Here is the problem. Suppose $T$ is the UMVUE of $\theta$, please prove the UMVUE of $ET^k$ is $T^k$(k=1,2,3,...), where $ET^{2k}<\infty$ holds.
I have a fake proof by using L-S theorem: Because T is the UMVUE of $\theta$, T should be sufficient and complete, (which is not true). Then use L-S theorem, it proved.
But I don't know how to proof it correctly. Please help me.