Daniel McLaury
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 Apr22 asked Passing an iterator Sep18 awarded Caucus Aug19 accepted Is there a clean way to extract the subspaces invariant under a list of matrices? Aug15 awarded Nice Question Jun8 comment Is there a clean way to extract the subspaces invariant under a list of matrices? @J.M., thanks for the reference. It actually looks like the paper by Arapura and Peters in its bibliography is much closer to what I want. I'm going to try to implement that now. Jun7 comment Is there a clean way to extract the subspaces invariant under a list of matrices? @J.M., care to elaborate? When I look at the documentation, there seems to be a "with respect to a" option, but no description of what this means or what it does. Jun7 comment Is there a clean way to extract the subspaces invariant under a list of matrices? @DanielLichtblau, C^n is (always) a valid answer. It'd be fine if it was returned as an answer, but it can also be deduced that it's invariant, since it's the direct sum of invariant subspaces already present in the list. Jun7 comment Is there a clean way to extract the subspaces invariant under a list of matrices? @rcollyer, if the matrices are all invertible, then being invariant under the matrices is the same as being invariant under the subgroup of $GL_n$ they generate. Otherwise, they generate a monoid. Alternatively, since being an invariant subspace isn't affected by adding scalar matrices to the $A_i$, you could modify them and reduce the question to the case where they are invertible, though I'm not sure that this accomplishes anything. Jun7 comment Is there a clean way to extract the subspaces invariant under a list of matrices? If they're all scalar multiples of the identity, then we have something of a problem, because then there would be infinitely many distinct invariant subspaces. This could be indicated in a few different ways -- whatever's convenient. (When this happens, it will always be the case that you can just take any subspace of a particular subspace and it will be invariant, so you just return something indicating "This space, together with all its subspaces," either by returning a separate list of these, or tagging them somehow.) Jun7 comment Is there a clean way to extract the subspaces invariant under a list of matrices? It depends on what's on the diagonal. If every diagonal entry is different, and they're 3x3, then the output should be { { { 1 , 0, 0 } }, { { 0, 1, 0 } }, { { 0, 0, 1 } } }. (The list could also contain two- and three- dimensional combinations of these if necessary, although that information would be redundant.) Jun7 comment Is there a clean way to extract the subspaces invariant under a list of matrices? A subspace $W \subseteq V$ is invariant under a matrix $A_i$ if, for each $w \in W$, $A_i w \in W$. I want to find all subspaces invariant under all the matrices simultaneously. In practice, you'd do this by looking at the (generalized) eigenvectors of each matrix (unless there's some nice matrix trick that I'm missing which makes it easier.) Jun7 asked Is there a clean way to extract the subspaces invariant under a list of matrices? Jun4 comment How do I make Mathematica understand roots of unity? Thanks! Upvoted both answers; sad I can only accept one. Jun4 awarded Supporter Jun4 awarded Scholar Jun4 awarded Editor Jun4 revised How do I make Mathematica understand roots of unity? deleted 1 characters in body Jun4 accepted How do I make Mathematica understand roots of unity? Jun4 awarded Student Jun4 asked How do I make Mathematica understand roots of unity?