1 ## Copyright (C) 2006 Sylvain Pelissier <sylvain.pelissier@gmail.com>
3 ## This program is free software; you can redistribute it and/or modify
4 ## it under the terms of the GNU General Public License as published by
5 ## the Free Software Foundation; either version 3 of the License, or
6 ## (at your option) any later version.
8 ## This program is distributed in the hope that it will be useful,
9 ## but WITHOUT ANY WARRANTY; without even the implied warranty of
10 ## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
11 ## GNU General Public License for more details.
13 ## You should have received a copy of the GNU General Public License
14 ## along with this program; If not, see <http://www.gnu.org/licenses/>.
17 ## @deftypefn {Function File} {@var{y} =} expint_Ei (@var{x})
18 ## Compute the exponential integral,
22 ## expint_Ei(x) = - | exp(t)/t dt
26 ## @seealso{expint, expint_E1}
29 function y = expint_Ei(x)
37 if(x(t)<0 && imag(x(t)) == 0)
38 y(t) = -quad(F,-x(t),Inf);
40 if(abs(x(t)) > 2 && imag(x(t)) == 0)
41 y(t) = expint_Ei(2) - quad(F,-x(t),-2);
49 y(t) = -(x(t).^2 - a1.*x(t) + a2)./((x(t).^2-b1.*x(t)+b2).*(-x(t)).*exp(-x(t)))-i.*pi;
51 y(t) = conj(expint_Ei(conj(x(t))));
56 y(t) = y(t) + x(t).^k./(k.*factorial(k));
58 y(t) = 0.577215664901532860606512090082402431 + log(x(t)) + y(t);