String diagrams are pictorial representations for morphisms of symmetric monoidal categories. They constitute an intuitive and expressive graphical syntax, which has found application in a very diverse range of fields including concurrency theory, quantum computing, control theory, machine learning, linguistics, and digital circuits. Rewriting theory for string diagrams relies on a combinatorial interpretation as double-pushout rewriting of certain hypergraphs. As previously studied, there is a `tension' in this interpretation: in order to make it sound and complete, we either need to add structure on string diagrams (in particular, Frobenius algebra structure) or pose restrictions on double-pushout rewriting (resulting in 'convex' rewriting). From the string diagram viewpoint, imposing a full Frobenius structure may not always be natural or desirable in applications, which motivates our study of a weaker requirement: commutative monoid structure. In this work we characterise string diagram rewriting modulo commutative monoid equations, via a sound and complete interpretation in a suitable notion of double-pushout rewriting of hypergraphs.

@article{paperbot3449,
  title = {Rewriting for Symmetric Monoidal Categories with Commutative (Co)Monoid Structure},
  author = {Aleksandar Milosavljevic and Robin Piedeleu and Fabio Zanasi},
  journal = {Logical Methods in Computer Science},
  volume = {Volume 21, Issue 1},
  year = {2025},
  doi = {10.46298/lmcs-21(1:12)2025}
}