All seminars take place in Lecture Room M2 (Department of Mathematics) in hybrid format.
Email hopega@math.auth.gr if you like to join the mailing list and receive seminar announcements.
(every Monday, 10 am - 1 pm, M2/online)
Organizers: Ch. Psaroudakis, G. Raptis
The Algebra & Topology Seminar will feature research talks by invited speakers and members of the group, as well as a seminar series devoted to the theory of stable infinity-categories and its applications.
05/10 10-12 pm V. Patzalis (AUTh) Seminar on stable ∞-categories: Introduction and Overview (program)
12/10 10-12 pm V. Patzalis (AUTh) Stable ∞-categories (1): Stability
19/10 10-12 pm Ch. Psaroudakis (AUTh) Stable ∞-categories (2): Exactness
12 - 1 pm A. Hayash (AUTh) What is Cartier duality?
26/10 No seminar (Holiday)
02/11 10-12 pm N. Müller (University of Mainz/AUTh) Stable ∞-categories (3): Spectra and Stabilization I [slides]
12 - 1 pm G. Raptis (AUTh) Acyclic maps I: General properties and characterizations
09/11 10-12 pm G. Raptis (AUTh) Stable ∞-categories (4): Spectra and Stabilization II
12 - 1 pm I. Goodbody (University of Warwick) Additive Invariants of Finite-dimensional DG-algebras
Abstract. Additive invariants of smooth finite-dimensional DG-algebras behave similarly to those of finite-dimensional algebras: Orlov showed their Grothendieck groups are free and of finite (but unknown!) rank and their Hochschild homology vanishes in non-zero degrees. In this talk I'll explain some results which hold in the non-smooth case and identify this rank for some natural classes of finite-dimensional DG-algebras.
16/11 10-12 pm Ch. Psaroudakis (AUTh) Stable ∞-categories (5): Localizations
12 - 1 pm G. Raptis (AUTh) Acyclic maps II: The plus construction
23/11 10-12 pm tba
12 - 1 pm T. Moulinos (Université de Paris-Saclay) Filtrations, formal groups and synthetic spectra
Abstract. Filtrations are ubiquitous in mathematics. In this talk, I will begin by explaining a modern, geometric perspective on the Rees equivalence: a filtered complex —more generally a filtered spectrum— is a (derived) quasi-coherent sheaf on the stack obtained by quotienting the affine line by the action of the multiplicative group. As a central example, I will discuss Hochschild homology with its HKR filtration; I will explain how this can be understood through the geometry of the “filtered circle”.
I will then switch gears and talk about the theory of synthetic spectra. Roughly, these are filtered spectra which are modules over the sphere spectrum equipped with its Adams filtration. I will describe how this relates to the moduli stack of formal groups, and I will conclude by describing a lift of the filtered circle to this setting.
30/11 10-12 pm tba
12 - 1 pm K. Golfis (NKU Athens) tba
07/12
21/12 10-11 am L. Moser (University of Hamburg) tba
11 - 1 pm O. Giatagantzidis (University of Stuttgart) tba
11/01 10-12 pm
12 - 1 pm M. Palmer (Mathematical Institute of the Romanian Academy) tba
(every Friday, 3 pm - 6 pm, M2/online)
Organizer: G. Raptis
The Topology & Geometry Seminar will feature research talks by invited speakers and members of the group, as well as a seminar series devoted to the interactions between manifold topology and algebraic K-theory.
09/10 3-6 pm G. Raptis (AUTh) SK-groups and SK-invariants
16/10 3-6 pm M. Maridakis (AUTh) Handle presentations and applications [postponed]
23/10 3-5 pm M. Maridakis (AUTh) Handle presentations and applications
5-6 pm C. Campagnolo (UA Madrid) A vanishing criterion for bounded cohomology
Abstract. Bounded cohomology is an invariant of groups and spaces first introduced by Johnson in the 70s in the context of Banach algebras, and then mightily developed by Gromov in the 80s. Since then, it has been widely studied and proved useful in several areas, from geometry and topology to group theory. Its vanishing and non-vanishing depending on the family of coefficients under consideration allows one to characterize, for instance, finite, amenable, and Gromov hyperbolic groups. I will explain a purely algebraic vanishing criterion obtained in joint work with F. Fournier-Facio, Y. Lodha and M. Moraschini for a big family of coefficients. It applies to many different types of groups, of geometric, topological and algebraic origin.
30/10 3-5 pm I. Gkeneralis (AUTh) Surgery theory I: From Cell Attachments to Intersection Forms
5-6 pm V. Patzalis (AUTh) Bounded products in the heart of Banach spaces
Abstract. Bounded cohomology is an invariant of spaces and groups introduced by Johnson in the 1970s and further developed by Gromov in the 1980s. Although it is a variant of ordinary singular and group cohomology, its behavior can be quite different; for instance, bounded cohomology does not commute with products in general. In this talk, we will discuss a refinement of bounded cohomology taking values in the heart associated to the (quasi-abelian) category of Banach spaces, with better formal properties. In particular, this refined version commutes with products, allowing us to formulate the Serre spectral sequence of Li, Moraschini and Raptis without additional assumptions on the fiber and to establish a Milnor-type short exact sequence. This is joint work in progress with G. Raptis.
06/11 3-5 pm I. Gkeneralis (AUTh) Surgery theory II: Surgery Obstructions, L-grours and Rigidity
5-6 pm J. Semikina (University of Lille) Scissors congruence K-theory for equivariant manifolds
Abstract. The generalized Hibert's third problem asks about the invariants preserved under the scissors congruence operation: given a polytope P in ℝn, one can cut P into a finite number of smaller polytopes and reassemble these to form Q. Kreck, Neumann and Ossa introduced and studied an analogous notion of cut-and-paste relation for manifolds called the SK-equivalence ("schneiden und kleben" is German for "cut and paste"). We introduce a scissors congruence K-theory spectrum which lifts the equivariant SK-groups for compact G-manifolds with boundary, and we show that on π0 this is the source of a spectrum level lift of the Burnside ring valued equivariant Euler characteristic of a compact G-manifold. We also discuss a Mackey functor structure on equivariant SK-groups, which is a shadow of a higher genuine equivariant structure.
13/11 3-4 pm G. Raptis (AUTh) Additivity of the signature
4-6 pm G. Raptis (AUTh) SK-groups and surgery
20/11
27/11
04/12
11/12
18/12
15/01
Workshop: Algebra & Geometry Days in Thessaloniki, 14-15 December 2026