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MPGrav(i) uses Xuse(i) and mass M(i) and calculates a 3 vector force from 3 vector Xuse(i,j)
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MPGrav(i) = S M(i)*M(j)*( Xuse(j)-Xuse(i))
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where ri,j = | Xuse(i)-Xuse(j) | is distance between particle i and particle j
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This calculation involves communication as N-Nproc of values of j (and parameters Xuse(j) M(j) ) are stored outside the processor holding the i'th processor
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We describe algorithms with increasing complexity and efficiency!
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Note in discussions, we are rather sloppy as to whether i,j are "local" (1...N/Nproc) or "global" (1...N) indices
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