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Dec 13, 2022 at 16:57 comment added Toni Mhax This is still unclear, is it $B=\begin{pmatrix}A&AU\\AU&AU\end{pmatrix}$ for a $\pm 1$ diagonal matrix $U$...
Dec 10, 2022 at 17:50 comment added ABB Your notation is also so helpful.
Dec 10, 2022 at 17:48 comment added ABB Thanks for your comment. I have done.
Dec 10, 2022 at 17:46 history edited ABB CC BY-SA 4.0
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Dec 10, 2022 at 13:29 comment added Toni Mhax I don't think you defined the fourth block as $B=\begin{pmatrix}A&AU\\AU&*\end{pmatrix}$?
Dec 9, 2022 at 15:25 comment added ABB $B$ is supposed to be symmetric, so to find $B$ we need only to know left side entries that are known for us.
Dec 9, 2022 at 14:33 comment added Antoine Labelle What is $b_{n+k,n+l}$? Also it seems to me that if $A$ isn't symmetric then B won't be neither since the top left $n \times n$ block of $B$ is exactly $A$.
Dec 9, 2022 at 9:38 history edited ABB
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Dec 9, 2022 at 9:35 comment added ABB Yes, extending eigenvectors of the submatrix $A$ to $B$ would be also considered as a question. But, the converse process is my concern for now.
Dec 9, 2022 at 9:10 comment added Beni Bogosel I would assume the opposite. If you know the eigenvectors of $A$ then you could try to build eigenvectors for $B$, changing the sign of certain components. With a bit of luck, you could use one eigenvector for $A$ to generate two different eigenvectors for $B$. Anyways, did you try $n=2$ and test some simple cases by hand??
Dec 9, 2022 at 8:47 history asked ABB CC BY-SA 4.0