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expint
Exponential integral
Y = expint(X)The exponential integral 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 expint as follows:
expint(-x+i*0) = -Ei(x) - i*pi Ei(x) = real(-expint(-x))
Y = expint(X)
evaluates the exponential integral for each element of X.
For elements of X in the domain 
, expint uses a series expansion representation (equation 5.1.11 in [1]):
For all other elements of X, expint uses a continued fraction representation (equation 5.1.22 in [1]):
[1] Abramowitz, M. and I. A. Stegun. Handbook of Mathematical Functions. Chapter 5, New York: Dover Publications, 1965.