Lecture 19: Introduction To Topology

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1 Chris Tralie, Duke University 3/24/2016

2 Announcements Group Assignment 2 Due Wednesday 3/30 First project milestone Friday 4/8/2016 Welcome to unit 3!

3 Table of Contents The Euler Characteristic Spherical Polytopes / Platonic Solids Fundamental Polygons, Tori Connected Sums, Genus

4 Graphs Review

5 Planar Graphs

6 The Euler Characteristic χ = V E + F

7 The Euler Characteristic Planar graphs? χ = V E + F

8 The Euler Characteristic Planar graphs? χ = V E + F = 2

9 The Euler Characteristic: Proof

10 Table of Contents The Euler Characteristic Spherical Polytopes / Platonic Solids Fundamental Polygons, Tori Connected Sums, Genus

11 Regular Polygons

12 Stereographic Projection eppstein/junkyard/euler/

13 Regular Polyhedra (Platonic Solids) The Tetrahedron: 4 Vertices, 4 Faces, Triangle Faces

14 Regular Polyhedra (Platonic Solids) The Cube: 8 Vertices, 6 Faces, Square Faces

15 Regular Polyhedra (Platonic Solids) The Octahedron: 6 Vertices, 8 Faces, Triangle Faces

16 Regular Polyhedra (Platonic Solids) The Dodecahedron: 20 Vertices, 12 Faces, Pentagonal Faces

17 Regular Polyhedra (Platonic Solids) The Icosahedron: 12 Vertices, 20 Faces, Triangle Faces

18 Constructing The Tetrahedron

19 Constructing The Icosahedron

20 Platonic Solids: Is This it?? Let p be the number of sides per face, q be the degree of each vertex

21 Platonic Solids: Is This it?? Let p be the number of sides per face, q be the degree of each vertex pf = 2E = qv

22 Platonic Solids: Is This it?? Let p be the number of sides per face, q be the degree of each vertex pf = 2E = qv Combine with V E + F = 2 2E q E + 2E p = 2

23 Platonic Solids: Is This it?? Let p be the number of sides per face, q be the degree of each vertex pf = 2E = qv Combine with V E + F = 2 2E q E + 2E p = 2 1 q + 1 p = E

24 Platonic Solids: Is This it?? Let p be the number of sides per face, q be the degree of each vertex pf = 2E = qv Combine with V E + F = 2 2E q E + 2E p = 2 1 q + 1 p = E = 1 q + 1 p > 1 2

25 Flattening To Plane We don t need convex polygons, as long as they are sphere-like

26 Flattening To Plane We don t need convex polygons, as long as they are sphere-like

27 Flattening To Plane

28 Flattening To Plane

29 Flattening To Plane

30 Table of Contents The Euler Characteristic Spherical Polytopes / Platonic Solids Fundamental Polygons, Tori Connected Sums, Genus

31 The Torus

32 Constructing Torus Show Video

33 Torus Fundamental Polygon

34 Torus Fundamental Polygon What is the Euler characteristic of a torus?

35 Intermezzo: Rhythm And Tori / Grateful Dead

36 Table of Contents The Euler Characteristic Spherical Polytopes / Platonic Solids Fundamental Polygons, Tori Connected Sums, Genus

37 Duplicating Spheres What s the euler characteristic of two spheres?

38 Duplicating Tori What s the euler characteristic of two tori?

39 Connected Sum T 1 #T 1 = T 2

40 Connected Sum T 1 #T 1 = T 2 What is the Euler characteristic?

41 Connected Sum: g Tori What is the Euler characteristic of T N = T 1 #T 1 #... #T 1 g times?

42 Connected Sum: g Tori What is the Euler characteristic of T N = T 1 #T 1 #... #T 1 g times? χ = 2 2g

43 Connected Sum: g Tori What is the Euler characteristic of T N = T 1 #T 1 #... #T 1 g times? g is known as the genus χ = 2 2g

44 Connected Sum with Spheres What is the connected sum of a sphere with a sphere?

45 Connected Sum with Spheres What is the connected sum of a torus with a sphere?

46 Euler Characteristic: Homology χ = β 0 β 1 + β 2 β 0 : Number of connected components β 1 : Number of independent loops/cycles β 2 Number of independent voids

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