Sharpness in the k-nearest neighbours random geometric graph model

Let $S_{n,k}$ denote the random geometric graph obtained by placing points in a square box of area $n$ according to a Poisson process of intensity 1 and joining each point to its $k$ nearest neighbours. Balister, Bollob\'as, Sarkar and Walters conjectured that for every $0< \epsilon <1$ and all $n$ sufficiently large there exists $C=C(\epsilon)$ such that whenever the probability $S_{n,k}$ is connected is at least $\epsilon$ then the probability $S_{n,k+C}$ is connected is at least $1-\epsilon$. In this paper we prove this conjecture. As a corollary we prove that there is a constant $C'$ such that whenever $k=k(n)$ is a sequence of integers such that the probability $S_{n,k(n)}$ is connected tends to one as $n$ tends to infinity, then for any $s(n)$ with $s(n)=o(\log n)$, the probability that $S_{n,k(n)+C's\log \log n}$ is $s$-connected tends to one This proves another conjecture of Balister, Bollob\'as, Sarkar and Walters.