Exponential Integral En
    N      X       Actual       EXPINT(N,X)
    0    1.00   0.367879E+00   0.367879E+00
    2    0.00   0.100000E+01   0.100000E+01
    3    0.00   0.500000E+00   0.500000E+00
    4    0.00   0.333333E+00   0.333333E+00
    2    0.50   0.326644E+00   0.326644E+00
    3    0.50   0.221604E+00   0.221604E+00
    4    0.50   0.165243E+00   0.165243E+00
   10    0.50   0.634583E-01   0.634583E-01
   20    0.50   0.310612E-01   0.310612E-01
    2    5.00   0.996469E-03   0.996469E-03
   20    5.00   0.278275E-03   0.278275E-03
    2   50.00   0.371178E-23   0.371178E-23
   20   50.00   0.276642E-23   0.276642E-23