function [ n_data, x, fx ] = e1_values ( n_data )

%% E1_VALUES returns some values of the exponential integral function E1(X).
%
%  Definition:
%
%    The exponential integral E1(X) is defined by the formula:
%
%      E1(X) = integral ( 1 <= T <= Infinity ) exp ( -X*T ) / T dT
%
%    In Mathematica, the function can be evaluated by:
%
%      ExpIntegralE[1,x]
%
%  Modified:
%
%    16 September 2004
%
%  Author:
%
%    John Burkardt
%
%  Reference:
%
%    Milton Abramowitz and Irene Stegun,
%    Handbook of Mathematical Functions,
%    US Department of Commerce, 1964.
%
%    Stephen Wolfram,
%    The Mathematica Book,
%    Fourth Edition,
%    Wolfram Media / Cambridge University Press, 1999.
%
%  Parameters:
%
%    Input/output, integer N_DATA.  The user sets N_DATA to 0 before the
%    first call.  On each call, the routine increments N_DATA by 1, and
%    returns the corresponding data; when there is no more data, the
%    output value of N_DATA will be 0 again.
%
%    Output, real X, the argument of the function.
%
%    Output, real FX, the value of the function.
%
  n_max = 16;

  fx_vec = [ ...
     0.5597735947761608E+00, ...
     0.4543795031894021E+00, ...
     0.3737688432335091E+00, ...
     0.3105965785455430E+00, ...
     0.2601839393259996E+00, ...
     0.2193839343955203E+00, ...
     0.1859909045360402E+00, ...
     0.1584084368514626E+00, ...
     0.1354509578491291E+00, ...
     0.1162193125713579E+00, ...
     0.1000195824066327E+00, ...
     0.8630833369753979E-01, ...
     0.7465464440125305E-01, ...
     0.6471312936386886E-01, ...
     0.5620437817453485E-01, ...
     0.4890051070806112E-01 ];

  x_vec = [ ...
     0.5E+00, ...
     0.6E+00, ...
     0.7E+00, ...
     0.8E+00, ...
     0.9E+00, ...
     1.0E+00, ...
     1.1E+00, ...
     1.2E+00, ...
     1.3E+00, ...
     1.4E+00, ...
     1.5E+00, ...
     1.6E+00, ...
     1.7E+00, ...
     1.8E+00, ...
     1.9E+00, ...
     2.0E+00 ];

  if ( n_data < 0 )
    n_data = 0;
  end

  n_data = n_data + 1;

  if ( n_max < n_data )
    n_data = 0;
    x = 0.0;
    fx = 0.0;
  else
    x = x_vec(n_data);
    fx = fx_vec(n_data);
  end