Math 2.0

Manifolds and cell complexes Topological manifolds: basic notions Example

Torus

Builds on Quotient topologyProduct topologyCircle as a quotient of the real line

Example

Let ∼\sim be the equivalence relation on the square [0,1]×[0,1][0, 1] \times [0, 1] generated by (0,y)∼(1,y)(0, y) \sim (1, y) and (x,0)∼(x,1)(x, 0) \sim (x, 1) for all x,y∈[0,1]x, y \in [0, 1]. The quotient space [0,1]2/∼[0, 1]^2 / {\sim}, with the quotient topology, is the torus T2T^2.

Remarks

The torus is homeomorphic to the product S1×S1S^1 \times S^1 of two circles, with the product topology, and to ℝ2/ℤ2\mathbb{R}^2 / \mathbb{Z}^2. It is also homeomorphic to a surface in ℝ3\mathbb{R}^3: turn a circle about an axis in its plane that does not meet the circle. The torus is a compact, connected surface. It is not homeomorphic to the sphere.

Reading path

To understand this page from the ground up, read these 63 pages in order. The foundations come first.

  1. Element definition
  2. Proposition concept
  3. Real numbers object
  4. Set definition
  5. Conjunction definition
  6. Disjunction definition
  7. Implication definition
  8. Predicate definition
  9. Empty set object
  10. Family of sets definition
  11. Ordered pair definition
  12. Intersection definition
  13. Union definition
  14. Set-builder notation concept
  15. Existential quantifier definition
  16. Universal quantifier definition
  17. Cartesian product definition
  18. Subset definition
  19. Union of a family of sets definition
  20. Uniqueness quantifier definition
  21. Binary relation definition
  22. Universal set definition
  23. Topological space definition
  24. Reflexive relation definition
  25. Symmetric relation definition
  26. Transitive relation definition
  27. Complement definition
  28. Function definition
  29. Basis of a topology definition
  30. Subspace topology definition
  31. Equivalence relation definition
  32. Metric space definition
  33. Product topology definition
  34. Preimage of a set definition
  35. Codomain definition
  36. Function composition definition
  37. Injective function definition
  38. Open ball definition
  39. Equivalence class definition
  40. Surjective function definition
  41. Metric topology definition
  42. Quotient set definition
  43. Bijective function definition
  44. Open set definition
  45. Quotient topology definition
  46. Inverse function definition
  47. Hausdorff space definition
  48. Open cover definition
  49. Closed set definition
  50. Continuous map of topological spaces definition
  51. Compact space definition
  52. Complement of an open set is closed statement
  53. Homeomorphism definition
  54. Universal property of the quotient topology statement
  55. Continuity via closed sets statement
  56. Compact set definition
  57. Compactness via open covers statement
  58. Compact subsets of a Hausdorff space are closed statement
  59. Closed subsets of compact sets are compact statement
  60. Continuous image of a compact set is compact statement
  61. Continuous bijection from a compact space to a Hausdorff space is a homeomorphism statement
  62. Circle as a quotient of the real line example
  63. Torus example

Where this leads

14 pages build on this one.

Further reading

  1. Web https://en.wikipedia.org/wiki/Torus, Topology
About this page
Tag03NM7DZXXC
URIm2://03NM7DZXXC
Kindexample
WritingW2 usable
MSC57K20
Instance ofTopological space
UsesQuotient topology, Product topology, Circle as a quotient of the real line
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