In this paper we consider the trees with fixed order n and diameter d ≤ 4. Among these trees we identify those trees whose index is minimal. © 2009 Elsevier B.V. All rights reserved.

For any k ∈ N, a graph G = (V,E) is said to be Zk-magic if there exists a labeling l : E(G) −→ Zk − {0} such that the induced vertex set labeling l : V (G) −→ Zk defined by l(v) = ∑ u∈N(v) l(uv) is a constant map. For a given graph G, the set of all k ∈ Z+ for which G is Zk-magic is called the integer-magic spectrum of G and is denoted by IM(G). In this paper we will consider trees whose diamet...

A graph H is said to be light in a family G of graphs if at least one member of G contains a copy of H and there exists an integer λ(H,G) such that each member G of G with a copy of H also has a copy K of H such that degG(v) ≤ λ(H,G) for all v ∈ V(K). In this paper, we study the light graphs in the class of graphs with small average degree, including the plane graphs with some restrictions on g...

We show that if a graph G with n ≥ 3 vertices can be drawn in the plane such that each of its edges is involved in at most four crossings, then G has at most 6n− 12 edges. This settles a conjecture of Pach, Radoičić, Tardos, and Tóth. As a corollary we also obtain a better bound for the Crossing Lemma which gives a lower bound for the minimum number of crossings in a drawing of a graph.

We characterise the primitive 2-closed groups G of rank at most four that are not automorphism group a graph or digraph and show if degree is least 2402 then there just two infinite families G⩽AΓL1(pd), 1-dimensional affine semilinear group. These first known examples non-regular digraph.