A survey of reachability trees of unbounded Petri nets

In recent years both industry and academia have paid much attention to the theory and applications of Petri nets. Reachability is a basic property of a Petri net, and many properties can be analyzed via it. However, analyzing the reachability problem of unbounded Petri nets by finite reachability trees has been an open problem since the inception of Petri nets. Researchers began to study the problem of reachability trees over 40 years ago. However, they made only limited progress over the last 20 years due to its complexity and difficulty. We present an overview of some important contributions toward its solution. The focuses are on four novel finite reachability trees: finite reachability tree (FRT), augmented reachability tree (ART), modified reachability tree (MRT) and new modified reachailbity tree (NMRT). The paper concludes with a discussion of directions for future research of the reachability problem of unbounded Petri nets.