# En(n,x)=Gamma(1-n,x)*x^(n-1) is the En() function;
# En(1,x)=E1(x) is the first En() function;
# En(x)=En(0,x)=E0(x);

En(x_):=exp(-x)/x;
En(0,x_):=exp(-x)/x;
En(1,x_):= -Ei(-x);
En(n_,inf):=0;