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diff --git a/octave_packages/m/specfun/isprime.m b/octave_packages/m/specfun/isprime.m
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+## Copyright (C) 2000-2012 Paul Kienzle
+## Copyright (C) 2010 VZLU Prague
+##
+## 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 3 of the License, 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, see
+## <http://www.gnu.org/licenses/>.
+
+## -*- texinfo -*-
+## @deftypefn {Function File} {} isprime (@var{x})
+## Return a logical array which is true where the elements of @var{x} are
+## prime numbers and false where they are not.
+##
+## If the maximum value in @var{x} is very large, then you should be using
+## special purpose factorization code.
+##
+## @example
+## @group
+## isprime (1:6)
+##     @result{} [0, 1, 1, 0, 1, 0]
+## @end group
+## @end example
+## @seealso{primes, factor, gcd, lcm}
+## @end deftypefn
+
+function t = isprime (x)
+
+  if (nargin == 1)
+    if (any ((x != floor (x) | x < 0)(:)))
+      error ("isprime: needs positive integers");
+    endif
+    maxn = max (x(:));
+    ## generate prime table of suitable length.
+    maxp = min (maxn, max (sqrt (maxn), 1e7)); # FIXME: threshold not optimized.
+    pr = primes (maxp);
+    ## quick search for table matches.
+    t = lookup (pr, x, "b");
+    ## take the rest.
+    m = x(x > maxp);
+    if (! isempty (m))
+      ## there are still possible primes. filter them out by division.
+      if (maxn <= intmax ("uint32"))
+        m = uint32 (m);
+      elseif (maxn <= intmax ("uint64"))
+        m = uint64 (m);
+      else
+        warning ("isprime: too large integers being tested");
+      endif
+      pr = cast (pr(pr <= sqrt (maxn)), class (m));
+      for p = pr
+        m = m(rem (m, p) != 0);
+        if (length (m) < length (pr) / 10)
+          break;
+        endif
+      endfor
+      pr = pr(pr > p);
+      mm = arrayfun (@(x) all (rem (x, pr)), m);
+      m = m(mm);
+      if (! isempty (m))
+        m = cast (sort (m), class (x));
+        t |= lookup (m, x, "b");
+      endif
+    endif
+
+  else
+    print_usage ();
+  endif
+
+endfunction
+
+
+%!assert (isprime (4), logical (0));
+%!assert (isprime (3), logical (1));
+%!assert (isprime (magic (3)), logical ([0, 0, 0; 1, 1, 1; 0, 0, 1]));
+%!error isprime ()
+%!error isprime (1, 2)