Prime-denominator large sieve

When $N\le P$,An arc of length $1/P$ contains at most three points of each grid of denominator $p\le 2P$. The Chebyshev estimate gives $O(P/\log P)$ such primes. Apply the local-multiplicity large sieve. Using only the separation of distinct reduced fractions gives the weaker $O(P^2)\sum |a_n{|}^2$.