  We consider the problem of approximating the arboricity of a graph $G=
(V,E)$, which we denote by $\mathsf{arb}(G)$, in sublinear time, where the
arboricity of a graph is the minimal number of forests required to cover its
edges. An algorithm for this problem may perform degree and neighbor queries,
and is allowed a small error probability. We design an algorithm that outputs
an estimate $\hat{\alpha}$, such that with probability $1-1/\textrm{poly}(n)$,
$\mathsf{arb}(G)/c\log^2 n \leq \hat{\alpha} \leq \mathsf{arb}(G)$, where
$n=|V|$ and $c$ is a constant. The expected query complexity and running time
of the algorithm are
  $O(n/\mathsf{arb}(G))\cdot \textrm{poly}(\log n)$, and this upper bound also
holds with high probability. %($\widetilde{O}(\cdot)$ is used to suppress
$\textrm{poly}(\log n)$ dependencies).
  This bound is optimal for such an approximation up to a $\textrm{poly}(\log
n)$ factor.
