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Equations whose unknown is a matrix, such as, for instance, algebraic Riccati equations $XAX+XB+CX+D=0$ or matrix differential equations (e.g. $\dot X(t)=AX(t)$. This tag is *not* meant for general systems of linear equations $Ax=b$.

9 votes

Solving the matrix equation $XX^t = A$ for binary matrix $X$

For general $A,X~$ this is a very difficult problem, but the condition you give that the rows of $X~$ have sum 2 makes it much easier. Consider each row to be an edge of a graph $G~$ (i.e. the two on …
Brendan McKay's user avatar
9 votes

A variant of Cholesky decomposition involving binary matrices

Even the case where $B$ is constant on the diagonal and constant off the diagonal is extremely difficult. For example, it includes the question of for which orders a finite projective plane exists. If …
Brendan McKay's user avatar
6 votes

The number of 0-1 normal matrices

For orders 1 to 9: 2, 8, 68, 1124, 36112, 2263268, 281249824, 70329901860, 35546752694048. I computed these numbers by finding representatives of the isomorphism classes of normal digraphs plus the …
Brendan McKay's user avatar
5 votes

A problem about determinant and matrix

If there is a rational nonzero solution, there is an integer nonzero solution by multiplying up. At least one of the integers can be assumed odd by dividing out a common power of two. The determinant …
Brendan McKay's user avatar