## DIMACS TR: 2008-14

## Friendship two-graphs

### Authors: Endre Boros, Vladimir A. Gurvich and Igor E. Zverovich

**
ABSTRACT
**

A friendship graph is a graph in which every two distinct vertices have exactly one common neighbor. All finite friendship graphs are known, each of them consists of triangles having a common vertex.
We extend friendship graphs to two-graphs, a two-graph being an ordered pair G = (G0, G1) of edge-disjoint graphs G0 and G1 on the same vertex-set V(G0) =V(G1). One may think that the edges of G are colored with colors 0 and 1.
In a friendship two-graph, every unordered pair of distinct vertices u, v is connected by a unique bicolored 2-path. Friendship two-graphs are solutions to the matrix equation AB + BA = J - I, where A and B are n*n symmetric 0-1 matrices of the same dimension, J is an n*n matrix with every entry being 1, and I is the identity n*n matrix.

We show that there are no finite friendship two-graph with minimum vertex degree at most two. However, we construct an infinite such graph, and the construction can be extended to an infinite family. Also, we find a finite friendship two-graph, and conjecture that it is unique.

Paper Available at:
ftp://dimacs.rutgers.edu/pub/dimacs/TechnicalReports/TechReports/2008/2008-14.pdf

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