Precalculus: Concepts Through Functions, A Unit Circle Approach to Trigonometry (3rd Edition)

$P(7,0) = 1$
Use the formula $P(n, r) =\dfrac{n!}{(n-r)!}$: $P(7,0) = \dfrac{7!}{(7-0)!} = \frac{7!}{7!} = 1$. Note that when $r=0, P(n,r) = 1$.