Graph compositions are related to compositions of positive integers and partitions of finite sets, and have applications in electrical networks. This paper provides extensions of a previously known result which states that C(K N ) = B(N) B(N 2), where B(N) represents the N th Bell number and C(K N ) is the complete graph on N vertices with one edge deleted.

We analyze three combinatorial games played on simple undirected graphs. A move consists of deleting a single edge; which edges can be deleted depends on the parity of each edge’s endpoints. While determining the possible values of each game, we show relationships between these games and graph matchings, octal games, vertex deletion games, and graph colourings.

The energy of a graph is the sum of the singular values of its adjacency matrix. We are interested in how the energy of a graph changes when edges are deleted. Examples show that all cases are possible: increased, decreased, unchanged. Our goal is to find possible graph theoretical descriptions and to provide an infinite family of graphs for each case. The main tool is a singular value inequali...

In the Perfect Completion problem one wishes to add the fewest possible edges to a graph in order to obtain a perfect graph. How large can the size of the added edge set be compared to the size of the edge set of the original graph? We show that with high probability the smallest perfect graph containing a random graph G(n; n 0:6) has (n 1:8) edges. We also show that with high probability the m...

The bipartite vertex (resp. edge) frustration of a graph G, denoted by ψ(G) (resp. φ(G)), is the smallest number of vertices (resp. edges) that have to be deleted from G to obtain a bipartite subgraph of G. A sharp lower bound of the bipartite vertex frustration of the line graph L(G) of every graph G is given. In addition, the exact value of ψ(L(G)) is calculated when G is a forest.

The problem of deciding whether an arbitrary graph G has a homomorphism into a given graph H has been widely studied and has turned out to be very difficult. Hell and Nešetril proved that the decision problem is NP-complete unless H is bipartite. We consider a restricted problem where G is an arbitrary triangle-free hexagonal graph and H is a Kneser graph or its induced subgraph. We give an exp...

Abstract. The energy of a graph is the sum of the absolute values of its eigenvalues. We propose a new problem on graph energy change due to any single edge deletion. Then we survey the literature for existing partial solution of the problem, and mention a conjecture based on numerical evidence. Moreover, we prove in three different ways that the energy of a cycle graph decreases when an arbitr...