# 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(a_*log(x_)):=li(x^a);
Ei(0) := -inf;
Ei(-inf) := 0;
Ei(inf) := inf;