Inverse problems in quantum graphs and accidental degeneracy
A general treatment of the spectral problem of quantum graphs and tight-binding models in finite Hilbert spaces is given. The direct spectral problem and the inverse spectral problem are written in terms of simple algebraic equations containing information on the topology of a quantum graph. The inverse problem is shown to be combinatorial, and some low dimensional examples are explicitly solved. For a {\it window\ }graph, a commutator and anticommutator algebra (superalgebra) is identified as the culprit behind accidental degeneracy in the form of triplets, where configurational symmetry {\it alone\ }fails to explain the result. For a Möbius cycloacene graph, it is found that the accidental triplet cannot be explained with a superalgebra, but that the graph can be built unambiguously from the spectrum using combinatorial methods. These examples are compared with a more symmetric but less degenerate system, i.e. a {\it car wheel\ } graph which possesses neither triplets, nor superalgebra.