Equitable Edge Coloring of Splitting Graph of Some Classes of Wheel Graphs

Jagannathan. M1, Vernold Vivin. J2, Veninstine Vivik. J3
1Department of Mathematics, RVS College of Engineering and Technology, Coimbatore-641 402, Tamil Nadu, India.
2Department of Mathematics, University College of Engineering Nagercoil, (Anna University Constituent College), Nagercoil – 629 004, Tamil Nadu, India.
3Department of Mathematics, Karunya Institute of Technology and Sciences, Coimbatore-641 114, Tamil Nadu, India

Abstract

The coloring of all the edges of a graph G with the minimum number of colors, such that the adjacent edges are allotted a different color is known as the proper edge coloring. It is said to be equitable, if the number of edges in any two color classes differ by atmost one. In this paper, we obtain the equitable edge coloring of splitting graph of Wn, DWn and Gn by determining its edge chromatic number.

Keywords: Splitting graph, Equitable edge coloring, Double wheel, Gear

1. Introduction

Let G=(V,E) be a graph with vertex set V(G) and edge set E(G). We denote the maximum degree of G by Δ(G). The first paper on edge coloring was written by Tait in 1880. Since all edges incident to the same vertex must be assigned different colors, obviously χ(G)Δ(G).

In 1964, Vizing [1] proved that χ(G)Δ(G)+1. In 1973, Meyer [2] presented the concept of equitable coloring and equitable chromatic number. The idea of equitable edge coloring was defined by Hilton and de Werra [3] in 1994. It is the assignment of colors to all the edges of G, so that the edges can be partitioned into color classes with the difference between any two classes is atmost one.

Sampathkumar et.al [4] introduced the concept of splitting graph of a graph G denoted by S(G). It is framed by adding to each vertex v, a new vertex v such that v is adjacent to every vertex that is adjacent to v in G. We construct the splitting graph of some families of wheel graph and obtain the equitable edge coloring of such graphs.

In our day to day life many problems on optimization, network designing, scheduling problems, allocation of memory in computer networks and so on are related to edge coloring. For example, consider the time-tabling problem for the proper utilization of facilities, i.e., the minimum number of rooms needed at any one time can be scheduled by equitable edge coloring. In this paper, we determine the equitable edge chromatic number for splitting graph of Wn, DWn and Gn.

2. Preliminaries

Theorem 1. [1] Let G be a graph. Then Δ(G)χ(G)Δ(G)+1.

Definition 1. [5] For any integer n4, the wheel graph Wn is the n-vertex graph obtained by joining a vertex v to each of the n1 vertices {v1,v2,,vn} of the cycle graph Cn1.

Definition 2. [6] A double-wheel graph DWn of size n can be composed of 2Cn+K1, which consists of two cycles of size n, where the vertices of the two cycles are all connected to a common hub.

Definition 3. The gear graph Gn is the graph obtained from a wheel graph Wn by adding a vertex to each edge of the n1 cycle in Wn.

Definition 4. [4] For each vertex v of a graph G, take a new vertex v. Join v to all vertices of G adjacent to v. The graph S(G) thus obtained is called the splitting graph of G.

Definition 5. [3,7,8] An equitable edge coloring of a graph G is a k-proper edge coloring , if ||Ei||Ej||1, i,j=1,2,,k, where Ei(G), Ej(G) is the set of edges of color i and j repectively. The minimum of such k is called the equitable edge chromatic number of G and is denoted by χ=(G).

Lemma 1. [9]For any simple graph G(V,E), χ=(G)Δ(G).

Lemma 2. [9] For any simple graph G and H, χ=(G)=χ(G) and if HG then χ(H)χ(G), where χ(G) is the proper edge chromatic number of G.

Lemma 3. [10] Let G be a graph and let k2. If kd(v) for each vV(G), then G has an equitable edge-coloring with k colors.

Lemma 4. [10] Let G be a graph and let k2. If the k-core of G is a set of isolated vertices, then G has an equitable edge-coloring with k colors.

Additional graph theory terminology used in this paper can be found in [1].

In the following section, the equitable edge chromatic number of wheel Wn, double wheel DWn and gear graph Gn are determined.

3. Equitable edge coloring of wheel, double wheel and gear graphs

Theorem 2. The equitable edge chromatic number of splitting graph of wheel graph is χ=(S(Wn))=2(n1), for n5.

Proof. The wheel graph Wn consists of n vertices and 2(n1) edges. Let V(Wn)={v}{vi:1in1}and E(Wn)={ei:1in1}{en}{ei+n1:2in2}{e2(n1)} where ei is the edge vvi(1in1), en is the edge v1v2, e{i+n1} is the edge vivi+1(2in2) and e2(n1) is the edge vn1v1.

For the construction of splitting graph of wheel graph, new vertices and edges are chosen. It consists of 2n vertices and 6(n1) edges. Let V(S(Wn))={v}{v}{vi:1in1}{vi:1in1}and E(S(Wn))={ek},where k={i+j(n1),0j3,1in1i+j(n1),j=4,1in2i+j(n1)2,j=5,2in1i+j(n1)2,j=6,1i2

The edges {ek} between the vertices of the splitting graph of wheel graph are tabulated as follows:

Table 1. Classification of edges of S(Wn)
Divisions of k Values of j Ranges of i Edges between
the vertices
i+j(n1) 0 1in1 vvi
i+j(n1) 1 1in2,i=n1 vivi+1,vn1v1
i+j(n1) 2 1in1 vvi
i+j(n1) 3 1in1 vvi
i+j(n1) 4 1in2 vivi+1
i+j(n1)2 5 2in1 vivi1
i+j(n1)2 6 i=1,i=2 v1vn1,vn1v1

The coloring of edges is defined as f:E(S(Wn))C,whereC={1,2,,2(n1)}. f(ei+k(n1))={i, k=0, 1in1n1, k=1, i=1i1, k=1, 2in1i+n1, 2k3, 1in1f(ei+4(n1))={n+2, i=1i+1, 2in2f(ei+5(n1)2)={i+n+1, 2in3n, i=n21, i=n1f(ei+6(n1)2)={2, i=1n+1, i=2 By this procedure, the assignment of colors to all the edges is attained with 2(n1) colors. The edges of splitting graph of wheel graph are classified into independent color classes as E(S(Wn))={E1,E2,,E2(n1)} such that |E1|=|E2|==|E2(n1)|=3 for any n4. Hence it is clear that ||Ei||Ej||1 for ij, thus satisfying equitablility condition. Therefore χ=(S(Wn))2(n1). Since Δ=2(n1) and by lemma 1, it follows that χ=(S(Wn))χ(S(Wn))Δ. Hence χ=(S(Wn))=2(n1)◻

Theorem 3. The equitable edge chromatic number of splitting graph of double wheel graph is χ=(S(DWn))=4(n1), for n5.

Proof. The double wheel graph DWn consists of 2n1 vertices and 4(n1) edges. Let V(DWn)={v}{vi:1in1}{ui:1in1} and E(DWn)={ei:1in1}{en}{ei+n1:2in2}{e2(n1)}{ei+2(n1):1in1}{e3(n1)+1}{ei+3(n1):2in2}{e4(n1)}

where ei is the edge vvi(1in1), en is the edge v1v2, ei+n1 is the edge vivi+1(2in2), e2(n1) is the edge vn1v1, ei+2(n1) is the edge vui(1in1), e3(n1)+1 is the edge u1u2, ei+3(n1) is the edge uiui+1(2in2) and e4(n1) is the edge un1u1.

The splitting graph of double wheel graph is constructed by adding new vertices and edges, which consists of 4n2 vertices and 12(n1) edges. Let V(S(Wn))={v}{v}{vi:1in1}{vi:1in1}{ui:1in1}{ui:1in1} and E(S(DWn))={ek},where k={i+j(n1),0j3,6j9,1in1i+j(n1),j=4,10,1in2i+j(n1)2,j=5,11,2in1i+j(n1)2,j=6,12,1i2 The edges {ek} of the splitting graph of double wheel graph lying between the vertices are summarized as follows:

Table 2. Classification of edges of S(DWn)
Divisions of k Values of j Ranges of i Edges between
the vertices
i+j(n1) 0 1in1 vvi
i+j(n1) 1 1in2,i=n1 vivi+1,vn1v1
i+j(n1) 2 1in1 vvi
i+j(n1) 3 1in1 vvi
i+j(n1) 4 1in2 vivi+1
i+j(n1)2 5 2in1 vivi1
i+j(n1)2 6 i=1,i=2 v1vn1,vn1v1
i+j(n1) 6 1in1 vui
i+j(n1) 7 1in2,i=n1 uiui+1,un1u1
i+j(n1) 8 1in1 vui
i+j(n1) 9 1in1 vui
i+j(n1) 10 1in2 uiui+1
i+j(n1)2 11 2in1 uiui1
i+j(n1)2 12 i=1,i=2 u1un1,un1u1

The coloring of edges is defined as

f:E(S(DWn))C,whereC={1,2,,4(n1)}.

f(ei+k(n1))={i, k=0, 1in1n1, k=1, i=1i1, k=1, 2in1i+n1, 2k3, 1in1f(ei+4(n1))={n+2, i=1i+1, 2in2f(ei+5(n1)2)={i+n+1, 2in3n, i=n21, i=n1f(ei+6(n1)2)={2, i=1n+1, i=2f(ei+k(n1))={i+2(n1), k=6, 1in13(n1), k=7, i=1i1+2(n1), k=7, 2in1i+3(n1), 8k9, 1in1f(ei+10(n1))={3n, i=1i+1+2(n1), 2in2f(ei+11(n1)2)={i1+3n, 2in33n2, i=n22n1, i=n1f(ei+12(n1)2)={2n, i=13n1, i=2

All the edges of this splitting graph are assigned colors by the above process with 4(n1) different colors. The edges are classified into E(S(DWn))={E1,E2,,E4(n1)} such that each of the independent sets |E1|=|E2|==|E4(n1)|=3 for any n4. It is clear that ||Ei||Ej||1 for ij, thus satisfying equitablility condition. Hence it is observed that χ=(S(DWn))4(n1). By lemma 1, it follows that χ=(S(DWn))χ(S(DWn))Δ=4(n1). Therefore χ=(S(DWn))=4(n1)◻

Theorem 4. The equitable edge chromatic number of splitting graph of gear graph is χ=(S(Gn))=2(n1), for n4.

Proof. The gear graph Gn consists of 2n1 vertices and 3(n1) edges. Let V(S(Gn))={v}{vi:1in1}{vi:1in1}and E(Gn)={ei:1in1}{Ei:1in1}{Ei:1in2}{En1} where Ei is the edge vvi(1in1) , Ei is the edge vivi(1in1), Ei is the edge vivi+1(1in2) and En1 is the edge vn1v1.

The construction of splitting graph of gear graph consists of 4n2 vertices and 9(n1) edges. Let V(S(Gn))={v}{v}{vi:1in1}{ui:1in1}{vi:1in1}{ui:1in1} and

E(S(Gn))={Ek:k=i+j(n1),1in1,0j8}

The edges {Ek} connecting each of the vertices of the splitting graph of gear graph is obtained by substituting k=i+j(n1) and are tabulated as follows:

Table 3. Classification of edges of S(Gn)
Values of j Ranges of i Edges between the vertices
0 1in1 vvi
1 1in1 viui
2 1in2,i=n1 uivi+1 , un1v1
3 1in1 viui
4 1in2,i=n1 vi+1ui , v1un1
5 1in1 vvi
6 1in1 vvi
7 1in1 viui
8 1in2,i=n1 vi+1ui , v1un1

The edge coloring of splitting graph of gear graph is defined as f:E(S(Gn))C,where the colorsetC={1,2,,2(n1)}.

f(Ei)=i, 1in1f(Ei+(n1))={i+1, 1in21, i=n1f(Ei+2(n1))={i, 1in2n1, i=n1f(Ei+3(n1))={i+2, 1in3i+3n, n2in1f(Ei+4(n1))={i+1+n, 1in3n, i=n2n+1, i=n1f(Ei+5(n1))=i+(n1), 1in1f(Ei+6(n1))=i+(n1), 1in1f(Ei+7(n1))={i+n, 1in2n, i=n1f(Ei+8(n1))={i+(n1), 1in22(n1), i=n1 By the above method of coloring all the edges of the splitting graph of gear graph are allocated with 2(n1) colors. The edges are classified into E(S(Gn))={E1,E2,,E2(n1)} such that each of the independent sets |E1|=|E2|==|E(n1)|=4 and |En|=|En+1|==|E2(n1)|=5 for all n4. Hence it is observed that ||Ei||Ej||1 for ij, satisfying equitablility condition. This implies χ=(S(Gn))2(n1). By lemma 1, it follows that χ=(S(Gn))χ(S(Gn))Δ=2(n1). Therefore χ=(S(Gn))=2(n1)◻

4. Conclusion

In this paper, the equitable edge chromatic number of wheel graph families such as wheel Wn, double wheel DWn and gear graph Gn are obtained, the proofs provides an optimal solution to the equitable edge coloration of these splitting graphs. The equitable edge coloring in the area of splitting graphs and other product graphs is vast explorable and the focus on the determination of equitability in allocation can be extended to other families of graphs.

Acknowledgements

The authors sincerely thank the Referee for the careful reading, corrections and suggestions that have resulted in the improvement of the quality of this manuscript.

Conflict of Interest

The author declares no conflict of interests.

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