  An $r$-identifying code in a graph $G = (V,E)$ is a subset $C \subseteq V$
such that for each $u \in V$ the intersection of $C$ and the ball of radius $r$
centered at $u$ is non-empty and unique. Previously, $r$-identifying codes have
been studied in various grids. In particular, it has been shown that there
exists a 2-identifying code in the hexagonal grid with density 4/19 and that
there are no 2-identifying codes with density smaller than 2/11. Recently, the
lower bound has been improved to 1/5 by Martin and Stanton (2010). In this
paper, we prove that the 2-identifying code with density 4/19 is optimal, i.e.
that there does not exist a 2-identifying code in the hexagonal grid with
smaller density.
