A geometric construction of isospectral magnetic graphs

Abstract

We present a geometrical construction of families of finite isospectral graphs labelled by different partitions of a natural number $r$ of given length $s$ (the number of summands). Isospectrality here refers to the discrete magnetic Laplacian with normalised weights (including standard weights). The construction begins with an arbitrary finite graph $G$ with normalised weight and magnetic potential as a building block from which we construct, in a first step, a family of so-called frame graphs $(F_a{)}_{a\in \mathbb{N}}$. A frame graph $F_a$ is constructed contracting $a$ copies of $G$ along a subset of vertices $V_0$. In a second step, for any partition $A=(a_1,\dots ,a_s)$ of length $s$ of a natural number $r$ (i.e., $r=a_1+\cdots +a_s$) we construct a new graph $F_A$ contracting now the frames $F_{a_1},\dots ,F_{a_s}$ selected by $A$ along a proper subset of vertices $V_1\subset V_0$. All the graphs obtained by different $s$-partitions of $r\ge 4$ (for any choice of $V_0$ and $V_1$) are isospectral and non-isomorphic. In particular, we obtain increasing finite families of graphs which are isospectral for given $r$ and $s$ for different types of magnetic Laplacians including the standard Laplacian, the signless standard Laplacian, certain kinds of signed Laplacians and, also, for the (unbounded) Kirchhoff Laplacian of the underlying equilateral metric graph. The spectrum of the isospectral graphs is determined by the spectrum of the Laplacian of the building block $G$ and the spectrum for the Laplacian with Dirichlet conditions on the set of vertices $V_0$ and $V_1$ with multiplicities determined by the numbers $r$ and $s$ of the partition.

arXiv comment31 pages, 10 figures, 5 tables. To appear in Analysis and Mathematical Physics. Final version: new references to the bibliography, new example with vertex virtualisation, Fig.4 improved

References in corpus (13)

• Can One Hear the Shape of a Graph?
• 100 years of Weyl's law
• The Isospectral Fruits of Representation Theory: Quantum Graphs and Drums
• Resolving isospectral "drums" by counting nodal domains
• Linear Representations and Isospectrality with Boundary Conditions
• Survey of the inverse spectral problem
• Frustration index and Cheeger inequalities for discrete and continuous magnetic Laplacians
• Spectral gaps and discrete magnetic Laplacians
• Isospectral graphs with identical nodal counts
• The sound of symmetry
• Spectral preorder and perturbations of discrete weighted graphs
• Matching number, Hamiltonian graphs and discrete magnetic Laplacians
• Hearing the shape of a quantum boundary condition

Cited by in corpus (2)

• Families of isospectral and isoscattering quantum graphs
• Geometric and spectral analysis on weighted digraphs