,
Hugo Paquet
Creative Commons Attribution 4.0 International license
It is an established idea in concurrency theory that every Petri net admits an unfolding semantics. This is a denotational object that represents its domain of possible executions. Unfoldings play an important role in practical analysis and verification. This paper is concerned with the following well-known problem: while the unfolding resembles a universal construction in the category of Petri nets, it generally fails to satisfy the expected universal property. This is because the unfolding construction overlooks the net’s internal symmetries. There are two solutions: make these symmetries explicit to obtain a weak universal property (one that holds only "up to symmetry"); or break the symmetries by assigning individual identities to components of the net. We review these two solutions and establish, in each case, a universal unfolding of Petri nets to event structures. This paper demonstrates a 2-categorical approach to Petri net unfoldings. We show that each unfolding semantics determines a 2-categorical relative adjunction involving Petri nets and event structures. Viewed in this way, the above two constructions can be related formally via an appropriate morphism of adjunctions. We exhibit a 2-density property of event structures which implies that unfolding functors are essentially unique.
@InProceedings{lechenne_et_al:LIPIcs.FSCD.2026.23,
author = {Lechenne, Serge and Paquet, Hugo},
title = {{Universal Properties of Petri Net Unfoldings}},
booktitle = {11th International Conference on Formal Structures for Computation and Deduction (FSCD 2026)},
pages = {23:1--23:19},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {978-3-95977-433-8},
ISSN = {1868-8969},
year = {2026},
volume = {378},
editor = {Pfenning, Frank},
publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
address = {Dagstuhl, Germany},
URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.FSCD.2026.23},
URN = {urn:nbn:de:0030-drops-263734},
doi = {10.4230/LIPIcs.FSCD.2026.23},
annote = {Keywords: Petri nets, Event structures, Symmetry, Unfolding, 2-Categories}
}