For given a graph $H$, a graphic sequence $\pi =(d_1,d_2,...,d_n)$ is said to be potentially $H$-graphic if there is a realization of $\pi$ containing $H$ as a subgraph. In this paper, we characterize the potentially $(K_5-e)$-positive graphic sequences and give two simple necessary and sufficient conditions for a positive graphic sequence $\pi$ to be potentially $K_5$-graphic, where $K_r$ is a complete graph on $r$ vertices and $K_r-e$ is a graph obtained from $K_r$ by deleting one edge. Moreover, we also give a simple necessary and sufficient condition for...

Let $K_m-H$ be the graph obtained from $K_m$ by removing the edges set $E\left(H\right)$ of $H$ where $H$ is a subgraph of $K_m$. In this paper, we characterize the potentially $K_5-P_4$ and $K_5-Y_4$-graphic sequences where $Y_4$ is a tree on 5 vertices and 3 leaves.

A matrix $𝒜$ whose entries come from the set $\{+,-,0\}$ is called a sign pattern matrix, or sign pattern. A sign pattern is said to be potentially nilpotent if it has a nilpotent realization. In this paper, the characterization problem for some potentially nilpotent double star sign patterns is discussed. A class of double star sign patterns, denoted by $𝒟SSP(m,2)$, is introduced. We determine all potentially nilpotent sign patterns in $𝒟SSP(3,2)$ and $𝒟SSP(5,2)$, and prove that one sign pattern in $𝒟SSP(3,2)$ is potentially stable.

A graph $G$ of order $n$ is said to be a prime graph if its vertices can be labeled with the first $n$ positive integers in such a way that the labels of any two adjacent vertices in $G$ are relatively prime. If such a labeling on $G$ exists then it is called a prime labeling. In this paper we seek prime labeling for union of tadpole graphs. We derive a necessary condition for the existence of prime labelings of graphs that are union of tadpole graphs and further show that the condition is also sufficient...

If $G$ is a connected graph with distance function $d$, then by a step in $G$ is meant an ordered triple $(u,x,v)$ of vertices of $G$ such that $d(u,x)=1$ and $d(u,v)=d(x,v)+1$. A characterization of the set of all steps in a connected graph was published by the present author in 1997. In Section 1 of this paper, a new and shorter proof of that characterization is presented. A stronger result for a certain type of connected graphs is proved in Section 2.

A graph is called 1-planar if there exists a drawing in the plane so that each edge contains at most one crossing. We study maximal 1-planar graphs from the point of view of properties of their diagrams, local structure and hamiltonicity.

A graph having a perfect matching is called $r$-extendable if every matching of size $r$ can be extended to a perfect matching. It is proved that in the hypercube $Q_n$, a matching $S$ with $\left|S\right|\le n$ can be extended to a perfect matching if and only if it does not saturate the neighbourhood of any unsaturated vertex. In particular, $Q_n$ is $r$-extendable for every $r$ with $1\le r\le n-1.$

We define digraphs minimal, critical, and maximal by three types of radii. Some of these classes are completely characterized, while for the others it is shown that they are large in terms of induced subgraphs.

The paper gives an overview of results for radially minimal, critical, maximal and stable graphs and digraphs.

We introduce the rainbowness of a polyhedron as the minimum number $k$ such that any colouring of vertices of the polyhedron using at least $k$ colours involves a face all vertices of which have different colours. We determine the rainbowness of Platonic solids, prisms, antiprisms and ten Archimedean solids. For the remaining three Archimedean solids this parameter is estimated.

For graphs F, G and H, we write F → (G,H) to mean that any red-blue coloring of the edges of F contains a red copy of G or a blue copy of H. The graph F is Ramsey (G,H)-minimal if F → (G,H) but F* ↛ (G,H) for any proper subgraph F* ⊂ F. We present an infinite family of Ramsey $(K_{1,2},C₄)$-minimal graphs of any diameter ≥ 4.