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Nested Dissection by METIS/ParMETIS

Nested Dissection (ND) is a popular method to compute fill-reducing orderings
for sparse matrices. It can also be used for other ordering purposes.
The algorithm recursively finds a separator (bisector) in a graph,
orders the nodes in the two subsets first, and nodes in the separator last.
In METIS/ParMETIS, the recursion is stopped when the graph is smaller than
a certain size, and some version of minimum degree ordering is applied
to the remaining graph.
METIS computes a (local) ordering of the objects (currently only for serial runs), while
ParMETIS performs a global ordering of all the objects. ParMETIS currently
(version 3.1, supported by Zoltan) requires that the number of processors is a power
of two.

If ORDER_METHOD=PARMETIS
ParMETIS is called, but if it is METIS, METIS is called.

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