In: Scientia Sinica Series A - Mathematical Physical Astronomical & Techincal Sciences, Vol. 31, No. 2, pages 216-224. February 1988.
Abstract: The concurrent systems modelled by Petri nets have been studied. A frozen token is a token that is unused in some place while a subpart of the net goes on working for ever. A procedure to find the frozen tokens is presented by virtue of the coverability graph of the net.