On the principal minors of Fourier matrices

For the $N$-dimensional Fourier matrix ${\mathcal{F}}_N$, we prove that if $N\ge 4$ is square-free, then every $2\times 2$ and $3\times 3$ principal minor of ${\mathcal{F}}_N$ is nonzero. We also show that if $N\ge 4$ is not square-free, then ${\mathcal{F}}_N$ has zero principal minors of all sizes. Moreover, based on numerical experiments, we conjecture that if $N$ is square-free, then all principal minors of ${\mathcal{F}}_N$ are nonzero.