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250137 VO Moduli of n points on the projective line (2021W)

3.00 ECTS (2.00 SWS), SPL 25 - Mathematik
Di 25.01. 17:00-18:30 Digital


Hinweis: Ihr Anmeldezeitpunkt innerhalb der Frist hat keine Auswirkungen auf die Platzvergabe (kein "first come, first served").


Sprache: Englisch


Termine (iCal) - nächster Termin ist mit N markiert

The course is currently scheduled (online via Zoom + recordings available afterwards) on

Tuesdays from 5 - 6,30 pm,

starting on October 5. As it seems that some interested people have conflicts at that time, we may change the day and/or the time, e.g. to Tuesday morning or early afternoon, or Monday afternoon. Please send an e-mail to herwig.hauser@univie.ac.at if you are interested in following the course but prefer a change of schedule.

Dienstag 05.10. 17:00 - 18:30 Digital
Dienstag 12.10. 17:00 - 18:30 Digital
Dienstag 19.10. 17:00 - 18:30 Digital
Dienstag 09.11. 17:00 - 18:30 Digital
Dienstag 16.11. 17:00 - 18:30 Digital
Dienstag 23.11. 17:00 - 18:30 Digital
Dienstag 30.11. 17:00 - 18:30 Digital
Dienstag 07.12. 17:00 - 18:30 Digital
Dienstag 14.12. 17:00 - 18:30 Digital
Dienstag 11.01. 17:00 - 18:30 Digital
Dienstag 18.01. 17:00 - 18:30 Digital


Ziele, Inhalte und Methode der Lehrveranstaltung

International Lecture Course Fall 2021:

"Moduli of n points on the projective line"

Outset: This course is designed for both students and (non-expert) researchers who wish to see how apparently remote mathematical theories come together and interlace. It will be given in digital form via Zoom from October 5th, 2021, until January 25, 2022.

Up to technical obstructions we plan to give the course on a transparent blackboard, the so-called lightboard, which allows the audience to simultaneously follow the hand-writing and the lecturer from the front.

The trailer for this course can be found on


The prospective schedule is Tuesdays, from 5 pm till 6.30. The link to the online transmission will be distributed in due time. If you are potentially interested, please send an e-mail to


or go to the course website


in order to receive updated information (you may unsubscribe at any moment).

Topic: The problem of constructing normal forms and moduli spaces for various geometric objects goes back (at least, and among many others) to the Italian geometers (Enriques, Chisini, Severi, ...). A highlight was reached in the 1960 and 70ies when Deligne, Mumford and Knudsen constructed the moduli space of stable curves of genus g. These spectacular works had a huge impact, though the techniques from algebraic geometry they applied were quite challenging.

In the course, we wish to offer a gentle and hopefully fascinating introduction to these results, restricting always to curves of genus

Contents: We start by discussing the concept of (coarse and fine) moduli spaces and universal families, providing also the philosophical background thereof: why is it natural to study such questions, and why the given axiomatic framework is the correct one? Once we have become familiar with these foundations (seeing many examples on the way), we will concentrate on n points in P1 and the action of PGL2 on them by Möbius transformations. This is part of classical projective geometry and very beautiful. As long as the n points are pairwise distinct, things are easy, and a moduli space is easily constructed. Things become tricky as the points start to move and thus become closer to each other until they collide and coalesce. What are the limiting configurations of the points one has to expect in this variation? This question has a long history –Grothendieck proposed a convincing answer: n-pointed stable curves. Then, Deligne, Mumford and Knudsen built a fascinating and multi-faceted theory for them, their famous compactification of M–(0,n).

We will take at the beginning a different approach by proposing an alternative version of limit. Namely, we embed the space of (PGL2-orbits of) n distinct points into a large projective space and then take limits therein via the Zariski-closure. That’s a one page construction of a compact space Xn. It opens the door to the theory of phylogenetic trees: they are certain finite graphs with leaves and inner vertices as a tree in a forest. Their combinatorial structure will become the guiding principle to design many proofs for our moduli spaces. Working with phylogenetic trees can be a very pleasing occupation: we will draw, glue, cut and compose these trees and thus get surprising constructions and insights.

At that point a miracle happens: When considering the above Zariski-closure Xn and the associated phylogenetic trees, the stable curves of Grothendieck, Deligne, Mumford, Knudsen pop up on their own. We don’t even have to define them – they are just
there. So the circle closes up, as we then get an isomorphism from Xn to the compactification of M-(0,n) . In this way our journey is now able to reprove many of the classical results in an easy going and appealing manner.

Art der Leistungskontrolle und erlaubte Hilfsmittel

Mindestanforderungen und Beurteilungsmaßstab

The course will presuppose little prerequisites. Basic courses (Bachelor level) in algebra, geometry, topology and graph theory suffice.



Zuordnung im Vorlesungsverzeichnis


Letzte Änderung: Mo 20.09.2021 12:49