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Iosif Pinelis
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In view of a result by Scott, there exist infinitely many coprime natural numbers $p,q$ such that $q$ is odd and $|p-\tfrac\pi2\,q|<1/q$. For such $p$ and $q$, letting $q\to\infty$, we have $\sin p=1-O(1/q^2)=1-O(1/p^2)$ and hence $\sin^p p\to1$. So, $f(1)=1$.

Iosif Pinelis
  • 127.9k
  • 8
  • 107
  • 229