Exponential integrals arise in many engineering applications. At least three different definitions have been used
in the literature. Here, we use the most common definition:
The E1 function is related to the exponential integral by the relation
E1(x) = Ei(x).
The logarithmic integral is related to the exponential integral by the relation
li(x) = Ei(ln x).
The sine and cosine integrals are related to the exponential integral by the relation
Ei(ix) = Ci(x) + iSi(x).
The Special class
provides static methods for evaluating the exponential integral and related functions for real arguments, as listed
in the table below.