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Federico Poloni
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Matrices that are Hadamard products of $X$ and $X^{-T}$

What are the matrices that you can write in the form $X \odot X^{-T}$, for a complex square matrix $X$, where $X^{-T}$ is the inverse of the complex transpose (not conjugate) and $\odot$ is the Hadamard (component-by-component) product?

In the $2\times 2$ case, you get the group of matrices in the form \begin{bmatrix}a & b\\ b & a \end{bmatrix}, which are the hyperbolic rotations $\oplus$ a multiplicative factor. In larger dimension, one sees that all the obtained matrices have the vector of all ones as both a right and left eigenvector. Is this the only restriction? Do the matrices that we obtain form a group? Is this problem known and studied?

Origin: motivated from this MO question.

Federico Poloni
  • 20.2k
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  • 82
  • 120