A graph \(G\) is a sum graph if there is a labeling \(o\) of its vertices with distinct positive integers, so that for any two distinct vertices \(u\) and \(v\), \(uv\) is an edge of \(G\) if and only if \(\sigma(u) +\sigma(v) = \sigma(w)\) for some other vertex \(w\). Every sum graph has at least one isolated vertex (the vertex with the largest label). Harary has conjectured that any tree can be made into a sum graph with the addition of a single isolated vertex. We prove this conjecture.

An \(H\)-decomposition of a graph \(G\) is a representation of \(G\) as an edge disjoint union of subgraphs, all of which are isomorphic to another graph \(H\). We study the case where \(H\) is \(P_3 \cup tK_2\) – the vertex disjoint union of a simple path of length 2 (edges) and \(t\) isolated edges – and prove that a set of three obviously necessary conditions for \(G = (V, E)\) to admit an \(H\)-decomposition, is also sufficient if \(|E|\) exceeds a certain function of \(t\). A polynomial time algorithm to test \(H\)-decomposability of an input graph \(G\) immediately follows.

In this paper we consider group divisible designs with equal-sized holes \((HGDD)\) which is a generalization of modified group divisible designs \([1]\) and \(HMOLS\). We prove that the obvious necessary conditions for the existence of the \(HGDD\) is sufficient when the block size is three, which generalizes the result of Assaf[1].

An obvious necessary condition for the existence of an almost resolvable \(B(k,k-1;v)\) is \(v \equiv 1 \pmod{k}\). We show in this paper that the necessary condition is also sufficient for \(k = 5\) or \(k = 6\), possibly excepting \(8\) values of \(v\) when \(k = 5\) and \(3\) values of \(v\) when \(k = 6\).

This paper gives two sufficient conditions for a \(2\)-connected graph to be pancyclic. The first one is that the degree sum of every pair of nonadjacent vertices should not be less than \(\frac{n}{2} + \delta\). The second is that the degree sum of every triple of independent vertices should not be less than \(n + \delta\), where \(n\) is the number of vertices and \(\delta\) is the minimum degree of the graph.

In this paper we will consider the Ramsey numbers for paths and cycles in graphs with unordered as well as ordered vertex sets.

Suppose that \(R = (V, A)\) is a diregular bipartite tournament of order \(p \geq 8\). Denote a cycle of length \(k\) by \(C_k\). Then for any \(e \in A(R)\), \(w \in V(R) \setminus V(e)\), there exists a pair of vertex-disjoint cycles \(C_4\) and \(C_{p-4}\) in \(R\) with \(e \in C_4\) and \(w \in C_{p-4}\), except \(R\) is isomorphic to a special digraph \(\tilde{F}_{4k}\).

We construct all four-chromatic triangle-free graphs on twelve vertices, and a triangle-free, uniquely three-colourable graph.

Let \(K\) be a maximal block of a graph \(G\) and let \(x\) and \(y\) be two nonadjacent vertices of \(G\). If \(|V(X)| \leq \frac{1}{2}(n+3)\) and \(x\) and \(y\) are not cut vertices, we show that \(x\) is not adjacent to \(y\) in the closure \(c(G)\) of \(G\). We also show that, if \(x, y \notin V(K)\), then \(x\) is not adjacent to \(y\) in \(c(G)\).