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The Laplacian matrix is the representation of a graph in matrix form.

15 votes
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is it possible to have two non-isomorphic non-regular graphs with the same adjacent spectrum...

Yes, Brendan McKay showed that almost all trees have mates that are simultaneously cospectral in both adjacency and Laplacian spectra. And more. …
Gordon Royle's user avatar
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5 votes
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When do the nonzero eigenvalues of a directed graph Laplacian have the same absolute value?

This is false. Run this code in SageMath; you can do this at sagecell.sagemath.org if you do not have SageMath already installed on your own computer. h = DiGraph('DKCYW?') print(h.laplacian_matrix(). …
Gordon Royle's user avatar
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4 votes

Multiplicity of the smallest non-zero Laplacian eigenvalue for tree graphs

All the Laplacian eigenvalues of the star graph $K_{1,n}$ other than $n+1$ and $0$ are equal to $1$. Computer experimentation reveals a modest number of additional examples. …
Gordon Royle's user avatar
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