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Theory of Statistics - George Mason University

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50 1 Probability <strong>Theory</strong><br />

where EX represents expectation wrt the distribution <strong>of</strong> X. Other generating<br />

functions for g(X) are defined similarly.<br />

In some cases <strong>of</strong> interest, the Borel function may just be a projection. For a<br />

random variable X consisting <strong>of</strong> two components, (X1, X2), either component<br />

is just a projection <strong>of</strong> X. In this case, we can factor the generating functions<br />

to correspond to the two components.<br />

If ϕX(t) is the CF <strong>of</strong> X, and if we decompose the vector t to be conformable<br />

to X = (X1, X2), then we have the CF <strong>of</strong> X1 as ϕX1(t1) = ϕX(t1, 0).<br />

Note that ϕX1(t1) is not (necessarily) the CF <strong>of</strong> the marginal distribution<br />

<strong>of</strong> X1. The expectation is taken with respect to the joint distribution.<br />

Following equation (1.30), we see immediately that X1 and X2 are independent<br />

iff<br />

ϕX(t) = ϕX1(t1)ϕX2(t2). (1.102)<br />

Cumulant-Generating Function<br />

The cumulant-generating function, defined in terms <strong>of</strong> the characteristic function,<br />

can be used to generate the cumulants if they exist.<br />

Definition 1.30 (cumulant-generating function)<br />

For the random variable X with characteristic function ϕ(t) the cumulantgenerating<br />

function is<br />

K(t) = log(ϕ(t)). (1.103)<br />

(The “K” in the notation for the cumulant-generating function is the Greek<br />

letter kappa.) The cumulant-generating function is <strong>of</strong>ten called the “second<br />

characteristic function”.<br />

The derivatives <strong>of</strong> the cumulant-generating function can be used to evaluate<br />

the cumulants, similarly to the use <strong>of</strong> the CF to generate the raw moments,<br />

as in equation (1.94).<br />

If Z = X + Y , given the random variables X and Y , we see that<br />

KZ(t) = KX(t) + KY (t). (1.104)<br />

The cumulant-generating function has useful properties for working with<br />

random variables <strong>of</strong> a form such as<br />

<br />

Yn = Xi − nµ / √ nσ,<br />

that appeared in the central limit theorem above. If X1, . . ., Xn are iid with<br />

cumulant-generating function KX(t), mean µ, and variance 0 < σ2 < ∞, then<br />

the cumulant-generating function <strong>of</strong> Yn is<br />

√ <br />

nµt t<br />

KYn(t) = − + nKX √nσ . (1.105)<br />

σ<br />

<strong>Theory</strong> <strong>of</strong> <strong>Statistics</strong> c○2000–2013 James E. Gentle

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