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Applicable Analysis and Discrete Mathematics available online at http://pefmath.etf.rs Appl. Anal. Discrete Math. 13 (2019), 178{202. https://doi.org/10.2298/AADM170803003P ON HAMILTONIAN DECOMPOSITIONS OF TENSOR PRODUCTS OF GRAPHS P. Paulraja and S. Sampath Kumar∗ Finding a hamiltonian decomposition of G is one of the challenging problems in graph theory. We do not know for what classes of graphs G and H; their tensor product G × H is hamiltonian decomposable. In this paper, we have proved that, if G is a hamiltonian decomposable circulant graph with certain properties and H is a hamiltonian decomposable multigraph, then G × H is hamiltonian decomposable. In particular, tensor products of certain sparse hamiltonian decomposable circulant graphs are hamiltonian decomposable. 1. INTRODUCTION A k-cycle is a subgraph of Kn with k distinct vertices x1; x2; x3; : : : ; xk and k edges x1x2; x2x3; : : : ; xk 1xk; xkx1: When k = n; a k-cycle is called a hamiltonian cycle. For two graphs G and− H their tensor product, denoted by G H; has vertex set V (G) V (H) in which two vertices (g ; h ) and (g ; h ) are adjacent× whenever × 1 1 2 2 g1g2 E(G) and h1h2 E(H): The wreath product of the graphs G and H; denoted by G2H; has vertex set2V (G) V (H) in which (g ; h )(g ; h ) is an edge whenever ◦ × 1 1 2 2 g1g2 is an edge in G; or g1 = g2 and h1h2 is an edge in H: Similarly, the cartesian product of the graphs G and H; denoted by G H; has vertex set V (G) V (H) in × which (g1; h1)(g2; h2) is an edge whenever g1 = g2 and h1h2 is an edge in H; or h = h and g g is an edge in G: The subgraph induced by S V (G) is denoted 1 2 1 2 ⊆ by G[S]: Similarly, the subgraph induced by E0 E(G) is denoted by G[E0]: For ⊆ ∗ Corresponding author. S. Sampath Kumar 2010 Mathematics Subject Classification. 05C45, 05C51, 05C76. Keywords and Phrases. Hamiltonian Decomposition, Tensor Product, Cayley Graphs. 178 On Hamiltonian Decompositions of Tensor Products of Graphs 179 a graph G; G(λ) is the graph obtained from G by replacing each edge of G by λ parallel edges. For a graph G; G∗ is the symmetric digraph of G: For two loopless multigraphs G(λ) and H(µ); their tensor product, denoted by G(λ) H(µ); has the vertex set V (G) V (H) and its edge set is described × × as follows: if e = g1g2 is an edge of multiplicity λ in G(λ) and f = h1h2 is an edge of multiplicity µ in H(µ); then corresponding to these edges there are edges (g ; h )(g ; h ) and (g ; h )(g ; h ) each of multiplicity λµ in G(λ) H(µ) 1 1 2 2 1 2 2 1 × and G(λ) H(µ) is isomorphic to (G H)(λµ): Hence G(λ) H ∼= G H(λ) ∼= (G H)(λ×): × × × × If H ;H ;:::;Hk are edge-disjoint subgraphs of G and E(G) = E(H ) 1 2 1 [ E(H ) ::: E(Hk); then we write G = H H ::: Hk: For a graph G; 2 [ [ 1 ⊕ 2 ⊕ ⊕ if E(G) can be partitioned into E1;E2; : : : ;Ek such that Ei ∼= H; for all i; 1 i k; then we say that H decomposes G; or H-decompositionh i of G exists.≤ If ≤ H1;H2;:::;Hk decomposes G; and each Hi is a hamiltonian cycle of G; then we call G is hamiltonian decomposable and we call H1;H2;:::;Hk is a hamiltonian decomposition of G: Clearly, the tensor productf is commutativeg and distributive over edge-disjoint union of graphs, that is, if G = H H ::: Hk; then 1 ⊕ 2 ⊕ ⊕ G G = (H G ) (H G ) ::: (Hk G ): × 1 1 × 1 ⊕ 2 × 1 ⊕ ⊕ × 1 Let G be a finite abelian group and let S be a symmetric subset of G (that is, s S implies s S): The vertices of the Cayley graph, Cay(S; G); are the elements2 of G and− there2 is an edge between x and y if and only if x y S: Note that Cay(S; G) is connected if and only if S generates the group G:− A2circulant X = Circ(n; L) is a graph with vertex set V (X) = u0; u1; : : : ; un 1 and edge f n − g set E(X) = uiui+` i Zn; ` L ; where L 1; 2;:::; 2 and Zn is the set of integers modulof n:j The2 elements2 g of L are called⊆ jumps. Clearly,b c every circulant graph of order n is a Cayley graph with the underlying group being Zn: The problem of finding hamiltonian decompositions of product graphs is not new. hamiltonian decompositions of various product graphs, including digraphs, have been studied by many authors; see, for example, [1, 6, 9, 10, 11, 13, 14, 15, 16, 18, 17]. It has been conjectured [6] that if both G and H are hamiltonian decomposable graphs, then GH is hamiltonian decomposable, where denotes the cartesian product of graphs. This conjecture has been verified to be true for a large classes of graphs [18]. Baranyai and Sz´asz[5] proved that if both G and H are even regular hamiltonian decomposable graphs, then G H is hamiltonian decomposable. In [16], Ng has obtained a partial solution to the◦ following conjec- ture of Alspach et al. [1]: If D1 and D2 are directed hamiltonian decomposable digraphs, then D D is directed hamiltonian decomposable. 1 ◦ 2 In [3], it had been proved that G H is not necessarily hamiltonian decom- posable even though G and H are hamiltonian× decomposable. Because of this, finding hamiltonian decompositions of the tensor products of hamiltonian decom- posable graphs is considered to be difficult. We believe that if the graphs G and H are suitably chosen, that is, with some suitable conditions imposed on them, then G H may have hamiltonian decomposition. In [2] and [13] it has been proved × that Kr Ks and Kr;r Km are hamiltonian decomposable; in [14] it is shown × × 180 P. Paulraja and S. Sampath Kumar that the tensor product of two regular complete multipartite graphs is hamiltonian decomposable. Hamiltonian decompositions of the tensor products of complete bi- partite graphs and complete multipartite graphs are dealt with in [15]. Also in [17], Paulraja and Sivasankar proved that (Kr Ks)∗; ((Kr Ks) Kn)∗; ((Kr Ks) × ◦ × × × Km)∗; ((Kr Ks) (Km Kn))∗ and (Kr;r (Km Kn))∗ are directed hamilto- ◦ × ◦ × ◦ nian decomposable. It can be observed that Kr;Kr; r;Kr Ks are circulant graphs. Based on the results of [10, 11, 13, 14, 15], Manikandan◦ and Paulraja conjectured the following: Conjecture 1.1. (Manikandan and Paulraja) [12]. If G and H are hamiltonian decomposable circulant graphs and at least one of them is non bipartite, then G H is hamiltonian decomposable. × Theorem 1.1. [6] If both G and H have hamiltonian decompositions and at least one of them is of odd order, then G H admits a hamiltonian decomposition. × One may naturally ask when G H is hamiltonian decomposable, if both G and H are of even order. In this paper× it is answered for some family of sparse graphs. We say that an even regular circulant graph X = Circ(n; L) has the property Q; if (1) the number of odd jumps and even jumps in L are equal; (2) odd jumps can be paired with even jumps so that the edges of each pair of jumps induce a con- L nected 4-regular graph. There are j2j such connected 4-regular graphs. It is known that every 4-regular connected Cayley graph is hamiltonian decomposable; see [7]. Consequently, every circulant graph with property Q is hamiltonian decomposable. Here we prove the following main Theorem. Theorem 1.2. Let G be a circulant graph with property Q and let H be any hamil- tonian decomposable multigraph, then G H is hamiltonian decomposable. × This theorem has many interesting consequences. In particular, we have the following corollary. Corollary 1.1. If G and H are even regular hamiltonian decomposable circulant graphs and at least one of them has the property Q; then G H is hamiltonian decomposable. × One of the consequences of Corollary 1.1 is that if G = (K n F ) and 4 +2 − H = (K2m F 0); where F and F 0 are 1-factors of K4n+2 and K2m; respectively, then G H−is hamiltonian decomposable. In particular, after deleting suitable × number of jumps of K4n+2 and K2m; the resulting graphs need not be dense but their tensor product is hamiltonian decomposable. This cannot be deduced from the existing results in this direction. On Hamiltonian Decompositions of Tensor Products of Graphs 181 2. NOTATION AND PRELIMINARIES First we present the necessary definitions here. The notation that we use here are from [7] and for the sake of completeness we give them. The Cayley graph Γ = Cay(S; G); where S = a; b is a generating set of the finite abelian group with 2a = 0; 2b = 0 and a = b;f is ag simple connected graph. Let x and x + a be two vertices6 of Γ;6 then we call6 the± edge x(x + a) of Γ an a-edge; the subgraph formed by the a-edges is a disjoint union of cycles, called a-cycles, each of length ka; the order of the element a: Similarly, we define b-edges and b- cycles; each b-cycle is of length kb; the order of the element b: In Γ; let us denote the number of a-cycles by α and the number of b-cycles by β: Since the length n n of each a-cycle (respectively b-cycle) is ka = α ; (respectively kb = β ;) n = αka (respectively n = βkb).