# 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(n_,0):= 1/(n-1);
Ei(0,x_):= exp(-x)/x;
Ei(1,x_) := -Ei(-x);
#Ei(0.5,x_) := sqrt(pi)*erfc(sqrt(x))/sqrt(x);
#Ei(-0.5,x_) := sqrt(pi)*erf(sqrt(x))*x^(-1.5);
Ei(n_,inf) := 0;
Ei(n_,-inf) := -inf;

Ei(log(x_)):=li(x);
Ei(0) := -inf;
Ei(-inf) := 0;
Ei(inf) := inf;