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Homotopy II (2020–2021)

Université de Paris. M2 Fundamental Mathematics (S2). lectures. 24 h. Video Notes Solved homework Exam (en) Exam (fr) Solved exam (fr)


The goal of this course is to introduce modern homotopy theory, its tools and applications. We will be particularly interested in two examples: chain complexes (see the previous Homology course) and topological spaces. We will present Quillen’s model categories, and we will explain the equivalence between topological spaces and simplicial sets. We will illustrate these methods with rational homotopy theory by showing that multiplicative structures of cochains encodes rational homotopy types of topological spaces.

 This is the page for the 2020–2021 lecture. See here for the 2021–2022 page.


It is recommended to have taken the courses Homology (by Emmanuel Wagner) and Homotopy I (by Bruno Vallette). It will be useful to have a certain familiarity with categorical language and with basic notions of algebraic topology and homological algebra.

Plan of the course

  1. Model categories.
  2. Chain complexes.
  3. Simplicial sets and topological spaces.
  4. Rational homotopy theory.

After this course


Lectures happened on Mondays 10:45–12:45 and Fridays 15:45–17:45. They began on January 11th and ended on February 19th. All lectures were online (via Zoom). The sessions were recorded and they are now available on YouTube.

The exam was on Friday, March 5th, 14:00–17:00, in room 207C of the Halle aux Farines. It was an open-book exam: students had access to printed and handwritten lecture notes. Electronic devices were forbidden.

The retake exam (rattrapage) was be on June 15th. It consisted in oral exams lasting around 30 minutes total per student. In-person sessions were in the morning between 9AM and 1PM, in the Sophie Germain building, room 1014. Online sessions for students that cannot be physically present in Paris at that time were in the afternoon.


Lecture 1

1/11/2021 10:45-12:45
Analogies between different homotopy theories: topological spaces, simplicial sets, chain complexes. Motivation for the definition of model categories. (Co)fibrations vs. injections/surjections. Definition of model categories. (Sections 1.1 and 1.2).

Lecture 2

1/15/2021 15:45-17:45
Definition of model categories and first examples (Section 1.3).

Lecture 3

1/18/2021 10:45-12:45
Definition and construction of the localization of a category with respect to a class of weak equivalence, definition of left homotopies, first properties, dual case of right homotopies (Sections 1.4.1 and 1.4.2).

Lecture 4

1/22/2021 15:45-17:45
End of Section 1.4: explicit description of the homotopy category of a model category, Whitehead theorem.

Lecture 5

1/25/2021 10:45-12:45
Section 1.5.1: The proof of the existence of the projective model structure on the category of bounded-below chain complexes.

Lecture 6

1/29/2021 15:45-17:45
Existence theorem for cofibration generated model categories (Section 1.5.2), Quillen adjunctions (beginning of Section 1.6).

Lecture 7

2/1/2021 10:45-12:45
Definition of Quillen equivalences and characterization, homotopy limits and colimits.

Lecture 8

2/5/2021 15:45-17:45
Reminders on simplicial sets and beginning of the proof of the existence of the Quillen model structure on the category of simplicial sets (Sections 2.1, 2.2, 2.3, and beginning of 2.4).

Lecture 9

2/8/2021 10:45-12:45
Continuation of the proof of the existence of the Quillen model structure on simplicial sets, anodynes extensions, simplicial homotopy groups.

Lecture 10

2/12/2021 15:45-17:45
End of the proof of the Quillen equivalence between simplicial sets and topological spaces. Left Bousfield localization and rational homotopy theory.

Lecture 11

2/15/2021 10:45-12:45
Transferred model structure on CDGAs and Sullivan algebras.

Lecture 12

2/19/2021 15:45-17:45
Equivalence between CDGAs and rational homotopy types through the PL forms. Applications of Sullivan's theory: models of spaces, dichotomy theorem, etc.


I have written lecture notes which are available here. I also gave a similar course last year; you can in particular find past exams on that webpage. Grégory Ginot gave a course in 2017–2018–2019 on the same subject. You can find on his page his lecture notes, as well as past exercise sheets and exams.

Works on homotopy theory:

  • William G. Dwyer et Jan Spaliński. “Homotopy theories and model categories”. In: Handbook of algebraic topology. Amsterdam: North-Holland, 1995, pp. 73–126. DOI:10.1016/B978-044481779-2/50003-1. MR1361887. Zbl:0869.55018. (Introduction to model categories.)
  • Yves Félix, Stephen Halperin et Jean-Claude Thomas. Rational Homotopy Theory. Graduate Texts in Mathematics 205. New York : Springer, 2001, p. xxxiv+535. ISBN: 0-387-95068-0. DOI: 10.1007/978-1-4613-0105-9. (Reference book on rational homotopy theory.)
  • Paul G. Goerss et John F. Jardine. Simplicial homotopy theory. Progress in Mathematics 174. Basel: Birkhäuser , 1999, pp. xvi+510. ISBN: 3-7643-6064-X. DOI:10.1007/978-3-0348-8707-6. MR1711612. Zbl:0949.55001. (Book on simplicial sets and their homotopical properties.)
  • Phillip Griffiths et John Morgan. Rational homotopy theory and differential forms. 2nd ed. Progress in Mathematics 16. New York: Springer, 2013. 224 p. ISBN: 978-1-4614-8467-7. DOI: 10.1007/978-1-4614-8468-4. (Notes on rational homotopy theory)
  • Kathryn Hess. “Rational homotopy theory: a brief introduction”. In: Interactions between homotopy theory and algebra. Contemp. Math. 436. Providence, RI: Amer. Math. Soc., 2007, pp. 175–202. DOI:10.1090/conm/436/08409. arXiv:math/0604626. MR2355774. Zbl:1128.55010. (Introduction to rational homotopy theory)
  • Mark Hovey. Model categories. Mathematical Surveys and Monographs 63. Providence, RI: American Mathematical Society, 1999, pp. xii+209. ISBN: 0-8218-1359-5. MR1650134. Zbl:0909.55001. (Book on model categories.)
  • Jacob Lurie. Higher topos theory. Annals of Mathematics Studies 170. Princeton, NJ: Princeton University Press, 2009, pp. xviii+925. ISBN: 978-0-691-14049-0. MR2522659. Zbl:1175.18001 (Very complete book on ∞-categories.)

Reminders on algebraic topology and homological algebra:

  • Glen E. Bredon. Topology and geometry. Graduate Texts in Mathematics 139. New York: Springer, 1993, pp. xiv+557. ISBN: 0-387-97926-3. DOI:10.1007/978-1-4757-6848-0. MR1224675. Zbl:0791.55001. ((Algebraic) topology and (differential) geometry textbook.)
  • Allen Hatcher. Algebraic topology. Cambridge: Cambridge University Press, 2002, pp. xii+544. ISBN: 0-521-79160-X. MR1867354. Zbl:1044.55001 (Algebraic topology textbook.)
  • Henri Paul de Saint-Gervais. Analysis Situs. (Lecture notes on algebraic topology, in French.)
  • Pierre Schapira. Categories and homological algebra. (Lecture notes on derived categories.)
  • Edwin Spanier. Algebraic topology. Berlin: Springer, 1995, pp. xiv+528. ISBN: 978-1-4684-9322-1. DOI:10.1007/978-1-4684-9322-1. MR210112. Zbl:0145.43303. (Algebraic topology textbook.)
  • Charles A. Weibel. An Introduction to homological algebra. Cambridge Studies in Advanced Mathematics 38. Cambridge: Cambridge University Press, 1994, pp. xiv+450. ISBN: 0-521-43500-5. MR1269324. Zbl:0797.18001. (Homological algebra textbook.)