z=x^y=e^{y*log(x)} より、Zx=(y/x)*(x^y)、Zxy=(1/x)*(x^y)+log(x)*(y/x)*(x^y)=x^(y-1)*{1+y*log(x)}
Zy=log(x)*(x^y)、Zyy={log(x)}^2*(x^y)