Reverse Cuthill-McKee ordering.
p = SYMRCM(S) returns a permutation p such that S(p,p)
tends to have smaller profile than S. This is a
good preordering for LU or Cholesky factorization
of matrices that come from "long, skinny" problems.
It works for both symmetric and asymmetric S.
See also SYMMMD3MKDHXL, COLMMD , COLPERM .