“…When the point sets are non-uniform, the non-uniform FFT (NUFFT) algorithms in [12,31] are able to evaluate (1) when Φ(x, ξ) = x • ξ with O(N log(N )) operations. For more general kernel functions, the butterfly factorization (BF) [20,24,26,27] can factorize the dense matrix e 2πiΦ(x,ξ) as a product of O(log(N )) sparse matrices, each of which has only O(N ) non-zero entries. Hence, storing and applying e 2πiΦ(x,ξ) via the BF for evaluating (1) take only O(N log(N )) complexity.…”