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+## Copyright (C) 1995, 1996, 1997  Kurt Hornik
+##
+## 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} {} geometric_inv (@var{x}, @var{p})
+## For each element of @var{x}, compute the quantile at @var{x} of the
+## geometric distribution with parameter @var{p}.
+## @end deftypefn
+
+## Author: KH <Kurt.Hornik@ci.tuwien.ac.at>
+## Description: Quantile function of the geometric distribution
+
+function inv = geometric_inv (x, p)
+
+  if (nargin != 2)
+    usage ("geometric_inv (x, p)");
+  endif
+
+  if (!isscalar (x) && !isscalar (p))
+    [retval, x, p] = common_size (x, p);
+    if (retval > 0)
+      error ("geometric_inv: x and p must be of common size or scalar");
+    endif
+  endif
+
+  inv = zeros (size (x));
+
+  k = find (!(x >= 0) | !(x <= 1) | !(p >= 0) | !(p <= 1));
+  if (any (k))
+    inv(k) = NaN;
+  endif
+
+  k = find ((x == 1) & (p >= 0) & (p <= 1));
+  if (any (k))
+    inv(k) = Inf;
+  endif
+
+  k = find ((x > 0) & (x < 1) & (p > 0) & (p <= 1));
+  if (any (k))
+    if (isscalar (x))
+      inv(k) = max (ceil (log (1 - x) ./ log (1 - p(k))) - 1, 0);
+    elseif (isscalar (p))
+      inv(k) = max (ceil (log (1 - x(k)) / log (1 - p)) - 1, 0);
+    else
+      inv(k) = max (ceil (log (1 - x(k)) ./ log (1 - p(k))) - 1, 0);
+    endif
+  endif
+
+endfunction