# Fall 2020 Seminars

**Time:**Monday, December 7, 2–3 PM (CST, UTC -6)

**Place:**Zoom

**Speaker:**Jason Parker (Brandon University)

**Title:**Isotropy Groups of Quasi-Equational Theories

**Slides:**here

**Abstract:**In [2], my PhD supervisors (Pieter Hofstra and Philip Scott) and I studied the new topos-theoretic phenomenon of isotropy (as introduced in [1]) in the context of single-sorted algebraic theories, and we gave a logical/syntactic characterization of the isotropy group of any such theory, thereby showing that it encodes a notion of inner automorphism or conjugation for the theory. In the present talk, I will summarize the results of my recent PhD thesis, in which I build on this earlier work by studying the isotropy groups of (multi-sorted) quasi-equational theories (also known as essentially algebraic, cartesian, or finite limit theories). In particular, I will show how to give a logical/syntactic characterization of the isotropy group of any such theory, and that it encodes a notion of inner automorphism or conjugation for the theory. I will also describe how I have used this characterization to exactly characterize the ‘inner automorphisms’ for several different examples of quasi-equational theories, most notably the theory of strict monoidal categories and the theory of presheaves valued in a category of models. In particular, the latter example provides a characterization of the (covariant) isotropy group of a category of set-valued presheaves, which had been an open question in the theory of categorical isotropy.

[1] J. Funk, P. Hofstra, B. Steinberg. Isotropy and crossed toposes. Theory and Applications of Categories 26, 660–709, 2012.

[2] P. Hofstra, J. Parker, P.J. Scott. Isotropy of algebraic theories. Electronic Notes in Theoretical Computer Science 341, 201–217, 2018.

**Time:**Monday, September 28, 2–3 PM (CDT, UTC -5)

**Place:**Zoom

**Speaker:**Bogdan Chornomaz

**Title:**Introduction to Möbius functions

**Slides:**here

**Abstract:**We will give a brief introduction to Möbius functions on lattices and discuss their basic properties. We will also prove that lattices with nonvanishing Möbius function are SSP and that geometric lattices have a nonvanishing (moreover, alternating) Möbius function.

**Time:**Monday, September 21, 2–3 PM (CDT, UTC -5)

**Place:**Zoom

**Speaker:**Bogdan Chornomaz

**Title:**SSP ?= RC

**Slides:**here

**Preprint:**here

**Abstract**

**:**We will talk about SSP ?= RC conjecture, which states that a finite lattice satisfies an analogue of Sauer-Shelah-Perles lemma iff it is relatively complemented. In particular, we discuss one strategy of attacking the conjecture using certain colored bipartite graphs. We will hit the wall by constructing a “counterexample” in the language of those graphs. Then we will try to lift it to the counterexample to the general conjecture using some “pumping” constructions and will hit the wall again.

**Time**: Monday, September 14, 2–3 PM (CDT, UTC -5)

**Place:**Zoom

**Speaker:**Adam Prenosil

**Title:**Four-valued logics of truth, non-falsity, and material equivalence

**Slides:**here

**Abstract:**the purpose of this talk is to demonstrate how to work with quasivarieties (universal Horn classes) in a relational signature which consists of more than one predicate symbol. While universal algebraists work with the binary equality predicate, and algebraic logicians work with the unary truth predicate, the four-valued Belnap–Dunn logic provides a natural setting where two unary predicates arise (namely the truth and non-falsity predicates), as well as an equality predicate. We show how to axiomatize the logic of truth and non-falsity, as well as the logic of truth and equality, determined by the four-valued algebraic semantics of Belnap and Dunn.

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