Limited codes associated with Petri nets

The purpose of this paper is to investigate the relationship between limited codes and Petri nets. The set M of all positive firing sequences which start from the positive initial marking μ of a Petri net and reach μ itself forms a pure monoid M whose base is a bifix code. Especially, the set of all elements in M which pass through only positive markings forms a submonoid N of M. Also N has a remarkable property that N is pure. Our main interest is in the base D of N. The family of pure monoids contains the family of very pure monoids, and the base of a very pure monoid is a circular code. Therefore, we can expect that D may be a limited code. In this paper, we examine "small" Petri nets and discuss under what conditions D is limited