Determinant of a polynomial matrix in the z^{-1} convention.
Coefficients are ordered as [z^0, z^{-1}, z^{-2}, …] along axis 2.
Uses an FFT-based approach: evaluate at DFT points, compute scalar det
at each frequency, then IFFT back.
- Parameters:
polynomial_matrix (ndarray) – shape (N, N, L), polynomial matrix entries.
- Returns:
- 1-D array of determinant polynomial coefficients,
ordered [z^0, z^{-1}, …], trimmed to the actual degree.
- Return type:
ndarray
