Y = expint(X)
Y = expint(X) evaluates
the exponential integral for each element of
The exponential integral computed by this function is defined as
Another common definition of the exponential integral function is the Cauchy principal value integral
which, for real positive x, is related to
This function fully supports tall arrays. For more information, see Tall Arrays.
 Abramowitz, M. and I. A. Stegun. Handbook of Mathematical Functions. Chapter 5, New York: Dover Publications, 1965.