Exponential integral Ei(x) is defined as a principal value of an integral
Exponential integral En(x) is defined as
ExponentialIntegralEI subroutine calculates Ei(x) using piecewise rational approximation. En(x) is calculated by using the ExponentialIntegralEN subroutine. This subroutine uses series expansion, continuous fraction approximation or asymptotic approximation depending on the values of x and n.
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