watching systems of triangular graphs

a watching system in a graph $g=(v, e)$ is a set $w={omega_{1}, omega_{2}, cdots, omega_{k}}$, where $omega_{i}=(v_{i}, z_{i}), v_{i}in v$ and $z_{i}$ is a subset of closed neighborhood of $v_{i}$ such that the sets $l_{w}(v)={omega_{i}: vin omega_{i}}$ are non-empty and distinct, for any $vin v$. in this paper, we study the watching systems of line graph $k_{n}$ which is called triangular graph and denoted by $t(n)$. the minimum size of a watching system of $g$ is denoted by $omega(g)$. we show that $omega(t(n))=lceilfrac{2n}{3}rceil$.