Serre's problem on the density of isotropic fibres in conic bundles

Abstract

Let $\pi:X\to \mathbb{P}^1_{\mathbb{Q}}$ be a non-singular conic bundle over $\mathbb{Q}$ having $n$ non-split fibres and denote by $N(\pi,B)$ the cardinality of the fibres of Weil height at most $B$ that possess a rational point. Serre showed in $1990$ that a direct application of the large sieve yields $$N(\pi,B)\ll B^2(\log B)^{-n/2}$$ and raised the problem of proving that this is the true order of magnitude of $N(\pi,B)$ under the necessary assumption that there exists at least one smooth fibre with a rational point. We solve this problem for all non-singular conic bundles of rank at most $3$. Our method comprises the use of Hooley neutralisers, estimating divisor sums over values of binary forms, and an application of the Rosser-Iwaniec sieve.