--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/scripts/statistics/distributions/poisspdf.m	Wed Jul 13 17:43:35 2005 +0000
@@ -0,0 +1,58 @@
+## 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} {} poisson_pdf (@var{x}, @var{lambda})
+## For each element of @var{x}, compute the probability density function
+## (PDF) at @var{x} of the poisson distribution with parameter @var{lambda}.
+## @end deftypefn
+
+## Author: KH <Kurt.Hornik@ci.tuwien.ac.at>
+## Description: PDF of the Poisson distribution
+
+function pdf = poisson_pdf (x, l)
+
+  if (nargin != 2)
+    usage ("poisson_pdf (x, lambda)");
+  endif
+
+  if (!isscalar (l))
+    [retval, x, l] = common_size (x, l);
+    if (retval > 0)
+      error ("poisson_pdf: x and lambda must be of common size or scalar");
+    endif
+  endif
+
+  pdf = zeros (size (x));
+
+  k = find (!(l > 0) | isnan (x));
+  if (any (k))
+    pdf(k) = NaN;
+  endif
+
+  k = find ((x >= 0) & (x < Inf) & (x == round (x)) & (l > 0));
+  if (any (k))
+    if (isscalar (l))
+      pdf(k) = exp (x(k) .* log (l) - l - gammaln (x(k) + 1));
+    else
+      pdf(k) = exp (x(k) .* log (l(k)) - l(k) - gammaln (x(k) + 1));
+    endif
+  endif
+
+endfunction