Fast Fourier Transform, Algorithms, Pattern Matching, Convolution, Combination Sum.
to multiply X inverse with R i.e. Note that in order to find the inverse of the matrix X, it needs to be a square matrix but there are 2n-1 columns and 2n rows. So we add a dummy column corresponding to the 2n-1 th coefficient and add c_(2n-1) = 0 in the C vector. It can be shown that the matrix C