On the quotient quantum graph with respect to the regular representation

Abstract

Given a quantum graph $ Γ$, a finite symmetry group $ G $ acting on it and a representation $ R $ of $ G $, the quotient quantum graph $ Γ/R $ is described and constructed in the literature [1, 2, 18]. In particular, it was shown that the quotient graph $ Γ/\mathbb{C}G $ is isospectral to $ Γ$ by using representation theory where $ \mathbb{C}G $ denotes the regular representation of $ G $ [18]. Further, it was conjectured that $ Γ$ can be obtained as a quotient $ Γ/\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 $ Γ$ and a finite symmetry group $ G $ acting on it, the quotient quantum graph $ Γ/ \mathbb{C}G $ is not only isospectral but rather identical to $ Γ$ 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|. $