Littlewood-Paley sum

returns the Littlewood-Paley sum for the 2-D filter banks in the 2-D wavelet scattering
decomposition `lpsum`

= littlewoodPaleySum(`sf`

)`sf`

. `lpsum`

is an
*M*-by-*N*-by-*Nfb* matrix, where
*M*-by-*N* is the matrix size of the padded filters and
*Nfb* is the number of filter banks.

Since the scattering transform is contractive, the Littlewood-Paley sums do not exceed 1.