Ei(x) Exponential Integral Function

Ei(x) = −Integral{−x, inf}[e^(−t) / t]dt.
For very large positive x, the asymptotic expansion of Ei(x) is used:

Ei(x) =~ exp(x) * (1/x + 1/x^2 + 2/x^3 + ... + k!/x^(k+1) + ... .
It is used if x >= lim = 10 + digits-desired * Ln(10). If x < lim, the series expansion of Ei(x) is used:

Ei(x) = Ln(|x|) + gamma + x + x^2/4 + x^3/18 + ... + x^k/(n*n!) + ... ,
where gamma is Euler's constant = 0.57721566... .

See: Exponential Integral -- From MathWorld
And: Wolfram Function Evaluation -- ExpIntegralEi
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