function EI(X: Double): Double;
Exponential integral Ei(x).
| # | Name | Description |
|---|---|---|
| 1 | X | Argument of the exponential integral |
| 2 | must not be zero. |
Returns: Double - the exponential integral Ei(x).
Remarks:
Computes the exponential integral, the Cauchy principal value of
Ei(x)=-integral _(-x)^(inf)(e^(-t))/t dt=p.v.integral _(-inf)^xe^t/t dt.
Domain: any real x \ne 0 (Ei has a logarithmic singularity at the origin). It is related to the complementary integral by Ei(x) = -E1(-x) for x > 0. Ei(x)<0 for x<0, crosses zero near x \approx 0.3725, and grows like e^{x}/x for large positive x.
See Also: SpecialFuncs.E1, SpecialFuncs.ExpEI