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eigenvalues of matrices or operators

7 votes
Accepted

Finite group is Dedekind iff for every irrep, every element acts as identity or has all eige...

The condition can be restated as: for every $g$ and $\rho$, the subspace $\mathrm{Ker}(\rho(g)-1)$ is a subrepresentation. If $G$ is Dedekind (meaning that every subgroup is normal, or equivalently th …
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5 votes

Eigenvalues of positive matrices in $\mathrm{SL}(d,\mathbb{Z})$

3 (found by checking all $3\times 3$ matrices with entries in $\{1,2,3\}$): the matrix $\begin{pmatrix}1&1&1\\2&1&2\\2&3&1\end{pmatrix}$, with characteristic polynomial $x^3 - 3x^2 - 7x - 1$ has the eigenvalues … $$4.577...,\; -1.424...,\; -0.153...$$ Another one has only positive eigenvalues: $\begin{pmatrix}3&1&1\\1&2&1\\2&1&1\end{pmatrix}$, with characteristic polynomial $x^3 - 6x^2 + 7x - 1$ with the eigenvalues
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