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Consider the standard multipole expansion for the coulomb potential often used in the fast multipole method algorithm (Theorem 5.2). A translation theorem is used in the algorithm (Theorem 5.3).

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Is O(n,m) the same as M(n,m) in Theorem 5.2 except shifted with coordinates rho', alpha', and beta'? This makes sense but I wanted to double check.

When evaluating the expansion in Theorem 5.3, the factorial terms in A(j-n,k-m) will contain negative integers (namely from k-m), giving an undefined result or complex infinity. Additionally, the O(j-n,k-m) term will be impossible to evaluate due to the spherical harmonic indices being impossible (in Y(n,m), n must be greater or equal to abs(m) and this is not the case for indices (j-n,k-m)). What do we do about this? How is Theorem 5.3 evaluated correctly?

source for Theorems: https://math.nyu.edu/faculty/greengar/shortcourse_fmm.pdf

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  • $\begingroup$ Consider migration of this question to Computational Science SE. If it does not get enough attention, flag for the moderator. $\endgroup$ – Anton Menshov May 2 '20 at 22:45
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This question was answered in SE! The indices of the summation need to be adjusted:

https://scicomp.stackexchange.com/questions/34969/fast-multipole-method-fmm-3d-laplace

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