# On the quotient quantum graph with respect to the regular representation

## TL;DR

This paper proves that the quotient quantum graph with respect to the regular representation of a symmetry group is identical to the original graph, confirming a conjecture and clarifying the role of basis choice in this construction.

## Contribution

It demonstrates that the quotient quantum graph with respect to the regular representation is identical to the original graph for a specific basis, resolving an open problem.

## Key findings

- The quotient graph with respect to the regular representation is identical to the original graph for a particular basis.
- The result holds for any permutation representation of degree |G|.
- The result does not hold for permutation representations of degree greater than |G|.

## Abstract

Given a quantum graph $ \Gamma $, a finite symmetry group $ G $ acting on it and a representation $ R $ of $ G $, the quotient quantum graph $ \Gamma /R $ is described and constructed in the literature [1, 2, 18]. In particular, it was shown that the quotient graph $ \Gamma/\mathbb{C}G $ is isospectral to $ \Gamma $ by using representation theory where $ \mathbb{C}G $ denotes the regular representation of $ G $ [18]. Further, it was conjectured that $ \Gamma $ can be obtained as a quotient $ \Gamma/\mathbb{C}G $ [18]. However, proving this by construction of the quotient quantum graphs has remained as an open problem. In this paper, we solve this problem by proving by construction that for a quantum graph $ \Gamma $ and a finite symmetry group $ G $ acting on it, the quotient quantum graph $ \Gamma / \mathbb{C}G $ is not only isospectral but rather identical to $ \Gamma $ for a particular choice of a basis for $ \mathbb{C}G $. Furthermore, we prove that, this result holds for an arbitrary permutation representation of $ G $ with degree $ |G| $, whereas it doesn't hold for a permutation representation of $ G $ with degree greater than $|G|. $