A Note on Acyclic Edge Colouring of Star Graph Families

A proper edge colouring f of a graph G is called acyclic if there are no bichromatic cycles in the graph. The acyclic edge chromatic number or acyclic chromatic index, denoted by ( ) ′ a G , is the minimum number of colours in an acyclic edge colouring of G. In this paper, we discuss the acyclic edge colouring of middle, central, total and line graphs of prime related star graph families. Also exact values of acyclic chromatic indices of such graphs are derived and some of their structural properties are discussed.


Introduction
All graphs considered in this paper are finite, undirected and simple.The concept of acyclic colouring of a graph was introduced by B. Grunbaum [1].A proper edge colouring of a graph G = (V, E) with vertex set V and edge set E, is a map f: E C → , where C is the set of colours with f(x) ≠ f(y) for any adjacent edges x, y of E. The minimum number of colours needed to properly colour the edges of G, is called the chromatic index of G and is denoted by A proper edge colouring f is called acyclic if there are no bichromatic cycles in the graph.The acyclic edge chromatic number or acyclic chromatic index, denoted by ( ) a G ′ , is the minimum number of colours in an acyclicedge colouring of G.
Consider the set X of lines of a graph G with at least one line as a family of 2-point subsets of V(G).The line graph [2] of graph G, denoted by L(G), is the intersection graph ( ) . Thus the points of L(G) are the lines of G, with two points of L(G) which are adjacent whenever the corresponding lines of G are.
Let G be a graph with vertex set V(G) and edge set E(G).The middle graph [3] of G, denoted by M(G) is a graph with vertex set ( ) ( ) Let G be a finite simple graph.The central graph [4] of a graph G, denoted by C(G) is obtained by subdividing each edge of G exactly once and joining all the non-adjacent vertices of G.
Let G be a graph with vertex set V(G) and edge set E(G).The total graph [2] [3] of G, denoted by T(G) is a graph with vertex set ( ) ( ) V G E G  in which two vertices , x y are adjacent in T(G) if one of the following holds.(i) , x y are in V(G) and x is adjacent to y in G; (ii) , x y are in E(G) and , x y are adjacent in G; ′ is a hard problem both from a theoretical and from an algorithmic point of view.Even for the simple and highly-structured class of complete graphs, the value of ( ) a G ′ is still not determined exactly.It has also been shown by Alon and Zaks [5] that determining whether Alon, Sudakov and Zaks [6] proved that ( ) for almost all ∆-regular graphs.This result was improved by Nesetril and Wormald [7] who showed that for a random ∆-regular graph ( ) In view of the discussion relating acyclic edge colouring to perfect 1-factorization conjecture, it may be inferred that finding the exact values of ( ) n a K ′ for every n seems hard.However, Alon et al. [8] designed an algorithm that can acyclically edge colour p K .Through this work, they constructively showed that ( )

Acyclic Edge Colouring of Line Graph of a Star Graph Theorem
The acyclic chromatic index, ( ) , As the line graph of the star graph is isomorphic to the complete graph and by Alon et al. [8], ( )

Acyclic Edge Colouring of Middle Graph of a Star Graph Theorem
For the star graph 1, 1 p K − the acyclic chromatic index, ( ) , , , , , , , , , with 0 v as the root vertex.By definition, in the middle graph ( ) From the definition of middle graph the vertices { } Assign the colour i to the edges { } , x y as follows.
{ } ( ) , if and One can easily check that it is an acyclic edge colouring of ( ) Example 3.1.

The Structural Properties Central Graph of Star Graph
 The maximum degree in the graph ( )  The minimum degree in the graph ( ) .
 The number of vertices in ( )

Theorem
For the graph 1, p K the acyclic chromatic index ( ) , , , , , , with p v as the root vertex.In central graph ( ) is subdivided by the i u in ( ) . Now the vertices { } One can easily check that it is an acyclic edge colouring of ( ) , 3 Example 4.2.

Acyclic Edge Colouring of Total Graph of a Star Graph
5.1.Structural Properties of ( )  The maximum degree in the graph ( )  The minimum degree in the graph ( )  The number of edges in the graph ( )  The number of vertices in ( )

Theorem
For any star graph 1, 1 p K − the acyclic chromatic index, ( ) ( ) , , , , , , , , , with 0 v as the root vertex.By definition, in the total graph ( ) . Now the vertices { }   One can easily check that it is an acyclic edge colouring of ( ) 1, 1 p T K − and hence ( ) ( )

Figure 1 ,
given below.Now assign a proper colouring to the vertices of ( ) 1, 1 p M K − as follows.Consider the colour class

Figure 2 ..
Now assign a proper colouring to the vertices of ( ) Assign the colour i to the edges { } , x y as follows.{} (

3 .
of order p , say p K in ( ) 1, 1 p T K − .See Figure Now assign a proper colouring to the vertices of ( ) 1, 1 p T K − as follows.Consider the colour class , say.Assign the colour i to the edges { } , x y as follows.