Inverse Fast Fourier transform.
a A matrix of real or complex numbers.
u A matrix each of whose columns is the inverse Fast Fourier transform of the corresponding column of the input.

If the number of rows in the input is not a power of two, prior to processing, the input is appended with sufficient number of rows with zero elements so that the number of rows in the modified matrix is a power of two. The columns of the output are the inverse transfroms of the corresponding columns of this modified matrix.