it is straightforward to see that each dimension may be separately applied. This reduces the cost of applying the operator to O(Mk4 ) and reduces the storage to O(Mk2 ). The computational cost may be further reduced by using singular value decomposition to exploit the low operator rank away from the singularity. The length of the expansion (M ) is referred to as the separation rank. It is critical that this rank be as small as possible, and also that the coefficients in
the expansion be small and of the same sign so as to avoid loss of precision.