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When does $det begin{pmatrix} A & X \ X^T & A end{pmatrix} = (det A)^2 + (det X)^2$?

MathOverflow Asked on December 8, 2021

Let $A$ be an $n times n$ real symmetric matrix.
Let
$$
M = begin{pmatrix} A & X \ X^T & A end{pmatrix}
$$

where $X$ is a real invertible $n times n$ matrix. I am interested in finding $X$ such that
$$
det M = (det A)^2 + (det X)^2.
$$

Given $A$, is there a systematic way of finding such $X$ under some hypotheses?

Any suggestions, comments or reference for a related topic is very much appreciated. Thank you!

One Answer

$$B=left[matrix{1&1&1&1\1&1&-1&-1\-1&-1&-1&1\-1&1&-1&1}right]$$ and probably many other solutions. I'm also voting to close because you didn't pose a research-level problem. If you have an interesting general case, pose that.

Answered by Brendan McKay on December 8, 2021

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