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## Copyright (C) 1995, 1996, 1997 Kurt Hornik
##
## This program is free software; you can redistribute it and/or modify
## it under the terms of the GNU General Public License as published by
## the Free Software Foundation; either version 2, or (at your option)
## any later version.
##
## This program is distributed in the hope that it will be useful, but
## WITHOUT ANY WARRANTY; without even the implied warranty of
## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
## General Public License for more details.
##
## You should have received a copy of the GNU General Public License
## along with this file. If not, write to the Free Software Foundation,
## 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
## usage: chisquare_inv (x, n)
##
## For each element of x, compute the quantile (the inverse of the CDF)
## at x of the chisquare distribution with n degrees of freedom.
## Author: TT <Teresa.Twaroch@ci.tuwien.ac.at>
## Description: Quantile function of the chi-square distribution
function inv = chisquare_inv (x, n)
if (nargin != 2)
usage ("chisquare_inv (x, n)");
endif
[retval, x, n] = common_size (x, n);
if (retval > 0)
error ("chisquare_inv: x and n must be of common size or scalar");
endif
inv = gamma_inv (x, n / 2, 1 / 2);
## Allow only for (positive) integer n.
k = find (n != round (n));
if any (k)
fprintf (stderr, ...
"WARNING: n should be positive integer\n");
[r, c] = size (x);
inv = reshape (inv, 1, r * c);
inv(k) = NaN * ones (1, length (k));
inv = reshape (inv, r, c);
endif
endfunction
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