On the principal minors of Fourier matrices

For the $N$-dimensional Fourier matrix $\mathcal F_N$, we show that if $N \geq 2$, then all $2\times 2$ principal minors of $\mathcal F_N$ are nonzero if and only if $N$ is square-free. Additionally, we show that if $N > 4$, then all $3\times 3$ principal minors of $\mathcal F_N$ are nonzero if and only if $N$ is square-free. Moreover, based on numerical experiments, we conjecture that if $N$ is square-free, then all principal minors of $\mathcal F_N$ are nonzero.