Let \(X_1,\ldots ,X_n\) be independent identically distributed Poisson random variables for each \(n\) with parameter \(\lambda _n\).
(a) Show that \(X_1+\cdots +X_n\) is asymptotically normal if and only if \(n\lambda _n\to \infty \).
(b) Can you weaken the condition that the X’s are identically distributed for each \(n\)?