Algorithm 385: Exponential integral E i (x)
Kathleen A. Paciorek · Communications of the ACM · 1970
The classical exponential integral is defined bywhere the integral is to be interpreted as the Cauchy principal value.Except for the sign, it represents the natural extension of the function El(z) ~ f~ e-$ --at=-E,(-z), largzl< 3. z t to the negative real axis.The rational approximations and corresponding intervals used in this routine are: Eu,(x) --e~ I1