User:WillemienH/Draft Pseudosphere new

In its general interpretation, a pseudosphere of radius R is any surface of curvature −1/R², by analogy with the sphere of radius R, which is a surface of curvature 1/R². The term was introduced by Eugenio Beltrami in his 1868 paper on models of hyperbolic geometry.^[1]

 

The term is also used to refer to a certain surface called the tractricoid: the result of revolving a tractrix about its asymptote. As an example, the (half) pseudosphere (with radius 1) is the surface of revolution of the tractrix parametrized by^[2]

${\displaystyle t\mapsto \left(t-\tanh {t},\operatorname {sech} \,{t}\right),\quad \quad 0\leq t<\infty .}$ 

It is a singular space (the equator is a singularity), but away from the singularities, it has constant negative Gaussian curvature and therefore is locally isometric to a hyperbolic plane.

The name "pseudosphere" comes about because it is a two-dimensional surface of constant negative curvature just like a sphere with positive Gauss curvature. Just as the sphere has at every point a positively curved geometry of a dome the whole pseudosphere has at every point the negatively curved geometry of a saddle.

As early as 1693 Christiaan Huygens found that the volume and the surface area of the pseudosphere are finite,^[3] despite the infinite extent of the shape along the axis of rotation. For a given edge radius R, the area is 4πR² just as it is for the sphere, while the volume is ⁠2/3⁠πR³ and therefore half that of a sphere of that radius.^[4][5]

The half pseudosphere of curvature −1 is covered by the portion of the hyperbolic upper half-plane with y ≥ 1.^[6] The covering map is periodic in the x direction of period 2π, and takes the horocycles y = c to the meridians of the pseudosphere and the vertical geodesics x = c to the tractrices that generate the pseudosphere. This mapping is a local isometry, and thus exhibits the portion y ≥ 1 of the upper half-plane as the universal covering space of the pseudosphere. The precise mapping is

${\displaystyle (x,y)\mapsto (v(\operatorname {arcosh} y)\cos x,v(\operatorname {arcosh} y)\sin x,u(\operatorname {arcosh} y))}$ 

where ${\displaystyle t\mapsto (u(t),v(t))}$  is the arclength parametrization of the tractrix above.

In some sources that use the hyperboloid model of the hyperbolic plane, the hyperboloid is referred to as a pseudosphere.^[7] This usage of the word is because the hyperboloid can be thought of as a sphere of imaginary radius, embedded in a Minkowski space.

• Dini's surface
• Gabriel's Horn
• Hyperboloid
• Hyperboloid structure
• Sine–Gordon equation
• Sphere
• Surface of revolution

• Stillwell J: Sources of Hyperbolic Geometry, 1996, Amer. Math. Soc & London Math. Soc.
• Henderson, D. W. and Taimina, D. (2006). "Experiencing Geometry: Euclidean and Non-Euclidean with History". Aesthetics and Mathematics. Springer-Verlag. {{cite book}}: External link in |chapter= (help)CS1 maint: multiple names: authors list (link)
• Edward Kasner & James Newman (1940) Mathematics and the Imagination, pp 140,145,155, Simon & Schuster.

• Non Euclid
• Crocheting the Hyperbolic Plane: An Interview with David Henderson and Daina Taimina
• Prof. C.T.J. Dodson's web site at University of Manchester
• Interactive demonstration of the pseudosphere (at the University of Manchester)
• Norman Wildberger lecture 16, History of Mathematics, University of New South Wales. YouTube. 2012 May.
• Pseudospherical surfaces at the virtual math museum.

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--Category:Hyperbolic geometry]]

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