Add a useful package (from Source forge) for octave

[CreaPhase.git] / octave_packages / specfun-1.1.0 / expint_E1.m

diff --git a/octave_packages/specfun-1.1.0/expint_E1.m b/octave_packages/specfun-1.1.0/expint_E1.m
new file mode 100644 (file)
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+++ b/octave_packages/specfun-1.1.0/expint_E1.m
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+## Copyright (C) 2006   Sylvain Pelissier   <sylvain.pelissier@gmail.com>
+##
+## 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 3 of the License, 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 program; If not, see <http://www.gnu.org/licenses/>.
+
+## -*- texinfo -*-
+## @deftypefn {Function File} {@var{y} =} expint_E1 (@var{x})
+## Compute the exponential integral,
+## @verbatim
+##                    infinity
+##                   /
+##       expint(x) = | exp(t)/t dt
+##                   /
+##                  x
+## @end verbatim
+## @seealso{expint, expint_Ei}
+## @end deftypefn
+
+function y = expint_E1(x)
+  if (nargin != 1)
+    print_usage;
+  endif
+  y = x;
+  y(imag(x) > 0 & imag(x) != 0) = -expint_Ei(-y(imag(x) > 0 & imag(x) != 0)) -i.*pi;
+  y(imag(x) < 0 & imag(x) != 0) = -expint_Ei(-y(imag(x) < 0 & imag(x) != 0)) +i.*pi;
+  y(real(x) >= 0 & imag(x)==0) = -expint_Ei(-y(real(x) >= 0 & imag(x)==0));
+  y(real(x) < 0 & imag(x)==0) = -expint_Ei(-y(real(x) < 0 & imag(x)==0)) -i.*pi;
+endfunction