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function p = cs_nd (A)
%CS_ND generalized nested dissection ordering.
% p = cs_nd(A) computes the nested dissection ordering of a matrix. Small
% submatrices (order 500 or less) are ordered via cs_amd. A must be sparse
% and symmetric (use p = cs_nd(A|A') if it is not symmetric).
%
% Example:
% A = delsq (numgrid ('L', 300)) ; % matrix used in 'bench'
% p = cs_nd (A) ;
% cspy (A (p,p)) ;
%
% See also CS_AMD, CS_SEP, CS_ESEP, CS_NSEP, AMD.
% CSparse, Copyright (c) 2006-2022, Timothy A. Davis. All Rights Reserved.
% SPDX-License-Identifier: LGPL-2.1+
n = size (A,1) ;
if (n == 1)
p = 1 ;
elseif (n < 500)
p = cs_amd (A) ; % use cs_amd on small graphs
else
[s a b] = cs_nsep (A) ; % find a node separator
a = a (cs_nd (A (a,a))) ; % order A(a,a) recursively
b = b (cs_nd (A (b,b))) ; % order A(b,b) recursively
p = [a b s] ; % concatenate to obtain the final ordering
end
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