Toeplitz-Block-Toeplitz inversion

[momchilmm]

Currently, the permittivity FT matrix inversion for the PWE and the GME is done using numpy.linalg.inv. However, the matrix has the structure of a Toeplitz-Block-Toeplitz matrix, which can be used for a faster inversion as per this paper . I also have a MATLAB code for that. It could be implemented at some point (ideally working with the autograd backend too) as this operation could become costly for large elementary cells.