A note on 1-planar graphs

On the M-polynomial of planar chemical graphs

Let $G$ be a graph and let $m_{i,j}(G)$, $i,jge 1$, be the number of edges $uv$ of $G$ such that ${d_v(G), d_u(G)} = {i,j}$. The $M$-polynomial of $G$ is $M(G;x,y) = sum_{ile j} m_{i,j}(G)x^iy^j$. With $M(G;x,y)$ in hands, numerous degree-based topological indices of $G$ can be routinely computed. In this note a formula for the $M$-polynomial of planar (chemical) graphs which have only vertices...

A Note on a graph associated to a commutative ring

The rings considered in this article are commutative with identity. This article is motivated by the work on comaximal graphs of rings.  In this article, with any ring $R$, we associate an undirected graph denoted by $G(R)$, whose vertex set is the set of all elements of $R$ and distinct vertices $x,y$ are joined by an edge in $G(R)$ if and only if $Rxcap Ry = Rxy$.  In Section 2 of this articl...

A note on the NP-completeness of the precoloring extension coloring problem in triangle free planar graphs

The precoloring extension coloring problem consists in deciding, given a positive integer k, a graph G = (V,E) and k pairwise disjoint subsets V0, . . . , Vk−1 of V , if there exists a (vertex) coloring S = (S0, . . . , Sk−1) of G such that Vi ⊆ Si, for all i = 0, . . . , k − 1. In this note, we show that the precoloring extension coloring problem is NP-complete in triangle free planar graphs w...

$n$-Array Jacobson graphs

We generalize the notion of Jacobson graphs into $n$-array columns called $n$-array Jacobson graphs and determine their connectivities and diameters. Also, we will study forbidden structures of these graphs and determine when an $n$-array Jacobson graph is planar, outer planar, projective, perfect or domination perfect.

Class two 1-planar graphs with maximum degree six or seven

A graph is 1-planar if it can be drawn on the plane so that each edge is crossed by at most one other edge. In this note we give examples of class two 1-planar graphs with maximum degree six or seven.

A Note on Strongly Quotient Graphs