Pinched torus

In mathematics, and especially topology and differential geometry, a pinched torus (or croissant surface) is a kind of two-dimensional surface. It gets its name from its resemblance to a torus that has been pinched at a single point. A pinched torus is an example of an orientable, compact 2-dimensional pseudomanifold.[1]

A Pinched Torus

Parametrisation

A pinched torus is easily parametrisable. Let us write g(x,y) = 2 + sin(x/2).cos(y). An example of such a parametrisation − which was used to plot the picture − is given by ƒ : [0,2π)2 → R3 where:

Topology

Topologically, the pinched torus is homotopy equivalent to the wedge of a sphere and a circle.[2][3] It is homeomorphic to a sphere with two distinct points being identified.[2][3]

Homology

Let P denote the pinched torus. The homology groups of P over the integers can be calculated. They are given by:

Cohomology

The cohomology groups of P over the integers can be calculated. They are given by:

References
  1. Brasselet, J. P. (1996). "Intersection of Algebraic Cycles". Journal of Mathematical Sciences. Springer New York. 82 (5): 3625–3632. doi:10.1007/bf02362566.
  2. Hatcher, Allen (2001), Algebraic Topology, Cambridge University Press, ISBN 0-521-79540-0
  3. Allen Hatcher. "Chapter 0: Algebraic Topology" (PDF). Retrieved August 6, 2010.
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