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# Bounds on inverse domination in squares of graphs

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IJRET : International Journal of Research in Engineering and Technology is an international peer reviewed, online journal published by eSAT Publishing House for the enhancement of research in various disciplines of Engineering and Technology. The aim and scope of the journal is to provide an academic medium and an important reference for the advancement and dissemination of research results that support high-level learning, teaching and research in the fields of Engineering and Technology. We bring together Scientists, Academician, Field Engineers, Scholars and Students of related fields of Engineering and Technology.

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### Bounds on inverse domination in squares of graphs

1. 1. IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308 _______________________________________________________________________________________ Volume: 02 Issue: 10 | Oct-2013, Available @ http://www.ijret.org 552 BOUNDS ON INVERSE DOMINATION IN SQUARES OF GRAPHS M. H. Muddebihal1 , Srinivasa G2 1 Professor, Department of Mathematics, Gulbarga University, Karnataka, India, mhmuddebihal@yahoo.co.in 2 Assistant Professor, Department of Mathematics, B. N. M. I. T , Karnataka, India, gsgraphtheory@yahoo.com Abstract Let be a minimum dominating set of a square graph . If ( ) − contains another dominating set of , then is called an inverse dominating set with respect to . The minimum cardinality of vertices in such a set is called an inverse domination number of and is denoted by ( ). In this paper, many bounds on ( ) were obtained in terms of elements of . Also its relationship with other domination parameters was obtained. Key words: Square graph, dominating set, inverse dominating set, Inverse domination number. Subject Classification Number: AMS-05C69, 05C70 --------------------------------------------------------------------***---------------------------------------------------------------------- 1. INTRODUCTION In this paper, we follow the notations of [1]. We consider only finite undirected graphs without loops or multiple edges. In general, we use < > to denote the subgraph induced by the set of vertices and ( ) and [ ] denote the open and closed neighborhoods of a vertex . The minimum (maximum) degree among the vertices of is denoted by ( ) ∆( ) . A vertex of degree one is called an end vertex. The term ( )( ( )) denotes the minimum number of vertices(edges) cover of . Further, ( )( ( )) represents the vertex(edge) independence number of . A vertex with degree one is called an end vertex. The distance between two vertices and is the length of the shortest uv-path in . The maximum distance between any two vertices in is called the diameter of and is denoted by !"#( ). The square of a graph denoted by , has the same vertices as in and the two vertices and are joined in if and only if they are joined in by a path of length one or two. The concept of squares of graphs was introduced in [2]. A set \$ ⊆ ( ) is said to be a dominating set of , if every vertex in ( − \$) is adjacent to some vertex in \$. The minimum cardinality of vertices in such a set is called the domination number of and is denoted by ( ). Further, if the subgraph < \$ > is independent, then \$ is called an independent dominating set of . The independent omination number of , denoted by &( ) is the minimum cardinality of an independent dominating set of . A dominating set \$ of is said to be a connected dominating set, if the subgraph < \$ > is connected in . The minimum cardinality of vertices in such a set is called the connected domination number of and is denoted by '( ). A dominating set \$ is called total dominating set, if for every vertex ∈ , there exists a vertex ∈ \$, ∉ such that is adjacent to . The total domination number of , denoted by *( ) is the minimum cardinality of total dominating set of . Further, a dominating set \$ is called an end dominating set of , if \$ contains all the end vertices in . The minimum cardinality of vertices in such a set is called the end domination number of and is denoted by +( ).
2. 2. IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308 _______________________________________________________________________________________ Volume: 02 Issue: 10 | Oct-2013, Available @ http://www.ijret.org 553 Domination related parameters are now well studied in graph theory (see [3] and [9]). Let \$ be a minimum dominating set of , if the compliment − \$ of \$ contains a dominating set \$′ , then \$′ is called an inverse dominating set of with respect to \$. The minimum cardinality of vertices in such a set is called an inverse domination number of and is denoted by ( ). Inverse domination was introduced by V. R. Kulli and S. C. Sigarkanti [10]. A set ⊆ ( ) is said to be a dominating set of , if every vertex not in is adjacent to a vertex in . The minimum cardinality of vertices in such a set is called the domination number of and is denoted by ( )(see [4]). A set of is said to be connected dominating set of if every vertex not in is adjacent to at least one vertex in and the set induced by is connected. The minimum cardinality of a connected dominating set of is called connected domination number of (see [5]). A dominating set of is said to be total dominating set, if for every vertex ∈ ( ), there exists a vertex ∈ , ≠ , such that is adjacent to . The total domination number of denoted by *( ) is the minimum cardinality of a total dominating set of (see [6]). A dominating set of is said to be restrained dominating set, if for every vertex not in is adjacent to a vertex in and to a vertex in ( − ). The restrained domination number of , denoted by -+( ) is the minimum cardinality of a restrained dominating set of . Further, a dominating set of is said to be double dominating set, if for every vertex ∈ ( ) is dominated by at least two vertices of . The double domination number of , denoted by .( ) is the minimum cardinality of a double dominating set of (see [7] and [8]). Analogously, let be a minimum dominating set of a square graph . If ( ) − contains another dominating set ′ of , then ′ is called an inverse dominating set with respect to . The minimum cardinality of vertices in such a set is called an inverse domination number of and is denoted by ( ). In this paper, many bounds on ( ) were obtained in terms of elements of . Also its relationship with other domination parameters was obtained. 2. RESULTS Theorem 2.1: For any connected graph , ( ) + *( ) ≤ ( ) + *( ). Proof: Let \$ = { , , … , 4} ⊆ ( ) be the minimum set of vertices which covers all the vertices in G. Suppose the subgraph < \$ > has no isolated vertices, then \$ itself is a * - set of . Otherwise, let 6 = { , , … , 7} ⊆ \$ be the set of vertices with deg( &) = , 1 ≤ ! ≤ <. Now make deg ( &) = 1 by adding vertices { &} ⊆ ( ) − \$ and ( &) ∈ { &}. Clearly, \$ = \$ ∪ 6 ∪ { &} forms a minimal total dominating set of . Since ( ) = ( ), let * = { , , … , >} ⊆ \$ be the minimal *-set of . Suppose ⊆ \$ is a minimal dominating set of . Then there exists a vertex set = { , , … , >} ⊆ ( ) − which covers all the vertices in . Clearly, forms a minimal inverse dominating set . Since for any graph , there exists atleast one vertex ∈ ( ), such that deg( ) = ( ), it follows that, | | ∪ | *| ≤ |\$ | + deg ( ). Therefore, ( ) + *( ) ≤ *( ) + ( ). Theorem 2.2: Let and be, -set and -set of respectively. If ( ) = ( ), then each vertex in is of maximum degree. Proof: For @ = 2, the result is obvious. Let @ ≥ 3. Since, ( ) = ( ) such that doesnot contain any end vertex, let = { , , … , 7} be a dominating set of . If there exists a vertex ∈ such that is adjacent to some vertices in ( ) − . Then every vertex D ∈ − { } is an end vertex in < >. Further, if D is adjacent to a vertex
3. 3. IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308 _______________________________________________________________________________________ Volume: 02 Issue: 10 | Oct-2013, Available @ http://www.ijret.org 554 ∈ ( ) − . Then = − { , D} ∪ { } is a dominating set of , a contradiction. Hence each vertex in is of maximum degree in . Theorem 2.3: For any connected graph , ( ) = 1 if and only if has at least two vertices of degree (@ − 1). Proof: To prove this result, we consider the following two cases. Case 1. Suppose has exactly one vertex of deg( ) = @ − 1. Then in this case = { } is a -set of . Clearly, − = − { }. Further, if = { }E ( ) in ( ) − , deg( ) ≤ @ − 2 in . Then there exists at least one vertex D ∉ ( ) in such that = ∪ {D} forms an inverse dominating set of , a contradiction. Case 2. Suppose has at least two vertices and of deg( ) = @ − 1 = deg ( ) such that and are not adjacent. Then = { } dominates since deg( ) = @ − 1 and ( ) − = ( ) − { } . Further, since and are not adjacent, = { } ∪ where ⊆ ( ) − forms a -set of , a contradiction. Conversely, suppose deg( ) = @ − 1 = deg( ), such that and are adjacent to all the vertices in . = { }E ( ), where { } ⊆ ( ) − and vice-versa. In any case, we obtain | | = 1. Therefore ( ) = 1. Proposition 2.1: FG = HG = F ,G = FGI, GJ = 1. Theorem 2.4: For any connected (@, K)-graph without isolates, then ( ) + ( ) ≤ @. Proof : Suppose = { , , … , 7} ⊆ ( ) be the -set of , the = { , , … , 7} ⊆ ( ) − forms a minimal inverse dominating set of . Since | | ≤ L@/4O and | | ≤ L@/3O, it follows that, | | ∪ | | ≤ @. Therefore, ( ) + ( ) ≤ @. Suppose − is not independent, then there exists at least one vertex ∈ such that ( ) ⊆ − . Clearly, | | = |{ − } − { }| and hence, | | ∪ | ′| ≤ @, a contradiction. Conversely, if − is independent. Then in this case, | | = | − | in . Clearly, it follows that | | ∪ | | = @. Hence, ( ) + ( ) = @. Theorem 2.5: For any connected (@, K) -graph with @ ≥ 3 vertices, ( ) + &( ) ≤ @ − 1. Proof: For @ ≤ 2, ( ) + &( ) ≰ @ − 1. Consider @ ≥ 3, let R = { , , … , 4} be the minimum set of vertices such that for every two vertices , ∈ R, ( ) ∩ ( ) ∈ ( ) − R. Suppose there exists a vertex \$ = { , , … , >} ⊆ R which covers all the vertices in and if the subgraph < \$ > is totally disconnected. Then S forms the minimal independent dominating set of . Now in , since ( ) = ( ) and distance between two vertices is at most two in , there exists a vertex set = T , , … , UV ⊆ \$, which forms minimal -set of . Then the complementary set ( ) − contains another set such that ( ) = ( ). Clearly, forms an inverse dominating set of and it follows that | | ∪ |\$| ≤ @ − 1. Therefore, ( ) + &( ) ≤ @ − 1. Theorem 2.6: If every non end vertex of a tree is adjacent to at least one end vertex, then , (W ) ≤ X G 4 Y + 1. Proof: Let R = { , , … , 4} be the set of all end vertices in W such that vertex |R| = # and R ∈ (R). Suppose, vertex ⊆ R is a -set of . Then ⊆ ( ) − − R forms a minimal inverse dominating set of . Since every tree W contains at least one non end vertex, it follows that | | ≤ X G 4 Y + 1. Therefore, (W ) ≤ X G 4 Y + 1. Theorem 2.7: For any connected (@, K)-graph , ( ) + '( ) ≤ @ + ( ).
4. 4. IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308 _______________________________________________________________________________________ Volume: 02 Issue: 10 | Oct-2013, Available @ http://www.ijret.org 555 Proof: For Z ≤ 5, the result follows immediately. Let Z ≥ 6, suppose = { , , … , 7} , deg( &) ≥ 2, 1 ≤ ! ≤ < be a minimal dominating set of . Now we construct a connected dominating set ' from by adding in every step at most two components of forms a connected component in . Thus we get a connected dominating set ' after at most − 1 steps. Now in , ( ) = ( ). Suppose ⊆ be minimal -set of . Then there exists a vertex set = { , , … , >} ⊆ ( ) − , such that !]^( , ) ≥ 2, for all , ∈ , which covers all the vertices in . Clearly, forms a minimal inverse dominating set of . Hence it follows that | | ∪ | '| ≤ @ ∪ | |. Therefore, ( ) + '( ) ≤ @ + ( ). Theorem 2.8: For any connected (@, K) -graph , ( ) + '( ) ≤ ( ) + ( ). Equality holds for FG. Proof: Let 6 = { , , … , >} be the maximum set of vertices such that < 6 > is totally disconnected and |6| = 0 . Now in 2, let = 1, 2,…, <⊆ 2 be the minimum set of vertices such that ( ) = ( ). Clearly, forms a minimal -set of . Suppose the subgraph < > is connected, then itself is a connected dominating set of . Otherwise, construct the connected dominating set ' from by adding at most two vertices , ∉ between the vertices of such that ( ') = ( ) and < ' > is connected. Further, there exists a vertex set = { , , … , 7} ⊆ ( ) − which covers all the vertices in . Clearly, forms an inverse dominating set of with respect to -set of . Since !"#( , ) ≥ 2, ∀ , ∈ ( ), it follows that | | ∪ | '| ≤ | | ∪ |6|. Therefore, ( ) + '( ) ≤ ( ) + ( ). Suppose ≅ FG. Then in this case, |6| = | | = | '| = | | = 1. Therefore, ( ) + '( ) = ( ) + ( ). Theorem 2.9: For any connected (@, K)-graph , ( ) ≤ X G bc(d) Y + 3. Proof: Let R = { , , … , 7} be the set of all end vertices in and let ∉ [R] in . Suppose e ⊆ is a dominating set of the subgraph < >. Then R ∪ e forms a minimal end dominating set of . In , ( ) = ( ) . Suppose diam( ) ≤ 3, then = { , } is a minimal -set of with respect to -set of , and the result follows immediately. Suppose diam( ) ≥ 4, then = { , , … , >} ⊆ ( ) − − R is a minimal inverse dominating set of with respect to the dominating set of . Clearly, it follows that | | ≤ X G {i∪j} Y + 3. Therefore, ( ) ≤ X G bc(d) Y + 3. Theorem 2.10: For any connected (@, K)-graph , ( )+ -+( ) ≤ @ − ( ) + 1. Equality holds for FG , Zk. Proof: Let l = { , , … , &} be the minimal set of vertices which covers all the edges in such that |l| = ( ). Now in ,obtain e, the induced subgraph of , i.e.,e = < > such that e contains at least one end vertex ∈ e. Let R = { , , … , >} be the set of all such end vertices in . Suppose = ( ) − R and m ⊆ , such that diam ( , ) ≥ 2, for all ∈ m and ∈ R. Then = R ∪ m , where m ⊆ m, covers all the vertices in such that every vertex not in is adjacent to a vertex in and to a vertex in ( ) − . Clearly, forms a minimal restrained dominating set of . Let = {D , D , … , D7} ⊆ ( ) forms a minimal -set of with respect to dominating set of . Clearly, it follows that | | ∪ | | ≤ @ − |l| + 1. Therefore, ( )+ -+( ) ≤ @ − ( ) + 1. For equality, suppose ≅ FG. Then in this case, | | = 1 = | | and |l| = @ − 1. Therefore, | | ∪ | | = @ − |l| + 1 and hence ( ) + -+( ) = @ − ( ) + 1. Suppose ≅ Zk . Then in this case, | | = 2 = | | and |l| = G . Clearly, it follows that, ( ) + -+( ) = @ − ( ) + 1.
5. 5. IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308 _______________________________________________________________________________________ Volume: 02 Issue: 10 | Oct-2013, Available @ http://www.ijret.org 556 Theorem 2.11: For any connected (@, K)-graph , ( ) + .( ) ≤ 2@ − K + 1. Equality holds for FG, @ ≥ 4. Proof: Let m = { , , … , 7} ⊆ ( ) and let m ∈ (m), for every vertex v∈ m in . Now we form the vertex set ⊆ ( ) by random and independent choice of the vertices of m. Define = {m ∈ : | (m) ∩ | = ∅}, p = {m ∉ : | (m) ∩ | = ∅} and p = {m ∉ : | (m) ∩ | = m}. Further, if { } 0 0 I X X I ∈ ′ ′= U and { } 0 0 I Y Y I ∈ ′ ′= U . Then | | ≤ | | and |p | ≤ |p | . Clearly, the set . = ∪ 0′∪p0∪ p0′∪p1 forms a -set of 2. Let ′= 1, 2,…, q be the minimal -set of with respect to dominating set of . It follows that | | ∪ | .| ≤ 2@ − K + 1 and hence ( ) + .( ) ≤ 2@ − K + 1. For equality, suppose ≅ FG , with @ ≥ 4 vertices. Then in this case, | .| = 2 = 2(1) = 2 | | and q = s(s ) for FG. Clearly, | | ∪ | .| = 2@ − K + 1. Hence ( )+ .( ) = 2@ − K + 1. Theorem 2.12: For any connected (@, K) − graph , ( ) + !"# ( ) ≤ @ + ( ) − 1. Proof: Suppose ≅ F , then obviously, ( ) + !"# ( ) = @ + ( ) − 1. Let ( ) contains atleast two vertices and such that !]^( , ) forms a diametral path in . Clearly, !]^( , ) = !"#( ). Let R = { , , … , >} ⊆ ( ) be the set of vertices which are adjacent to all end vertices in . Suppose w = { , , … , 7} be the set of vertices such that !]^( &, U) ≥ 2, for all 1≤ ! ≤ q, 1 ≤ x ≤ <. Then R ∪ w where w ⊆ w forms a minimal -set of . Now in , suppose = { , , … , 4} ⊆ R ∪ w be the minimal -set of . Then the complementary set V(G ) − D contains the vertex set ⊆ V(G ) − D, which covers all the vertices in . Clearly, forms a minimal inverse dominating set of and it follows that | | + !"# ( ) ≤ @ + |R ∪ w | − 1. Hence ( ) + !"# ( ) ≤ @ + ( ) − 1. REFERENCES [1]. F. Harary, Graph Theory, Adison-Wesley, Reading, Mass., 1972. [2]. F. Harary and I. C. Ross, The Square of a Tree, Bell System Tech. J. 39, 641-647, 1960. [3]. M. A. Henning, Distance Domination in Graphs, in: T. W. Haynes, S. T. Hedetniemi and P. J. Slater, editors. Domination in Graphs: Advanced Topics, chapter 12, Marcel Dekker, Inc., New York, 1998. [4]. M. H. Muddebihal, G. Srinivasa and A. R. Sedamkar, Domination in Squares of Graphs, Ultra Scientist, 23(3)A, 795–800, 2011. [5]. M. H. Muddebihal and G. Srinivasa, Bounds on Connected Domination in Squares of Graphs, International Journal of Science and Technology, 1(4), 170–176, April 2012. [6]. M. H. Muddebihal and G. Srinivasa, Bounds on Total Domination in Squares of Graphs, International Journal of Advanced Computer and Mathematical Sciences, 4(1), 67–74, 2013. [7]. M. H. Muddebihal and G. Srinivasa, Bounds on Restrained Domination in Squares of Graphs, International Journal of Mathematical Sciences, 33(2), 1173–1178, August 2013. [8]. M. H. Muddebihal and G. Srinivasa, Bounds on Double Domination in Squares of Graphs, International Journal of Research in Engineering and Technology, 2(9), 454- 458, September 2013. [9]. T. W. Haynes, S. T. Hedetniemi and P. J. Slater, Fundamentals of Domination in Graphs, Marcel Dekker, Inc., New York, 1998. [10]. V. R. Kulli and S. C. Sigarkanti, Inverse Domination in Graphs. Nat. Acad. Sci. Lett., 14, 473-475, 1991.