# erf(x) is the error function;
# erf(n,x) is the generalized error function;

erf(n_,x_):=If(isnumber(n) and isnumber(x), gamma(n)*(gamma(1/n)-gamma(1/n,x^n))/sqrt(pi));
erf(0,x_) := x/sqrt(pi)/exp(1);
erf(1,x_) := 1/sqrt(pi)-exp(-x)/sqrt(pi);
erf(2,x_) := erf(x);