent if any two vertices of S are independent. The vertex independence number or simply the independence number, of a graph G, denoted by α(g) is the maximum cardinality among the independent sets of vertices of G. Definition 1.5 ([]). A subset M of the edge set of G, is called a matching in G if no two of the edges in M are adjacent. In other words, if for any two edges e f in M, both the end vertices of e are different from the end vertices of f. Definition 1.6 ([]). A perfect matching of a graph G is a matching of G containing n/ edges, the largest possible, meaning perfect matchings are only possible on graphs with an even number of vertices. A perfect matching sometimes called a complete matching or 1-factor. Definition 1.7 ([]). The matching number of a graph G, denoted by ν(g), is the size of a maximal independent edge set. It is also known as edge independence number. The matching number ν(g) satisfies the inequality ν(g) n. Equality occurs only for a perfect matching graph G has a perfect matching if only if G = ν(g), where G = n is the vertex count of G. Definition 1.8 ([]). A maximal independent set in a line graph corresponds to maximal matching in the original graph. In this paper, we discussed the sum product of the covering numbers of certain class of graphs their line graphs.. New Results Definition.1 ([13]). The line graph L(G) of a simple graph G is the graph whose vertices are in one-one correspondence with the edges of G, two vertices of L(G) being adjacent if only if the corresponding edges of G are adjacent. Theorem. ([1]). The independence number α(g) of a graph G vertex cover number β(g) are related by α(g) + β(g) = G, where G = n, the vertex count of G. Theorem.3 ([1]). The independence number of the line graph of a graph G is equal to the matching number of G. Communications in Mathematics Applications, Vol. 6, No. 1, pp. 9 16, 015

3 On the Sum Product of Covering Numbers of Graphs... : Susanth C S.J. Kalayathankal 11 Proposition.. For a complete graph K n, n 3, n β(k n ) + β(l(k n )) = n 1 ; if n is odd β(k n ) β(l(k n )) = n(n 1)(n ) (n 1) 3 Proof. The independence number of a complete graph K n on n vertices is 1, since each vertex is joined with every other vertex of the graph G. By Theorem., the covering number, β(k n ) of K n = n 1. By [11], α(l(k n )) = n n(n 1). Then, since there are vertices in L(K n ), by Theorem., β(l(k n )) = n(n 1) n(n 1) ( n 1 ) = (n 1), when n is odd. Therefore β(k n ) + β(l(k n )) = β(k n ) β(l(k n )) = n ; n 1 ; ; if n is odd n = n(n 1) n = n(n ), when n is even n(n 1)(n ) (n 1) 3 Proposition.5. For a complete bipartite graph K m,n, β(k m,n ) + β(l(k m,n )) = mn β(k m,n ) β(l(k m,n )) = m (n 1). Proof. Without the loss of generality, let m < n. The independence number of a complete bipartite graph, α(k m,n ) = max(m, n) = n. Since a complete bipartite graph consists of m + n number of vertices, by Theorem., β(k m,n ) = m + n n = m. The number of vertices in L(K m,n ) is mn α(l(k m,n )) = matching number of K m,n = ν(k m,n ) = min(m, n) = m. Then by Theorem., β(l(k m,n )) = mn m = m(n 1). Therefore, β(k m,n ) + β(l(k m,n )) = mn β(k m,n ) β(l(k m,n )) = m (n 1). Definition.6. [10] For n 3, a wheel graph W n+1 is the graph K 1 + C n. A wheel graph W n+1 has n + 1 vertices n edges. Theorem.7. For n 3, β(w n+1 ) + β(l(w n+1 )) = β(w n+1 ) β(l(w n+1 )) = n + 1 (n + 1) ; if n is odd 3n(n+) (n+3)(3n+1) Communications in Mathematics Applications, Vol. 6, No. 1, pp. 9 16, 015

4 1 On the Sum Product of Covering Numbers of Graphs... : Susanth C S.J. Kalayathankal Proof. By [11], the independence number of a wheel graph W n+1 is n. The number of vertices in W n+1 is n + 1. Then by theorem., β(w n+1 ) = (n + 1) n = (n + 1) n = (n+), if n is even (n + 1) (n 1) = (n+3), if n is odd. Now, consider the line graph of the wheel graph W n+1. By [11], the independence number, α(l(w n+1 )) = n. Since W n+1 consists of n edges, the line graph L(W n+1 ) have exactly n vertices, by theorem., β(l(w n+1 )) = n n = n n = 3n, if n is even n (n 1) = 3n+1, if n is odd. Therefore, β(w n+1 ) + β(l(w n+1 )) = n + 1 (n + 1) ; if n is odd β(w n+1 ) β(l(w n+1 )) = 3n(n+) (n+3)(3n+1) Definition.8 ([9]). Helm graphs are graphs obtained from a wheel by attaching one pendant edge to each vertex of the cycle. Theorem.9. For a helm graph H n, n 3, β(h n ) + β(l(h n )) = 3n β(h n ) β(l(h n )) = n. Proof. A helm graph (H n ) consists of n + 1 vertices 3n edges. By [11], the independence number of a helm graph, α(h n ) = n + 1. Since, the number of vertices of (H n ) is n + 1, by Theorem., β(h n ) = (n + 1) (n + 1) = n. Now consider the line graph of the helm graph H n. By theorem.3, the independence number of L(H n ) is equal to the matching number of H n = n. Since, the number of vertices of L((H n )) = 3n, by Theorem., β(l(h n )) = 3n n = n. Therefore, β(h n ) + β(l(h n )) = 3n β(h n ) β(l(h n )) = n. Definition.10 ([1]). Given a vertex x a set U of vertices, an x, U fan is a set of paths from x to U such that any two of them share only the vertex x. A U fan is denoted by F 1,n. Theorem.11. For a fan graph F 1,n, β(f 1,n ) + β(l(f 1,n )) = n n 1 ; if n is odd β(f 1,n ) β(l(f 1,n )) = (n+)(3n ) (n+1)(3n 3) Proof. A fan graph F 1,n is defined to be a graph K 1 +P n. By [11], the independence number of a fan graph F 1,n is either n n+1 or, depending on n is even or odd. Since the number of vertices of Communications in Mathematics Applications, Vol. 6, No. 1, pp. 9 16, 015

5 On the Sum Product of Covering Numbers of Graphs... : Susanth C S.J. Kalayathankal 13 F 1,n is n + 1, by Theorem., β(f 1,n ) = (n + 1) n = n+ (n+1), if n is even (n + 1) = n+1, if n is odd. Now consider the line graph of the fan graph F 1,n. By Theorem.3, the independence number of L(F 1,n ), α(l(f 1,n )) = ν(f 1,n ) is either n the number of vertices in F 1,n is n 1, by Theorem., β(l(f 1,n )) = (n 1) n = (3n ), if n is even (n 1) (n+1) = (3n 3), if n is odd. Therefore For a fan graph F 1,n, β(f 1,n ) + β(l(f 1,n )) = n n 1 ; if n is odd or n+1 depending on n is even or odd. Since β(f 1,n ) β(l(f 1,n )) = (n+)(3n ) (n+1)(3n 3) Definition.1 ([1, 1]). An n sun or a trampoline, denoted by S n, is a chordal graph on n vertices, where n 3, whose vertex set can be partitioned into two sets U = u 1, u, u 3,..., u n } W = w 1, w, w 3,..., w n } such that U is an independent set of G u i is adjacent to w j if only if j = i or j = i + 1(mod n). A complete sun is a sun G where the induced subgraph U is complete. Theorem.13. For a complete sun graph S n, n 3, β(s n ) + β(l(s n )) = n(n+3) β(s n ) β(l(s n )) = n (n+1). Proof. Let S n be a complete sun graph on n vertices. By [11], the independence number of S n, α(s n ) = n. Since S n consists of n vertices, by theorem., β(s n ) = n n = n. Now consider the line graph L(S n ) of S n. By [11], the independence number of L(S n ), αl(s n ) = n. The number of vertices of L(S n ) is the number of edges of S n = n(n 1) + n = n(n+3). Then by theorem., β(l(s n )) = [ n(n+3) ] n = n(n+1). Therefore, n 3, β(s n ) + β(l(s n )) = n(n+3) β(s n ) β(l(s n )) = n (n+1). Definition.1 ([1]). The n sunlet graph is the graph on n vertices obtained by attaching n pendant edges to a cycle graph C n is denoted by L n. Theorem.15. For a sunlet graph L n β(l n ) β(l(l n )) = n. on n vertices, n 3, β(l n ) + β(l(l n )) = n Proof. Let L n be a sunlet graph on n vertices. By [11], the independence number of L n, α(l n ) = n. Since L n consists of n vertices, by Theorem., β(l n ) = n n = n. Now consider the line graph L(L n ) of L n. The number of vertices of L(L n ) is the number of edges of L n = n. Then by theorem., β(l(l n )) = n n = n. Therefore, β(l n ) + β(l(l n )) = n β(l n ) β(l(l n )) = n. Definition.16 ([10]). The armed crown is a graph G obtained by adjoining a path P m to every vertex of a cycle C n. Communications in Mathematics Applications, Vol. 6, No. 1, pp. 9 16, 015

6 1 On the Sum Product of Covering Numbers of Graphs... : Susanth C S.J. Kalayathankal Theorem.17. For an armed crown graph G with a path P m a cycle C n, mn + 1 ; if m, n are odd β(g) + β(l(g)) = mn ; otherwise β(g) β(l(g)) = (mn+1) ; if m, n are odd m n ; otherwise. Proof. Note that the number of vertices of P m is m. By [11], α(g) = mn, except for m n are odd. The number of vertices of an armed crown graph is mn. By Theorem., β(g) = mn mn = mn. Now consider the line graph, L(G) of G. From [11], the independence number of the line graph of G is mn. The number of vertices of L(G) is the number of edges in G is equal to n(m 1) + n = mn. Then by Theorem., β(g) = mn mn = mn. Therefore, β(g) + β(l(g)) = mn + mn mn = mn β(g).β(l(g)) = mn = m n When m n are odd, by [11], the independence number of G, α(g) = n Theorem., [ n [ m + 1 β(g) = mn [( n 1 )( m + 1 = mn = mn + 1. ] n + ) + [ m 1 ]] ( n + 1 )( m 1 )]. [ m+1 ]+ n m 1 [ ]. By Now consider the line graph, L(G) of G. From [11], α(l(g)) = n( m 1 ) + n. The number of vertices of L(G) is the number of edges in G is equal to n(m 1)+ n = mn. Then by theorem., [ β(l(g)) = mn n [ = mn n = mn + 1. ( m 1 ( m 1 ) ) n ] + + n 1 ] That is, β(g) + β(l(g)) = mn+1 + mn+1 = mn + 1 β(g) β(l(g)) = mn+1 mn+1 = (mn+1). Therefore, for an armed crown graph, mn + 1 ; if m, n are odd β(g) + β(l(g)) = mn otherwise β(g) β(l(g)) = (mn+1) ; if m, n are odd m n ; otherwise. Communications in Mathematics Applications, Vol. 6, No. 1, pp. 9 16, 015

7 On the Sum Product of Covering Numbers of Graphs... : Susanth C S.J. Kalayathankal Conclusion The theoretical results obtained in this research may provide a better insight into the problems involving covering number independence number by improving the known lower upper bounds on sums products of independence numbers of a graph G an associated graph of G. More properties characteristics of operations on covering number also other graph parameters are yet to be investigated. The problems of establishing the inequalities on sums products of covering numbers for various graphs graph classes still remain unsettled. All these facts highlight a wide scope for further studies in this area. Acknowledgments This work is motivated by the inspiring talk given by Dr. J. Paulraj Joseph, Department of Mathematics, Manonmaniam Sundaranar University, TamilNadu, India, entitled Bounds on sum of graph parameters A survey, at the National Conference on Emerging Trends in Graph Connections (NCETGC-01), University of Kerala, Kerala, India. Competing Interests The authors declare that they have no competing interests. Authors Contributions Both the authors contributed equally significantly in writing this article. Both authors read approved the final manuscript. References [1] A. Brstadt, V.B. Le J.P. Spinard, Graph Classes: A Survey, SIAM, Philadelphia (1999). [] J.A. Bondy U.S.R. Murty, Graph Theory, Springer (008). [3] A.E. Brouwer, A.M. Cohen A. Neumaier, Distance-Regular Graphs, Springer-Verlag, New York (1989). [] G. Chartr P. Zhang, Chromatic Graph Theory, CRC Press, Western Michigan University Kalamazoo, MI, U.S.A. [5] J. Clark D.A. Holton, A First Look At Graph Theory, Allied Pub., India (1991). [6] K.L. Collins A. Trenk, Nordhaus-Gaddum theorem for the distinguishing chromatic number, The Electronic Journal of Combinatorics 16 (009), arxiv: v1 [math.co], 6 March 01. [7] N. Deo, Graph Theory with Applications to Engineering Computer Science, PHI Learning (197). [8] R. Diestel, Graph Theory, Springer-Verlag, New York 1997, (000). [9] J.A. Gallian, A dynamic survey of graph labeling, The Electronic Journal of Combinatorics 18 (011). Communications in Mathematics Applications, Vol. 6, No. 1, pp. 9 16, 015

8 16 On the Sum Product of Covering Numbers of Graphs... : Susanth C S.J. Kalayathankal [10] F. Harary, Graph Theory, Addison-Wesley Publishing Company Inc. (199). [11] C. Susanth S.J. Kalayathankal, The sum product of independence numbers of graphs their line graphs, (submitted). [1] D.B. West, Introduction to Graph Theory, Pearson Education Asia (00). [13] R.J. Wilson, Introduction to Graph Theory, Prentice Hall (1998). [1] Information System on Graph Classes their Inclusions, URL: graphclasses.org/smallgraphs Communications in Mathematics Applications, Vol. 6, No. 1, pp. 9 16, 015