Identifying code for directed graph

Identifying code is a concept for undirected graph and can be applied to target identification. Its ability of identification comes from a fact that different elements in a system may have different neighbors. In this paper we extend the concept to directed graph. Then the determination rules, the concept of optimal identifying code, the formulation with integer program, and the solving steps are all extended. In this way, the identifying code problem and the alarm placement problem in earlier literatures are linked together and solved together.