Group Cohomology, SS 2019
Prof. Dr. C. Löh
/
D. Fauser
/
J. Witzig
News

The lecture notes are updated (version of June 16).
The (straightforward) definition of commutator length is already
included (because it is needed on Sheet 8).

The new exercise sheet is online: Sheet 8
(sheet of June 17, submission before June 24, 10:00).

June 20 is a holiday; the corresponding exercise class is moved to:
Wednesday, June 19, 1416, H38.

The new etudes are online: Etudes 7
(sheet of June 13, no submission).

The new exercise sheet is online: Sheet 7
(sheet of June 10, submission before June 17, 10:00).

Organisational matters

If you plan to write a bachelor thesis under my supervision in WS 2019/20 (in
Topology/Geometry), you should participate in a seminar in the Global Analysis
and Geometry group before WS 2019/20.
Group Cohomology
Group cohomology is an invariant that connects algebraic and geometric
properties of groups in several ways. For example, group cohomology
admits descriptions in terms of homological algebra and also in terms
of topology. Different choices of coefficients for group cohomology
leads to different invariance properties, whence to different types
of applications.
Group cohomology naturally comes up in algebra, topology, and geometry.
For example, group cohomology allows to

generalise the Hilbert 90 theorem in Galois theory,

classify group extensions with given Abelian kernel,

generalise the classical grouptheoretic transfer,

generalise finiteness properties of groups
(such as finiteness, finite generation, finite presentability, ...),

characterise the class of amenable groups (which
are important in largescale geometry),

study which finite groups admit free actions on spheres,

characterise which groups admit nontrivial quasimorphisms,

prove that certain groups admit significantly different
dynamical systems,

prove rigidity results in topology and geometry

...
In this course, we will introduce the basics of group homology and
cohomology, starting with elementary descriptions and calculations.
Depending on the background of the audience, we will then either
focus on a more algebraic perspective or on a more topological one.
If all participants agree, this course can be held in German; solutions to the
exercises can be handed in in German or English.
Lecture notes:
pdf.
Topics covered so far:

Literature
 Introduction

What is group cohomology?

Why group cohomology?

Overview of this course

The basic view

Foundations: The group ring
[The group ring,
Modules over the group ring,
The domain categories for group (co)homology]

The basic definition of group (co)homology
[The simplicial and the bar resolution,
Group (co)homology]

Degree 0: (Co)Invariants

Degree 1: Abelianisations and homomorphisms
[Homology in degree 1: Abelianisation,
Cohomology in degree 1: Homomorphisms,
Application: Hilbert 90]

Degree 2: Presentations and extensions
[Homology in degree 2: Hopf's formula,
Cohomology in degree 2: Extensions]

Changing the resolution
[Projective resolutions,
The fundamental theorem of group (co)homology,
Example: Finite cyclic groups,
Example: Free groups]

(Co)Homology and subgroups
[Restriction and (co)induction,
The Shapiro lemma,
Transfer]

The geometric view

Foundations: Geometric group theory
[QuasiIsometry, Amenability]

Uniformly finite homology
[Uniformly finite homology of spaces,
Uniformly finite homology of groups,
Application: Ponzi schemes and amenability]

Bounded cohomology
[Bounded cohomology of groups,
Application: A characterisation of amenability,
Application: Quasimorphisms]

Appendix
Amalgamated free products
comments:

(02.05.)
When defining the bar resolution in the lecture, I wrote [g_2...g_{n}]
as last summand in the definition of the boundary operator. It should have
been [g_1...g_{n1}] (as in the lecture notes).

(02.05.)
The computation of H_1 today was unnecessarily obfuscated; in the lecture
notes, I provide a more transparent, algebraic, argument (the geometric
observation is still helpful, but now exiled into a separate remark).

(02.05.)
Diagram on p. 19 (computation of the bar cocomplex): In the lecture,
I wrote "g^{1} a" instead of "g a"; as we do not have to flip between
left and right, "g a" is better (but both descriptions lead to the
same object because in the category of \Zmodules the two descriptions
are isomorphic).

(20.05)
Proof of Theorem 1.6.15: proof of step 1: It is easy to reduce to
the case of 1 + p^{n1} (see the diagram in the lecture notes; the
right arrow is not the identity; thus, these extensions are not
equivalent, but they are "isomorphic").
When discussing this during the lecture, I was confused about
something that is not relevant to this comparison.

(06.06.2019)
The floored chain in the proof of Theorem 2.2.15 is
corrected (in the lectures the scoping of the floor was wrong).

(06.06.2019) Corrected the averaging in the cohomological transfer (and some
other typos here and there).
Time/Location
Monday, 1012, M 102,
Thursday, 1012, M 104.
Exercise classes
group 1: Thursday, 1214, M 102
group 2: Friday, 8:3010, H 32

The exercise classes start in the second week; in this first session, some
basics material will be discussed (as on the
sheet Etudes 0).

Organisational matters
Exercise sheets
Solutions can be submitted in English or German and in teams of up to two people.
Please do not forget to add your name to all your submissions!
Sheet 1,

of April 29, 2019,

submission before May 6, 2019 (10:00)

will be discussed in the exercise classes on May 9/10

Sheet 2,

of May 6, 2019,

submission before May 13, 2019 (10:00)

will be discussed in the exercise classes on May 16/17

Sheet 3,

of May 13, 2019,

submission before May 20, 2019 (10:00)

will be discussed in the exercise classes on May 23/24

Sheet 4,

of May 20, 2019,

submission before May 27, 2019 (10:00)

will be discussed in the exercise classes on May 29(!)/31

Sheet 5,

of May 27, 2019,

submission before June 3, 2019 (10:00)

will be discussed in the exercise classes on June 6/7

Sheet 6,

of June 3, 2019,

submission before June 11(!), 2019 (10:00)

will be discussed in the exercise classes on June 13/14

Sheet 7,

of June 10, 2019,

submission before June 17, 2019 (10:00)

will be discussed in the exercise classes on June 19/21

Sheet 8,

of June 17, 2019,

submission before June 24, 2019 (10:00)

will be discussed in the exercise classes on June 27/28

Etudes
These etudes help to train elementary techniques and
terminology. These problems should ideally be easy enough to be solved
within a few minutes. Solutions are not to be submitted and will not
be graded.
Sheet 0,

of April 25, 2019,

no submission,

will be discussed in the exercise classes on May 2/3.

Sheet 1,

of May 2, 2019

no submission


Sheet 2,

of May 9, 2019

no submission


Sheet 3,

of May 16, 2019

no submission


Sheet 4,

of May 23, 2019

no submission


Sheet 5,

of May 30, 2019

no submission


Sheet 6,

of June 6, 2019

no submission


Sheet 7,

of June 13, 2019

no submission


Literature
This course will not follow a single book. Therefore, you should
individually compose your own favourite selection of books.
A list of suitable books can be found in the lecture notes.
Prerequisites
All participants should have a firm background in Analysis I/II
(in particular, basic point set topology, e.g., as in
Analysis II in WS 2011/12),
in Linear Algebra I/II, and basic knowledge in group theory
(as covered in the lectures on Algebra).
Knowledge about manifolds as in Analysis IV is not necessary, but helpful.
Knowledge about basic homological algebra (as in the last two weeks
of
Kommutative Algebra)
is not necessary, but helpful.
Knowledge on algebraic topology (as in the
course
in WS 18/19) is not necessary, but helpful.
Exams
Please read the
information on organisation and formalities of this course.
Last change: June 16, 2019