The Delannoy Category


Snyder, N. (2024). The Delannoy Category. Perimeter Institute for Theoretical Physics.


Snyder, Noah. The Delannoy Category. Perimeter Institute for Theoretical Physics, Mar. 14, 2024,


          @misc{ scivideos_PIRSA:24030112,
            doi = {10.48660/24030112},
            url = {},
            author = {Snyder, Noah},
            keywords = {Mathematical physics},
            language = {en},
            title = {The Delannoy Category},
            publisher = {Perimeter Institute for Theoretical Physics},
            year = {2024},
            month = {mar},
            note = {PIRSA:24030112 see, \url{}}

Noah Snyder Indiana University

Source Repository PIRSA


The prototypical example of a symmetric tensor category is Rep(G) for G a compact group. The other main source of symmetric tensor categories are Deligne's interpolation categories (S_t, GL_t, and O_t) which extend a family of group representation categories defined at positive integers to all complex numbers. Harman and Snowden have developed a theory of symmetric tensor categories coming from Oligomorphic permutation groups together with a well-behaved measure on G-sets. This includes Deligne's S_t which comes from the infinite symmetric group together with a measure where the usual permutation G-set has volume t. The simplest new example of their theory is the group of order-preserving bijections of the real line with the measure given by Euler characteristic. In joint work with Harman and Snowden (arXiv:2211.15392) we give a detailed description of this new symmetric tensor category, which has a number of novel properties. We call this category the Delannoy category because the dimensions of Hom spaces are given by Delannoy numbers. In this talk I'll outline our main results, including the classification of simple objects, the tensor product rules, and a combinatorial model for the category using Delannoy paths.


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