1. Analysis and control of discrete event systems represented by Petri nets;Ichikawa,1987
2. Properties of Conflict-Free and Persistent Petri nets;Landweber;J. ACM,1978
3. Petri Net Theory and the Modeling of Systems;Peterson,1981
4. Ref. [3], pp. 97, 98.
5. Petri Nets;Reisig,1982