Platonic Solids and the Euler Characteristic

Transcription

1 Platonic Solids and the Euler Characteristic Keith Jones Sanford Society, SUNY Oneonta September 2013

2 What is a Platonic Solid? A Platonic Solid is a 3-dimensional object with flat faces and straight edges (a.k.a a polyhedron) satisfying:

3 What is a Platonic Solid? A Platonic Solid is a 3-dimensional object with flat faces and straight edges (a.k.a a polyhedron) satisfying: all faces are congruent,

4 What is a Platonic Solid? A Platonic Solid is a 3-dimensional object with flat faces and straight edges (a.k.a a polyhedron) satisfying: all faces are congruent, all faces are regular polygons,

5 What is a Platonic Solid? A Platonic Solid is a 3-dimensional object with flat faces and straight edges (a.k.a a polyhedron) satisfying: all faces are congruent, all faces are regular polygons, the same number of faces meet at each vertex, and

6 What is a Platonic Solid? A Platonic Solid is a 3-dimensional object with flat faces and straight edges (a.k.a a polyhedron) satisfying: all faces are congruent, all faces are regular polygons, the same number of faces meet at each vertex, and the solid is convex (no indentations).

7 What is a Platonic Solid? A Platonic Solid is a 3-dimensional object with flat faces and straight edges (a.k.a a polyhedron) satisfying: all faces are congruent, all faces are regular polygons, the same number of faces meet at each vertex, and the solid is convex (no indentations). Images from WikipediA.

8 The Five Platonic Solids There are only five solids satisfying these properties: 1. The tetrahedron has 4 triangular faces, 6 edges, and 4 vertices:

9 The Five Platonic Solids There are only five solids satisfying these properties: 2. The cube or hexahedron has 6 square faces, 12 edges, and 8 vertices:

10 The Five Platonic Solids There are only five solids satisfying these properties: 3. The octahedron has 8 triangular faces, 12 edges, and 6 vertices:

11 The Five Platonic Solids There are only five solids satisfying these properties: 4. The dodecahedron has 12 pentagonal faces, 30 edges, and 20 vertices.

12 The Five Platonic Solids There are only five solids satisfying these properties: 5. The icosahedron has 20 triangular faces, 30 edges, and 12 vertices.

13 Platonic Solids are not a New Idea According to a certain website, these stones are believed to date to 2000 B.C.

14 Platonic Solids are not a New Idea According to a certain website, these stones are believed to date to 2000 B.C. But this is a dubious website dedicated to conspiracy theories. No solid evidence these people knew of the Platonic solids.

15 Platonic Solids are not a New Idea According to a certain website, these stones are believed to date to 2000 B.C. But this is a dubious website dedicated to conspiracy theories. No solid evidence these people knew of the Platonic solids. Still, the platonic solids are named after Plato, after all.

16 How to count edges and vertices Suppose a solid has F faces, each face is an p-sided polygon, and q edges meet at each vertex.

17 How to count edges and vertices Suppose a solid has F faces, each face is an p-sided polygon, and q edges meet at each vertex. Then there are E = pf 2 edges and V = 2E q vertices.

18 How to count edges and vertices Suppose a solid has F faces, each face is an p-sided polygon, and q edges meet at each vertex. Then there are E = pf 2 edges and V = 2E q vertices. Example: The Dodecahedron. 12 faces, 5 12 = 30 edges, 2 30 = 20 vertices. 2 3

19 How to count edges and vertices Suppose a solid has F faces, each face is an p-sided polygon, and q edges meet at each vertex. Then there are E = pf 2 edges and V = 2E q vertices. Example: The Dodecahedron. 12 faces, 5 12 = 30 edges, 2 30 = 20 vertices. 2 3 If you have E edges, you have F = 2E p faces and V = 2E q vertices.

20 The Euler Characteristic The Euler Characteristic χ for the surface a polyhedron with F faces, E edges, and V vertices is: χ = F E + V

21 The Euler Characteristic The Euler Characteristic χ for the surface a polyhedron with F faces, E edges, and V vertices is: χ = F E + V or χ = F + V E

22 The Euler Characteristic The Euler Characteristic χ for the surface a polyhedron with F faces, E edges, and V vertices is: χ = F E + V or χ = F + V E tetrahedron: F = 4, E = 6, V = 4. So χ = = 2

23 The Euler Characteristic The Euler Characteristic χ for the surface a polyhedron with F faces, E edges, and V vertices is: χ = F E + V or χ = F + V E tetrahedron: F = 4, E = 6, V = 4. So χ = = 2 hexahedron: F = 6, E = 12, V = 8. So χ = = 2

24 The Euler Characteristic The Euler Characteristic χ for the surface a polyhedron with F faces, E edges, and V vertices is: χ = F E + V or χ = F + V E tetrahedron: F = 4, E = 6, V = 4. So χ = = 2 hexahedron: F = 6, E = 12, V = 8. So χ = = 2 octahedron: F = 8, E = 12, V = 6. So χ = = 2

25 The Euler Characteristic The Euler Characteristic χ for the surface a polyhedron with F faces, E edges, and V vertices is: χ = F E + V or χ = F + V E tetrahedron: F = 4, E = 6, V = 4. So χ = = 2 hexahedron: F = 6, E = 12, V = 8. So χ = = 2 octahedron: F = 8, E = 12, V = 6. So χ = = 2 dodecahedron: F = 12, E = 30, V = 20. So χ = 2

26 The Euler Characteristic The Euler Characteristic χ for the surface a polyhedron with F faces, E edges, and V vertices is: χ = F E + V or χ = F + V E tetrahedron: F = 4, E = 6, V = 4. So χ = = 2 hexahedron: F = 6, E = 12, V = 8. So χ = = 2 octahedron: F = 8, E = 12, V = 6. So χ = = 2 dodecahedron: F = 12, E = 30, V = 20. So χ = 2 icosahedron: F = 20, E = 30, V = 12. So χ = 2 In all cases, χ = 2! Why?

27 A Theorem for the Euler Characteristic For the surface of any convex polyhedron, χ = F + V E = 2. This is known as Euler s Polyhedron Formula.

28 There Can Be Only Five! How do we know these are the only five platonic solids?

29 There Can Be Only Five! How do we know these are the only five platonic solids? From Euler s Characteristic Formula, we know that F + V E = 2 > 0.

30 There Can Be Only Five! How do we know these are the only five platonic solids? From Euler s Characteristic Formula, we know that F + V E = 2 > 0. If each face meets p edges, and each vertex meets q edges, we can write F = 2E p and V = 2E q.

31 There Can Be Only Five! How do we know these are the only five platonic solids? From Euler s Characteristic Formula, we know that F + V E = 2 > 0. If each face meets p edges, and each vertex meets q edges, we can write F = 2E p and V = 2E q. So we have 2E p + 2E q E = 2 > 0.

32 Continuing the calculation... 2E p + 2E q E > 0

33 Continuing the calculation... 2E p + 2E q E > 0 1 p + 1 q 1 2 > 0

34 Continuing the calculation... 2E p + 2E q E > 0 1 p + 1 q 1 2 > 0 1 p + 1 q > 1 2

35 Continuing the calculation... 2E p + 2E q E > 0 1 p + 1 q 1 2 > 0 1 p + 1 q > 1 2 Now, p and q must both be larger than 2.

36 The only possible (p, q) pairs: p = 3, q = Tetrahedron

37 The only possible (p, q) pairs: p = 3, q = Tetrahedron p = 4, q = Hexahedron

38 The only possible (p, q) pairs: p = 3, q = Tetrahedron p = 4, q = Hexahedron p = 3, q = Octahedron

39 The only possible (p, q) pairs: p = 3, q = Tetrahedron p = 4, q = Hexahedron p = 3, q = Octahedron p = 5, q = Dodecahedron

40 The only possible (p, q) pairs: p = 3, q = Tetrahedron p = 4, q = Hexahedron p = 3, q = Octahedron p = 5, q = Dodecahedron p = 3, q = Icosahedron

41 Spot the Non-Platonic Solid!

42 Visit For Print and Fold instructions to make your own Platonic Solids!

43 Thanks for your time!