Pinched torus

A 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]

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:

$f(x,y) = \left( g(x,y)\cos x , g(x,y)\sin x , \sin\!\left(\frac{x}{2}\right)\sin y \right)$

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:

$H_0(P,\Z) \cong \Z, \ H_1(P,\Z) \cong \Z, \ \text{and} \ H_2(P,\Z) \cong \Z.$

Cohomology

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

$H^0(P,\Z) \cong \Z, \ H^1(P,\Z) \cong \Z, \ \text{and} \ H^2(P,\Z) \cong \Z.$

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.