A NOTE ON THE ASCENDING SUBGRAPH DECOMPOSITION PROBLEM

Discrete Mathematics 84 (1990) 315-318 North-Holland

315

NOTE

A NOTE ON THE ASCENDING SUBGRAPH

DECOMPOSITION PROBLEM

Hung-Lin FU

Department of Applied Mathematics, National Chiao Tung University, Hsinchu, Taiwan, People’s Rep. of China

Received 16 September 1987 Revised 14 October 1988

Let G be a graph with (” : ‘) edges. We say G has an ascending subgraph decomposition (ASD) if the edge set of G can be partitioned into n sets generating graphs G,, G,, . , G, such that IE(G,)I = i (for i = 1, 2, . . , n) and G, is isomorphic to a subgraph of G,+r for

i = 1,2, . . , n - 1.

In this note, we prove that if G is a graph of maximum degree d C [(n + 1)/2j on (” l ‘) edges, then G has an ASD. Moreover, we show that if d s [(n - 1)/2], then G has an ASD with each member a matching. Subsequently, we also verify that every regular graph of degree a prime power has an ASD.

1. Introduction

In [l] the authors give the following decomposition conjecture.

Conjecture. Let G be a graph with ( * : ‘) edges. Then the edge set of G can be partitioned into n sets generating graphs Gr, G2, . . . , G,, such that ]E(GJ] = i (for i=1,2,..., n) and Gi is isomorphic to a subgraph of Gi+, for i = 1, 2, . . . , n - 1.

A graph G that can be decomposed as described in the conjecture will be said to have an ascending subgraph decomposition (abbreviated ASD). The graphs Gr, Gz, . . . , G,, are said to be members of such a decomposition.

In [l, 21, the conjecture has been verified for star forests. Also, in [2] it is proved that if G is a graph of maximum degree d on (” : ‘) edges and n L 4d2 + 6d + 3, then G has an ASD with each member a matching.

In this note, we prove that if G is a graph of maximum degree d s [(n + 1)/2] on (” : ‘) edges, then G has an ASD. Moreover, we show that if d s [(n - 1)/2], then G has an ASD with each member a matching. As a special case we also verify that every regular graph of degree a prime power has an ASD.

2. Main results

Let N be the set (1, 2, . . . , n}, and Al, A*, . . . , Ak be mutually disjoint subsets of N such that lJf=“=, Ai = N. Let S(Ai) be the sum of all elements in 0012-365X/90/$03.50 @ 1990 - Elsevier Science Publishers B.V. (North-Holland)

316 H.-L. Fu

Ai(s(@) = 0). W e will say that N can be decomposed into subsets of type ( Sl, s2,. * . 9 Sk) if there exists a collection of mutually disjoint subsets of

N, A,, A2,. . . , Ak, such that their union is N and s(AJ = si, i = 1, 2, . . . , k.

Obviously, Cf==, si = (” z ‘). For example { 1, 2, . . . , 6) can be decomposed into subsets of type (3, 5, 6, 7). (A, = {3}, A2 = (1, 4}, A3 = {6}, A, = (2, 5}.)

An edge-coloring of a graph is an assignment of colors to its edges so that no two incident edges have the same color. If a graph G has an edge-coloring with k colors, then G is called k-colorable. (Let hi denote the number of edges in G which are colored cj, i = 1, 2, . . . , k.) After a bit of reflection, we have the following proposition. (Unless stated otherwise, we assume that G has (“t’) edges and that the number of edges that are colored ci is Si.)

Proposition 1. Let G be a k-colorable graph. Zf N can be decomposed into subsets

of type (4, d2,. . . , Sk),

We call an edge-coloring equalized if )6i - ajl s 1 (1 s i < j s k). McDiarmid [3] and de Werra [5] independently proved that if a graph has an edge-coloring with k colors then it has an equalized edge-coloring with k colors. We can easily prove the following result by using the above fact.

Proposition 2. Let G be a graph with maximum degree d c L(n - 1)/2], then G has an ASD with each member a matching.

i-roof. By Vizing’s Theorem [4] G has edge chromatic number x’(G) at most [(n - 1)/2] + 1. H ence we can color G with n/2 or (n + 1)/2 colors depending on whether n is even or odd. By the theorem of McDiarmid and de Werra, we obtain an equalized edge-coloring with n/2 or (n + 1)/2 colors as the case may be. If n is even, then each color occurs n + 1 times. Since, N can be decomposed into subsets of type (n + 1, n + 1, . . . , n + 1) (n/Ztuple), we conclude that G has an ASD with each member a matching by Proposition 1. Similarly, if n is odd, then each color occurs n times. Since N can be decomposed into subsets of type

( n, n, . . . , n) ((n + l)/Ztuple), we have the proof. Cl

As a matter of fact, if G is of class one, i.e. x’(G) = d, then we can let d s [(n + 1)/2] in Proposition 2. Actually, if we simply want to prove that G has an ASD, we can improve the upper bound of d a bit.

Proposition 3. Let G be a graph with maximum degree d s L(n + 1)/2], then G has an ASD.

The ascending subgraph decomposition problem 317

Proof. From Proposition 2, the only cases left are d = n/2 (n is even) and

d = (n + 1)/2 (n is odd). If n is even, then G is (n/2 + 1)-colorable. Since we have an equalized edge-coloring, hence we can color the edges by the way: n/2 colors occur n - 1 times and one color occurs n times. Since N can be decomposed into subsets of type (n - 1, n - 1, . . . , n - 1, n) ((n/2 + 1)-tuple), we are done. For the case when n is odd, G is ((n + 1)/2 + 1)-colorable. Similarly, we can color the edges in the following way: (n - 3)/2 colors occur (n - 2) times and 3 colors occur (n - 1) times. Without loss of generality, we let those three colors which occur (n - 1) times be ci, c2, and c3. It is not difficult to see {1,2, . . . , n - 3) can be decomposed into subsets of type (n - 2, n - 2, . . . , II - 2) ((n - 3)/2-tuple), therefore we can choose Gi, G2, . . . , Gn_3 subsequently. We conclude the proof by letting G,_, be the collection of edges colored c1 except for one edge e, G,_, be the collection of edges colored c2, and G,, be the collection of those edges colored c3 and e. Cl

From Proposition 3, it is easy to see every regular graph of degree a prime power has an ASD.

Proposition 4. Every regular graph of degree a prime power has an ASD.

Proof. Let the degree and order of G be d and v respectively. Then d - v =

n . (n + 1). Hence we have d ) n(n + 1). Since d is a prime power and the common divisor of n and n + 1 is 1, d 1 n or d 1 n + 1. If d < n, then d =z (n + 1)/2. By Proposition 3, G has an ASD. If d = n, then G = K,,,,. The theorem follows from the fact that K,,, has an ASD. q

As we have seen above, if the maximum degree of the graph is not too large, it has an ASD. In what follow we suggest a slightly different approach to the problem.

A vertex covering in a graph is any set of vertices such that each edge of the graph has at least one of its end vertices in the set. We will say (&, &, . . . T &) is a covering pattern for a graph G, if we can find a vertex covering

{ Vl, 212, . . . 9 vk} such that there are pi edges incident with the vertex vi,

i= 1,2,. . . , k and each edge can be counted only once. For example, Fig. 1 has a covering pattern (5,4,3,3).

Since the following proposition is easy to see, it will be stated without proof. Proposition 5. Let G be a graph with a covering pattern (PI, p2, . . . , t&s,). Zf N can be decomposed into subsets of type (PI, &, . . . , &), then G has an ASD with each member a star.

318 H.-L. Fu

Fig. 1.

The following proposition is also easy to prove, we simply state it.

Proposition 6. Zf a graph can be partitioned into edge disjoint paths of length rI, r2, . . , , r, respectively, and the set N can be decomposed into subsets of type

( rl, r2, . . . , rk), then G has an ASD with each member a path.

3. Acknowledgement

The author would like to express his appreciation to the referee for his helpful

comments and his patience in correcting errors.

References PI PI [31 141 PI

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R.J. Faudree, A. Gyarfas, and R.H. Schelp, Graphs which have an ascending subgraph

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C.J.H. McDiarmid, The solution of a time-tabling problem, J. Inst. Maths. Applies. 9 (1972)

23-24.

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