Uniquely $C_{4}^{+}$-saturated graphs

TL;DR

This paper characterizes uniquely $C_{4}^{+}$-saturated graphs, showing their girth properties, connection to strongly regular graphs, and limitations on the number of triangles they can contain.

Contribution

It provides a complete characterization of nontrivial uniquely $C_{4}^{+}$-saturated graphs, including girth conditions, their relation to strongly regular graphs, and bounds on the number of triangles.

Findings

Girth of such graphs is 3 or 4.

Graphs with girth 4 are strongly regular with specific parameters.

No such graphs exist beyond certain size and triangle count constraints.

Abstract

A graph $G$ is uniquely $H$-saturated if it contains no copy of a graph $H$ as a subgraph, but adding any new edge into $G$ creates exactly one copy of $H$. Let $C_{4}^{+}$ be the diamond graph consisting of a $4$-cycle $C_{4}$ with one chord and $C_{3}^{*}$ be the graph consisting of a triangle with a pendant edge. In this paper we prove that a nontrivial uniquely $C_{4}^{+}$-saturated graph $G$ has girth $3$ or $4$. Further, $G$ has girth $4$ if and only if it is a strongly regular graph with special parameters. For $n>18k^{2}-24k+10$ with $k\geq2$, there are no uniquely $C_{4}^{+}$-saturated graphs on $n$ vertices with $k$ triangles. In particular, $C_{3}^{*}$ is the only nontrivial uniquely $C_{4}^{+}$-saturated graph with one triangle, and there are no uniquely $C_{4}^{+}$-saturated graphs with two, three or four triangles.