Probability Generating Function of One

Theorem
Let $X$ be a discrete random variable whose codomain, $\Omega_X$, is a subset of the natural numbers $\N$.

Let $p_X$ be the probability mass function for $X$.

Let $\Pi_X \left({s}\right)$ be the probability generating function for $X$.

Then:
 * $\Pi_X \left({1}\right) = 1$