# Signed bicyclic graphs with minimal index

Document Type : Original paper

Authors

Dipartimento di Matematica e Applicazioni R. Caccioppoli. Universita&#039; di Napoli Federico II

Abstract

The index $\lambda_1(\Gamma)$ of a signed graph $\Gamma=(G,\sigma)$ is just the largest eigenvalue of its adjacency matrix. For any $n \geqslant 4$ we identify the signed graphs achieving the minimum index in the class of signed bicyclic graphs with $n$ vertices. Apart from the $n=4$ case, such graphs are obtained by considering a starlike tree with four branches of suitable length (i.e.\ four distinct paths joined at their end vertex $u$) with two additional negative independent edges pairwise joining the four vertices adjacent to $u$. As a by-product, all signed bicyclic graphs containing  a theta-graph and whose index is less than $2$ are detected.

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