Tag - Algebraic groups

Ian Tan: Tensor decompositions with applications to LU and SLOCC equivalence of multipartite pure states

30th September, 2024

We introduce a broad lemma, one consequence of which is the higher order singular value decomposition (HOSVD) of tensors defined by DeLathauwer, DeMoor and Vandewalle (2000). By an analogous application of the lemma, we find a complex orthogonal version of the HOSVD. Kraus' (2010) algorithm used the HOSVD to compute normal forms of almost all n-qubit pure states under the action of the local unitary group. Taking advantage of the double cover SL2(ℂ) × SL2(ℂ) → SO4(ℂ), we produce similar algorithms (distinguished by the parity of n) that compute normal forms for almost all n-qubit pure states under the action of the SLOCC group.

Artem Chernikov: Recognizing groups in Erdős geometry and model theory

15th March, 2024

Erdős-style geometry is concerned with difficult questions about simple geometric objects, such as counting incidences between finite sets of points, lines, etc. These questions can be viewed as asking for the possible number of intersections of a given algebraic variety with large finite grids of points. An influential theorem of Elekes and Szabó indicates that such intersections have maximal size only for varieties that are closely connected to algebraic groups. Techniques from model theory - variants of Hrushovski’s group configuration and of Zilber’s trichotomy principle - are very useful in recognizing these groups, and led to far reaching generalizations of Elekes-Szabó in the last decade. I will overview some of the recent developments in this area, in particular explaining how all of this is not just about polynomials and works for definable sets in o-minimal structures.

Nate Harman: Oligomorphic and Linearly Oligomorphic groups

11th September, 2023

Oligomorphic groups are a class of groups arising in model theory. I will discuss where these groups come from, highlight some of their interesting properties, and explain why I (a non-model-theorist) am interested in them. Then I will introduce a new notion of linearly oligomorphic groups, and give new examples of infinite-dimensional algebraic groups.

Michael Wibmer: Expansive endomorphisms of profinite groups

25th May, 2023

Étale algebraic groups over a field k are equivalent to finite groups with a continuous action of the absolute Galois group of k. The difference version of this well-known result asserts that étale difference algebraic groups over a difference field k (i.e., a field equipped with an endomorphism) are equivalent to profinite groups equipped with an expansive endomorphism and a certain compatible difference Galois action. In any case, understanding the structure of expansive endomorphisms of profinite groups seems a worthwhile endeavour and that's what this talk is about.

Michael Wibmer: Difference algebraic groups

25th May, 2023

Difference algebraic groups are a generalization of algebraic groups. Instead of just algebraic equations, one allows difference algebraic equations as the defining equations. Here one can think of a difference equation as a discrete version of a differential equation. Besides their intrinsic beauty, one of the main motivations for studying difference algebraic groups is that they occur as Galois groups in certain Galois theories. This talk will be an introduction to difference algebraic groups.

Ting Xue: Introduction to linear algebraic groups

20th June, 2022

Algebraic groups are fundamental objects in representation theory and number theory. The course will discuss the structure theory of linear algebraic groups over algebraically closed fields. Topics include tori, parabolic subgroups and Borel subgroups, Lie algebras, root data, Weyl group, and classification of simple algebraic groups. If time permits, we will briefly discuss relation between compact Lie groups and algebraic groups.

Camila Sehnem: Equilibrium on Toeplitz extensions of higher-dimensional non-commutative tori

1st April, 2022

The C∗-algebra generated by the left-regular representation of ℕn twisted by a 2-cocycle is a Toeplitz extension of an n-dimensional non-commutative torus, on which each vector r ∈ [0,∞)n determines a one-parameter subgroup of the gauge action. I will report on joint work with Z. Afsar, J. Ramagge and M. Laca, in which we show that the equilibrium states of the resulting C∗-dynamical system are parametrized by tracial states of the non-commutative torus corresponding to the restriction of the cocycle to the vanishing coordinates of r. These in turn correspond to probability measures on a classical torus whose dimension depends on a certain degeneracy index of the restricted cocycle. Our results generalize the phase transition on the Toeplitz non-commutative tori used as building blocks in work of Brownlowe, Hawkins and Sims, and of Afsar, an Huef, Raeburn and Sims.

James Parkinson: Automorphisms and opposition in spherical buildings

4th March, 2022

The geometry of elements fixed by an automorphism of a spherical building is a rich and well-studied object, intimately connected to the theory of Galois descent in buildings. In recent years, a complementary theory has emerged investigating the geometry of elements mapped onto opposite elements by a given automorphism. In this talk we will give an overview of this theory.