Solution
= Solution
The <matrix> $A$ is square by <completeness>. Column <orthogonality> says $A^*A=\operatorname{diag}(|Z(g_j)|)$. Taking determinants gives
$$
\boxed{|\det A|^2=\det(A^*A)=\prod_j|Z(g_j)|.}
$$
In particular its <determinant> is nonzero.