Oberseminar Algebra und Geometrie

Winter term 2023/24: Central Simple Algebras

Chaired by I. Halupczok, H. Kammeyer and B. Klopsch.

Organised by I. Halupczok

All talks take place on Fridays at 12:30 pm in 25.22.03.73.

If you want to participate but did not yet receive any mails related to the seminar, please get in touch with I. Halupczok so that your address is added to the mailing list.

Infos für Studierende

Das Oberseminar richtet sich an alle, die sich für das Thema interessieren, ist aber tendenziell eher für fortgeschrittene Studierende geeignet (ab Master). Besonders empfohlen wird die Teilnahme an Oberseminaren, wenn Sie sich vorstellen können zu promovieren.

Wenn Sie intessiert sind, können sich sich entweder einfach so ins Seminar reinsetzen (ohne Anmeldung), oder Sie können auch selbst einen Vortag halten (und sich das Seminar fürs Studium anrechnen lassen). Wenn Sie einen Vortrag halten wollen und/oder wenn Sie unsicher sind, ob Sie die nötigen Voraussetzungen haben, kontaktieren Sie einfach den/die Organisator:in. Er/sie kann Sie dann ggf. auch beraten, welcher Vortrag für Sie geeignet wäre.

Die Vorträge in diesem Oberseminar sind auf englisch. Üblicherweise nehmen an Oberseminaren auch viele Doktorand:innen, Postdocs und Professor:innen teil. Als Studi erhalten Sie das Privileg, sich einen besonders einfachen Vortrag aussuchen zu dürfen. Wahrscheinlich werden Sie von den anderen Vorträgen nicht alles verstehen; das passiert aber auch den fortgeschritteneren Teilnehmer:innen, und auch, wenn man nicht alles verstanden hat, hat man doch am Ende oft einen interessanten Einblick in ein neues Thema erhalten. Nebenbei erhalten Sie außerdem einen Einblick in die Arbeit von Mathematiker:innen.

Aims and Content - a short description

A field whose multiplication is not necessarily commutative is called a division algebra. There is a beautiful and classical theory of division algebras which are finite extensions of fields. It turns out that it is often more convenient to consider a slightly more general notion, namely central simple algebras (over a field $F$). In this seminar, we will see, among others:
  • Out of the central simple algebras over a field $F$, one can construct a group called the Brauer Group.
  • The only finite division algebras are fields. (This is a theorem by Wedderburn.)
  • Concrete constructions of various examples, in particular of quaternion algebras.
  • We will recall the notion of local fields, and as the final goal, we will describe all central simple algebras over local fields. (In other words, we will determine their Brauer groups.)

Schedule

Please volunteer for talks that have not yet been asigned to anyone!
13.10.23 1: Warm-up: Quaternion Algebras (Giada)

[Pie 1.6], [MR 2.1, up to Lemma 2.1.6]
Introduce quaternion algebras $A = (\frac{a,b}{F})$; mention that they are (the simplest non-trivial examples of) central simple algebras. If $A$ is a division algebra, then there is a quadratic field extension $E \subset A$ of $F$ [MR, Lemma 2.1.6].

20.10.23 2: The Brauer-group (Doris)

[MR 2.8]
Introduce central simple algebras and the Brauer-group

27.10.23 3: Wedderburn's structure Theorem (Daniel)

[MR 2.9]
Prove Wedderburn's structure Theorem [MR Thm 2.9.6] and the Skolem Noether Theorem [MR Thm 2.9.8] [Pie 12.6]. As an example application, show [MR, Thm 2.1.7] and [MR, Thm 2.1.8].

3.11.23 4: Isomorphisms of Quaternion Algebras (Marcelo)

[MR 2.3] [Pie 1.7]
Express when two quaternion algebras are isomorphic in terms of quadratic forms [MR Cor. 2.3.5] [Pie 1.7 Prop] As an example, classify all quaternion algebras over ℝ (see also [MR Thm 2.5.1])

10.11.23 5: Maximal subfields (Immi)

[Pie 13.1-13.3]; but restrict to the case where $A$ is a central simple algebra
[Pie 13.1]: Introduce the notion of (strictly) maximal subfield of a division algebra. Introduce the notion of degree (see [Pie 13.1], after Cor. a). Prove that in division algebra, maximal subfields are strictly maximal [Pie 13.1 Cor b]. Skip [Pie 13.1 Thm] (this has been done in a previous talk), but deduce that the Brauer group of $\mathbb{R}$ is $\mathbb{Z}/2\mathbb{Z}$. [Pie 13.2]: Introduce the notion of splitting field. Introduce the relative Brauer group $\mathbf{B}(E/F)$. Show that if $E$ is a maximal subfield of a division algebra $A$, then $A$ splits over $A$ [Pie 13.3 Theorem]

17.11.23 6: (a) Separability; (b) Infiniteness (Holger)

Part a: [Pie 13.5]
Prove that the Brauer group $\mathbf{B}(F)$ is the union of the $\mathbf{B}(E/F)$ for all Galois extensions $E$ of $F$ [Pie 13.5 Corollary]. (Or, if the proof is boring, just sketch it)
Part b: [Pie 13.6]
Let's have some fun now: Prove Wedderburns Finite Division Algebra Theorem [Pie 13.6] (stating that there are only fields)

24.11.23 7: The Brauer Group is a Galois Cohomology Group (Martina)

[Pie 14.1-14.3]
Sketch the proof that $\mathbf{B}(E/F)$ is isomorphic to $H^2(\operatorname{Gal}(E/F), E^\times)$ [Pie 14.2 Theorem]. In particular, introduce the crossed product $[(E, G, \Phi)]$ [Pie 14.1 Corollary].

1.12.23 8: Cyclic Algebras (Benjamin)

[Pie 15.1 15.4]
Introduce cyclic algebras and the notation $(E, \sigma, a)$ ([Pie 15.1, after Prop. a]); prove at least [Pie 15.1 Cor. a (i)] and [Pie 15.1 Prop b]
Present the example construction $(\frac{a,b}{F,\zeta})$ from [Pie 15.4] (if there is time left), as a generalization of quaternion algebras.

8.12.23 9: Valuations on division algebras (Olga)

[Pie 17.1-17.3]
Introduce the following notions for division algebras: valuations [Pie 17.1] (maybe give the example from Exercise 3 (c)); non-archimedean valuations [Pie 17.2]; valuation rings [Pie 17.3]
All this serves as an excuse to recall the corresponding material on fields.

15.12.23 10: Completions (Margherita)

[Pie 17.4, 17.5]
Introduce the valuation topology, completion, and Hensel's lemma [Pie 17.4]. Introduce local fields [e.g. Pie 17.5]

22.12.23 (no talk)
12.1.24 11: (a) Quaternion algebras over non-archimedean local fields and (b) extending valuations (Florian)

Part a: [MR 2.6]
Classify quaternion algebras over non-archimedean local fields [MR 2.6]. (You can also restrict to $\mathbb{Q}_p$, if you want. Don't do all details if it would take too much time.)
Part b: [Pie 17.6]
Prove that the valuation on a local field $F$ extends uniquely to any finite dimensional division algebra over $F$ [Pie 17.6 Prop].

19.1.24 12: Ramification (Immi)

[Pie 17.7, 17.8]
Introduce the notion of ramification and prove the formulas from [Pie 17.7 Prop]. Describe unramified extensions of non-archimedean local fields [Pie 17.8 Proposition].

26.1.24 13: The Brauer Group of Local Fields (Benjamin)

[Pie 17.9, 17.10]
Recall why we are interested in the norm factor group $F^\times/\mathrm{N}(K^\times)$ (namely, because of [Pie 15.1 Prop b]) and determine that group for an unramified extension $K$ of a local field $F$ [Pie 17.9 Prop].
Put things together to determine the Brauer group of non-archimedean local fields [Pie 17.10 Thm].

2.2.24 Discussion of topics for the next semester (all of us)

Literature

[MR] C. Maclachlan, A. Reid: The Arithmetic of Hyperbolic 3-Manifolds
[Pie] R. Pierce: Associative Algebras

Archive

SS 2023: Knot theory and quandles

WS 2022/23: Combinatorics and Commutative Algebra

SS 2022: Discrete Groups, Expanding Graphs and Invariant Measures

WS 2021/22: Superrigidity

SS 2021: Group cohomology

SS 2020 and WS 20/21: cancelled due to pandemic

WS 2019/20: Intersection theory

SS 2019: Knots and primes

WS 2018/19: The Grothendieck group of varieties and stacks

SS 2018: Arithmetic Groups - Basics and Selected Applications

WS 2017/18: Algebraic K-theory

SS 2017: Berkovich spaces

WS 16/17: Resolution of singularities and alterations

SS 2016: Modular Representation Theory

WS 15/16: The Milnor Conjectures

SS 2015: Rationality

WS 14/15: Essential Dimension

SS 2014: Varieties of Representations



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