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Si.m

## 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 2 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/>.

##SI compute the sine integral function define by:
##
##                    x
##                   /
##           Si(x) = | sin(t)/t dt
##                   /
##                   0
##

function y = Si(x)
      if (nargin != 1)
            usage ("Si(x)");
      endif
      y = zeros(size(x));
      if prod(size(x)) < 101
            for k = 1:prod(size(x))
            y(k) = sum(besselj([0:100]+0.5,(x(k)/2)).^2);
            endfor
            y = y.*pi;
      else
            for k=0:100
            y += besselj(k+0.5,x/2).^2;
            endfor
            y = y.*pi;
      endif 
endfunction;

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