PMF function: joint pmf question For the PMF of $X$ 
$$P(X = x) = (0.5)^x \;\text{ for }\;x\in\mathbb{Z}^+,$$
would the PMF of $Y= X-1$ just be
$$P(Y = x) = (0.5)^{x-1}.$$

$$
\Pr(Y=x) = \Pr(X-1=x) = \Pr(X=x+1) = 0.5^{x+1}.
$$
