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+## Copyright (C) 2000 Paul Kienzle
+##
+## This file is part of Octave.
+##
+## Octave 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.
+##
+## Octave 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 Octave; see the file COPYING.  If not, write to the Free
+## Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
+## 02110-1301, USA.
+
+## -*- texinfo -*-
+## @deftypefn {Function File} {} isprime (@var{n})
+##
+## Return true if @var{n} is a prime number, false otherwise.
+##
+## Something like the following is much faster if you need to test a lot
+## of small numbers:
+##
+## @example
+##    @var{t} = ismember (@var{n}, primes (max (@var{n} (:))));
+## @end example
+##
+## If max(n) is very large, then you should be using special purpose 
+## factorization code.
+##
+## @seealso{primes, factor, gcd, lcm}
+## @end deftypefn
+
+function t = isprime (n)
+  if (! isscalar (n))
+    nel = numel (n);
+    t = n;
+    for i = 1:nel
+      t(i) = isprime (t(i));
+    endfor
+  elseif (n != fix (n) || n < 2)
+    t = 0;
+  elseif (n < 9)
+    t = all (n != [4, 6, 8]);
+  else
+    q = n./[2, 3:2:sqrt(n)];
+    t = all (q != fix (q));
+  endif
+endfunction