mardi 29 septembre |
Thierry Coulbois
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Free Groups:
- Words, reduced words, definition
- Some computations in free groups: cyclically-reduced normal form, no torsion, commutative-transitivity
- Ping-pong in SL2(Z), (see Johanna Mangahas Office Hours,chapter 5)
- Playing ping-pong in the Hyperbolic Plane with two loxodromic Möbius transformations
- Cayley graph of a free group (see Matt Clay's Office Hours pp 30-33)
- Serre's definition of a graph
- A group acting freely on a tree is free
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Some exercices |
mardi 6 octobre |
Thierry Coulbois |
Group acting on trees:
- A group acting freely on a tree (without inversions) is free: proof (I followed the proof of Dan Margalit Office Hours pp 50-54 , but warning there is an erratum and even with the erratum...)
- Graphs as metric spaces
- Classification of isometries of trees, axes of loxodromic isometries
- Parallel with the classification of isometries of the hyperbolic plane
- Fundamental group of graphs (see section 4.1 of Matt Clay's Office Hours)
- Subgroup of free groups as fundamental group of graphs (see section 4.2 and 4.3 of Matt Clay's Office Hours)
- Stallings foldings (see section 4.2 and 4.3 of Matt Clay's Office Hours)
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One more exercice |
mardi 13 octobre |
Thierry Coulbois |
- An example of subgroup of the free group through foldings (Taken from Matt Clay's Office hours, project 1, p 83)
- Coverings and subgroups of finite index
- Dynamics of free group automorphisms : (Matt Clay's Office hours, chapter 6 rather deals with the question of train-tracks, but you will see there Perron-Frobenius theorem)
- Iterating tribonacci on letters to get infinite words
- Expansion of free group automorphisms
- Cooper cancellation bound
- Iterating tribonacci on trees
- Twisting the action
- Trees as metric spaces
- Renormalization and convergence of the distances
- R-trees
- Gromov four points condition (See Moon Ducin's Office hours pp 180-181)
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Even more exercices
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mardi 20 octobre |
Thierry Coulbois |
Hyperbolic surfaces, the hyperbolic aspect of this course is close to Moon DUCHIN's Office Hours 9.3
- Regular 4g-gones in the hyperbolic plane
- The genus g surface with a hyperbolic structure (g≥2)
- Fundamental group of a topological space. See Algebraic Topology of W. MASSEY or online: A. HATCHER's Algebraic Topology, Chapter 1.
- paths, loops, homotopies
- Deformation-retract
- Seifert - van Kampen Theorem
- The fundamental group of the surface Sg
- Free products (again this is covered in details by A. HATCHER's Algebraic Topology, Chapter 1)
- Universal property
- Free products as groups of reduced words
- Free products with amlagamation as groups of reduced words
- Universal covers
- Tiling of the hyperbolic plane by regular 4g-gones
- The Cayley graph of the fundamental group of Sg inside the hyperbolic plane
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mardi 8 décembre |
Exam and its correction
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