# Ei(x) is exponential integral; # Ei(x)= - integrate(exp(-t)/t,t from -x to inf); # Ei(n,x)=(d/dx)^n Ei(x) is the nth derivative of exponential integral Ei(x); Ei(log(x_)):=li(x); Ei(0) := -inf; Ei(-inf) := 0; Ei(inf) := inf;