A Note on Decomposition of Tensor Product of Complete Multipartite Graphs into Gregarious Kite

A. Tamil Elakkiya1
1Gobi Arts & Science College, Gobichettipalayam, Erode, Tamil Nadu, India

Abstract

A kite K is a graph which can be obtained by joining an edge to any vertex of K3. A kite with edge set {ab,bc,ca,cd} can be denoted as (a,b,c;cd). If every vertex of a kite in the decomposition lies in different partite sets, then we say that a kite decomposition of a multipartite graph is a gregarious kite decomposition. In this manuscript, it is shown that there exists a decomposition of (KmK¯n)×(KrK¯s) into gregarious kites if and only if
n2s2m(m1)r(r1)0(mod8),
where and × denote the wreath product and tensor product of graphs respectively. We denote a gregarious kite decomposition as GK-decomposition.

Keywords: Decomposition, Kite, Tensor product, Wreath Product

1. Introduction

Many authors have been studied on kite-design and kite decomposition/ kite factorization and their special properties such as gregarious kite decomposition/ gregarious kite factorization. Bermond and Schonheim [1] proved that a kite-design of order n exists if and only if n0,1(mod8). Wang and Chang [2,3] considered the problem of a resolvable (K3+e) and (K3+e,λ)-group divisible designs of type gtu1. Wang [4] have proven that the necessary conditions to find a resolvable (K3+e)-group divisible design of type gu are also sufficient. Fu et al. [5] have demonstrated that there exists a GK-decomposition of Km(n) if and only if n0,1(mod8) for odd m or n4 for even m. Gionfriddo and Milici [6] considered the uniformly resolvable decompositions of Kv and KvI into paths and kites. To know more about kite designs, refer [7-11]. Further, the authors A. Tamil Elakkiya and A. Muthusamy [12] have shown that there is a GK-decomposition of Km×Kn if and only if mn(m1)(n1)0(mod8). Moreover, the authors A. Tamil Elakkiya and A. Muthusamy [13] considered the existence of a GK factorization of tensor product of complete graphs.

In this way, our main concern is to find a existence of a GK-decomposition of (KmK¯n)×(KrK¯s). In this document, it is demonstrated that a GK-decomposition of (KmK¯n)×(KrK¯s) holds, if and only if, n2s2m(m1)r(r1)0(mod8).

2. Preliminary Results

Definition 1. [12] A partition of a graph G into a collection of subgraphs G1,G2,,Gr such that each one of it is distinct mutually by their edges together with E(G)=i=1rE(Gi) is called a decomposition of G; We then write G as G=G1G2Gr, where denotes edge-disjoint sum of subgraphs. For an integer s, sG denotes s copies of G.

Definition 2. [12] A kite K is a graph which can be obtained by joining an edge to any vertex of K3, for instance see Figure 1. A kite with edge set {ab,bc,ca,cd} can be denoted as (a,b,c;cd).

Figure 1

If every vertex of a kite in the decomposition lies in different partite sets, then we say that a kite decomposition of a multipartite graph is a gregarious kite decomposition. Here we may consider a Gregarious Kite decomposition as a GK-decomposition for short.

Definition 3. [12] The tensor product G×H and the wreath product GH of two graphs G and H are defined as follows: V(G×H)=V(GH) = {(u,v)uV(G),vV(H)}. E(G×H) = {(u,v)(x,y)uxE(G) and vyE(H)} and E(GH) = {(u,v)(x,y)u=x and vyE(H) or uxE(G)}.

As tensor product has commutative and distributive property over an edge-disjoint sum of subgraphs, if decomposition of G = G1G2Gr holds, we then write as G×H = (G1×H)(G2×H)(Gr×H).

Definition 4. [12] A graph G having partite sets V1,V2,,Vm with Vi∣=n, 1in and E(G) ={uv uVi and vVj,ij} is called complete m-partite graph and is denoted by Km(n). Note that Km(n) is same as the KmK¯n. where K¯n is the complement of a complete graph on n vertices.

In order to prove our key result, we require the following:

Theorem 1. [5] There exists a GK-decomposition of Km(n) if and only if m0,1(mod8), for all odd n (or) m4, for even n.

Remark 1. [5] Let K is a kite. Then there exists a GK-decomposition of KK¯s, for all s.

Lemma 1. [12] For all n, n2, there exists a GK-decomposition of K×Kn, where K is a kite.

Theorem 2. [12] There exists a GK-decomposition of Km×Kn if and only if mn(m1)(n1)0(mod8).

3. Existence of GK-decomposition of (KmK¯n)×(KrK¯s)

Lemma 2. A GK-decomposition of (KmK¯n)×(KrK¯s) exists, if m0,1(mod8), n,s1(mod2) and for all r, r2.

Proof. By Theorem 1, we have a GK-decomposition of (KmK¯n). Then (KmK¯n)×(KrK¯s) = (KKK)×(KrK¯s) = K×(KrK¯s)K×(KrK¯s)K×(KrK¯s). Now K×(KrK¯s)(K×Kr)K¯s. As we have a kite decomposition of K×Kr from Lemma 1, we then write as (K×Kr)K¯s = (KKK)K¯s = (KK¯s)(KK¯s)(KK¯s). A GK-decomposition of KK¯s can be obtained from Remark 1. Thus all together provides a GK-decomposition of (KmK¯n)×(KrK¯s). ◻

Lemma 3. A GK-decomposition of (KmK¯n)×(KrK¯s) exists, if n0(mod2) and for all m,r,s, {m,r}2.

Proof. Case 1: Let m=3, n=2b,b0 and for all r,s,(r2). We write (K3K¯n)×(KrK¯s)[(K3K¯n)×Kr]K¯s = [(K3K3K3)×Kr]K¯s = [(K3×Kr)(K3×Kr)(K3×Kr)]K¯s = [(K3×Kr)K¯s][(K3×Kr)K¯s][(K3×Kr)K¯s]. Theorem 2 gives a GK-decomposition for K3×Kr. Now, we write as (K3×Kr)K¯s= (KKK)K¯s = (KK¯s)(KK¯s)(KK¯s). A GK-decomposition exists for KK¯s according to Remark 1. As a consequence of it, a GK-decomposition of (K3K¯n)×(KrK¯s) has been derived.

Case 2: A GK-decomposition for (KmK¯n)×(KrK¯s) have been described as follows:

Let m4, n=2b,b0 and for all r,s,(r2). One can obtain a GK-decomposition of (KmK¯n) from Theorem 1. Then (KmK¯n)×(KrK¯s) = (KKK)×(KrK¯s) = K×(KrK¯s)K×(KrK¯s)K×(KrK¯s). Now K×(KrK¯s)(K×Kr)K¯s. A kite decomposition exits for K×Kr by using Lemma 1. Moreover, we then write, (K×Kr)K¯s = (KKK)K¯s = (KK¯s)(KK¯s)(KK¯s). According to Remark 1, a GK-decomposition of KK¯s has been proved.

Consequently, Cases 1 and 2 lead a GK-decomposition for (KmK¯n)×(KrK¯s) when n0(mod2) and for all m,r,s, {m,r}2. ◻

Lemma 4. A GK-decomposition of (KmK¯n)×(KrK¯s) exists, if n1(mod2), s0(mod2), and for all m,r, {m,r}2.

Proof. Case 1: Let m=3, n=2b+1,b0, s=2c,c0 and for all r,(r2). Let us consider (K3K¯n)×(KrK¯s)[(K3K¯n)×Kr]K¯s = [(K3K3K3)×Kr]K¯s = [(K3×Kr)(K3×Kr)(K3×Kr)]K¯s = [(K3×Kr)K¯s][(K3×Kr)K¯s][(K3×Kr)K¯s]. By Theorem 2, a GK-decomposition exists for K3×Kr. Now (K3×Kr)K¯s = (KKK)K¯s = (KK¯s)(KK¯s)(KK¯s). Now, a GK-decomposition of KK¯s follows from Remark 1. Thus, the above construction provides a GK-decomposition of (K3K¯n)×(KrK¯s).

Case 2: Let m4, n=2b+1,b0, s=2c,c0 and for all r,(r2). By Theorem 1, there is a GK-decomposition for (KrK¯s). Then (KmK¯n)×(KrK¯s) = (KmK¯n)×(KKK) = [(KmK¯n)×K][(KmK¯n)×K][(KmK¯n)×K].

Now [(KmK¯n)×K]K×(KmK¯n)(K×Km)K¯n. By Lemma 1, we have a kite decomposition of K×Km. Then (K×Km)K¯n = (KKK)K¯n = (KK¯n)(KK¯n)(KK¯n). A GK-decomposition of KK¯n follows from Remark 1. Thus, all together gives a GK-decomposition of (KmK¯n)×(KrK¯s). ◻

Lemma 5. A GK-decomposition of (KmK¯n)×(KrK¯s) exists, if m4,5(mod8), n,s1(mod2) and for all r, r2.

Proof. Let us consider (KmK¯n)×(KrK¯s)[(KmK¯n)×Kr]K¯s. We write as (KmK¯n)×KrKr×(KmK¯n)(Kr×Km)K¯n. By Theorem 2, we have a GK-decomposition of Kr×Km. Then (Kr×Km)K¯n = (KKK)K¯n = (KK¯n)(KK¯n)(KK¯n). By Remark 1, we can obtain a GK-decomposition of KK¯n. Further, [(KmK¯n)×Kr]K¯s = (KKK)K¯s = (KK¯s)(KK¯s)(KK¯s). A GK-decomposition of KK¯s follows from Remark 1. Thus, all together gives a GK-decomposition of (KmK¯n)×(KrK¯s). ◻

Lemma 6. A GK-decomposition of (KmK¯n)×(KrK¯s) exists, if m2,3(mod4), m2, n,s1(mod2) and r0,1(mod8).

Proof. A GK-decomposition of KrK¯s can be obtained from Theorem 1. We then write, (KmK¯n)×(KrK¯s) = (KmK¯n)×(KKK) = [(KmK¯n)×K][(KmK¯n)×K][(KmK¯n)×K]. Now, we write as [(KmK¯n)×K]K×(KmK¯n)(K×Km)K¯n. A kite decomposition of K×Km has been derived from Lemma 1. Then (K×Km)K¯n = (KKK)K¯n = (KK¯n)(KK¯n)(KK¯n). A GK-decomposition of KK¯n exists according to Remark 1. Consequently, a GK-decomposition exists for (KmK¯n)×(KrK¯s)◻

Lemma 7. A GK-decomposition of (KmK¯n)×(KrK¯s) exists, if m2,3(mod4), m2, n,s1(mod2) and r4,5(mod8).

Proof. We write (KmK¯n)×(KrK¯s)[(KmK¯n)×Kr]K¯s. Now (KmK¯n)×KrKr×(KmK¯n)(Kr×Km)K¯n. By Theorem 2, we have a GK-decomposition of Kr×Km. Then (Kr×Km)K¯n = (KKK)K¯n = (KK¯n)(KK¯n)(KK¯n). A GK-decomposition of KK¯n can be obtained from Remark 1. Further, [(KmK¯n)×Kr]K¯s = (KKK)K¯s = (KK¯s)(KK¯s)(KK¯s). A GK-decomposition of KK¯s follows from Remark 1. Thus the above describes a GK-decomposition for (KmK¯n)×(KrK¯s). ◻

4. Main Result

Theorem 3. There exists a GK-decomposition of (KmK¯n)×(KrK¯s) if and only if n2s2m(m1)r(r1)0(mod8).

Proof. Necessity: The number of edges of (KmK¯n)×(KrK¯s) is n2s2m(m1)r(r1)2 and number of edges of a kite K is 4. Therefore, the edge divisibility condition for a graph (KmK¯n)×(KrK¯s) is n2s2m(m1)r(r1)8.

Sufficiency: It has been derived from Lemmas 2 – 7. ◻

5. Conclusion

In Section 3, we give a complete solution for the existence of a GK-decomposition of (KmK¯n)×(KrK¯s). In future, one can find the existence of a GK-factorization of Tensor product complete multipartite graph.

Funding Information

No funds, grants, or other support were received during preparation of this manuscript.

Conflict of Interest

The author have no conflict of interest.

References:

  1. Bermond, J.C. and Schönheim, J., 1977. G-decomposition of Kn, where G has four vertices or less. Discrete Mathematics, 19(2), pp.113-120.
  2. Wang, H. and Chang, Y., 2006. Kite-group Divisible Designs of Type g t u 1. Graphs & Combinatorics, 22(4), 545-571.
  3. Wang, H. and Chang, Y., 2008. (K-3+ e, lambda)-group divisible designs of type g (t) u (1). Ars Combinatoria, 89, pp.63-88.
  4. Wang, L., 2010. On the Existence of Resolvable (K+ e)-Group Divisible Designs. Graphs & Combinatorics, 26(6), pp.879-889.
  5. Fu, C.M., Hsu, Y.F., Lo, S.W. and Huang, W.C., 2013. Some gregarious kite decompositions of complete equipartite graphs. Discrete Mathematics, 313(5), pp.726-732.
  6. Gionfriddo, M. and Milici, S., 2013. On the existence of uniformly resolvable decompositions of Kv and Kv- I into paths and kites. Discrete Mathematics, 313(23), pp.2830-2834.
  7. Colbourn, C.J., Ling, A. C. and Quattrocchi, G., 2005. Embedding path designs into kite systems. Discrete Mathematics, 297(1-3), pp.38-48.

  8. Gionfriddo, L. and Lindner, C. C., 2005. Nesting kite and 4-cycle systems. The Australasian Journal of Combinatorics, 33, pp.247-254.
  9. Kucukcifci, S. and Lindner, C.C., 2002. The Metamorphosis of Lambda-fold Block Designs with Block Size Four into Lambda-fold Kite Systems. Journal of Combinatorial Mathematics and Combinatorial Computing, 40, pp.241-252.
  10. Faro, G.L. and Tripodi, A., 2006. The Doyen–Wilson theorem for kite systems. Discrete mathematics, 306(21), pp.2695-2701.
  11. G. Ragusa, G., 2010. Complete simultaneous metamorphosis of λ-fold kite systems. Journal of Combinatorial Mathematics and Combinatorial Computing, 73, pp.159-180.
  12. Elakkiya, A.T. and Muthusamy, A., 2016. Gregarious kite decomposition of tensor product of complete graphs. Electronic Notes in Discrete Mathematics, 53, pp.83-96.
  13. Elakkiya, A. T. and Muthusamy, A., 2020. GK-factorization of tensor product of complete graphs. Discussiones Mathematicae Graph Theory, 40, pp.7-24.