# Ein(x) is the entire function; # Ein(x)=sum( -(-1)^k*x^k/(k!*k),k from 1 to inf) =integrate( (1-exp(-t))/t,t from 0 to x); Ein(x_,0):=0; Ein(x_,1):=0; Ein(x_,inf):=Ein(x); Ein(-x_):=log(x)+gamma-Ei(x);