Computing Hypermatrix Spectra with the Poisson Product Formula

We compute the spectrum of the "all ones" hypermatrix using the Poisson product formula. This computation includes a complete description of the eigenvalues' multiplicities, a seemingly elusive aspect of the spectral theory of tensors. We also give a general distributional picture of the spectrum as a point-set in the complex plane, and use our techniques to analyze the spectrum of "sunflower hypergraphs", a class that has played a prominent role in extremal hypergraph theory.