Let \(X_1,\ldots ,X_n\) be a random sample from \(\operatorname {Poisson}(\theta )\).
(a) Find a complete, sufficient statistic.
(b) Noting that \(\Prob _\theta (X_1=1)=\theta e^{-\theta }\), use the Rao–Blackwell and Lehmann–Scheffé theorems to find a UMVUE of \(\tau (\theta ):=\theta e^{-\theta }\).