## 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
