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(a) Consider a sequence of random variables X1, X2, . . . with cumulantgenerating functions KX1 , KX2 , . . ., and a random variable X with cumulant-generating function KX(t). Suppose, in addition, that all these cumulant-generating functions are well-defined for |t| < c. If KXn (t) → KX(t) as n → ∞ for all |t| < c, what can we conclude?

(b) Now suppose that Yn ∼ Pois(n) and we define Xn = (Yn − n)/ √ n . Show that KXn (t) → (t^2)/2 as n → ∞. What does this tell us about the distribution of Yn for large values of n?


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