Counting arcs in $\mathbb F_q^2$

An arc in $\mathbb F_q^2$ is a set $P \subset \mathbb F_q^2$ such that no three points of $P$ are collinear. We use the method of hypergraph containers to prove several counting results for arcs. Let $\mathcal A(q)$ denote the family of all arcs in $\mathbb F_q^2$. Our main result is the bound $|\mathcal A(q)| \leq 2^{(1+o(1))q}.$ This matches, up to the factor hidden in the $o(1)$ notation, the trivial lower bound that comes from considering all subsets of an arc of size $q$. We also give upper bounds for the number of arcs of a fixed (large) size. Let $k=q^t$ for some $t >2/3$, and let $\mathcal A(q,k)$ denote the family of all arcs in $\mathbb F_q^2$ with cardinality $k$. We prove that, for all $γ>0$ $|\mathcal A(q,k)| \leq \binom{(1+γ)q}{k}.$ This result improves a bound of Roche-Newton and Warren. A nearly matching lower bound $|\mathcal A(q,k)| \geq \binom{q}{k}$ follows by considering all subsets of size $k$ of an arc of size $q$.

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