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Goal:
Model a large number
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of
“self-gravitating” particles
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To
simulate evolution of a
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galaxy,
for instance
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Straightforward
solution
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Compute
all n(n-1)/2
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pairwise
forces
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For
each particle, we sum
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the
forces and compute its
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motion
for one time-step
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Recompute
pairwise forces
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and
repeat
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Straightforward
solution is
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too
slow for large n
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Idea:
Use hierarchical
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grouping
to speed up the
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calculation
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Calculations
for nearby
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particles
are exact
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Distant
particles are
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grouped
and act as if their
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total
mass is concentrated
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at
their center-of-mass
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