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Questions related to the spectrum of graphs, defined using one of the possible variants of the discrete Laplace operator or Laplacian matrix. See https://en.wikipedia.org/wiki/Discrete_Laplace_operator

3 votes

Upper bounds for the second largest eigenvalue in terms of degree?

There exists arbitrarily large $d$-regular graphs with a cut edge. A direct inspection shows that the expansion tends to $0$. By the Cheeger inequality, $λ_2$ is arbitrarily close to $d=λ_1$. There c …
LeechLattice's user avatar
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8 votes
Accepted

Are there graphs with irrational eigenvalues which are all $>1$?

Yes, the graph with adjacency matrix 0 0 1 0 1 1 1 1 0 0 0 1 0 0 0 1 1 0 0 0 1 1 1 1 0 1 0 0 0 …
LeechLattice's user avatar
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4 votes
Accepted

Strongly/distance regular graphs over $\mathbb{Z}_2^n$ with the same parameters

The answer is yes. There are many difference sets on the group $\mathbb Z_2^6$, which can be found in the La Jolla repository. Below is the SageMath code to generate two nonisomorphic $\mathbb Z_2^6$- …
LeechLattice's user avatar
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8 votes
Accepted

Graph embeddings in the projective plane: for the 35 forbidden minors, do we know their Coli...

Here's a table containing the Colin de Verdière numbers: Name Graph6 μ Reason K33 + K33 4 (components linklessly embeddable) K5 + K33 4 (components linkl …
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