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A denotes MPGrav and Ac denotes MPGrav_travel
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Case when i compared to i is not needed as particles dont interact with themselves
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At step k, interact particle i with particle j= 1 + mod((i+k-1),N)
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Accumulate force on i due to j in fixed Ai
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Accumulate negative of this as force on j due to i in circulating Acj
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At the end of the algorithm, add Ai and Aci
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In parallel version, Note that Ai will be calculated in "home processor for particle i but Aci will travel around the machine being accumulated in processor holding particle j
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Thus this violates the owner computes rule and so this parallel algorithm must be implemented by hand -- the compiler will not find it automatically
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