====== 확률생성함수 (Probability Generating Function : PGF) =======
===== 정의 =====
 [[계승적률생성함수]]([[Factorial Moment Generating Function]])이라고도 함.

  * $$ G(t) = E(t^{x}) $$

  * $$ G^{(t)}(1) = \left[ \frac{d^{(k)} G(t)}{dt^{k}} \right]_{t=1} = E[ \ X(X - 1) \cdots (X - k + 1) \ ] $$