Boy's surface
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geometry Geometry (; ) is a branch of mathematics concerned with properties of space such as the distance, shape, size, and relative position of figures. Geometry is, along with arithmetic, one of the oldest branches of mathematics. A mathematician w ...
, Boy's surface is an immersion of the real projective plane in
three-dimensional space In geometry, a three-dimensional space (3D space, 3-space or, rarely, tri-dimensional space) is a mathematical space in which three values ('' coordinates'') are required to determine the position of a point. Most commonly, it is the three- ...
. It was discovered in 1901 by the German mathematician Werner Boy, who had been tasked by his doctoral thesis advisor David Hilbert to prove that the projective plane ''could not'' be immersed in three-dimensional space. Boy's surface was first parametrized explicitly by Bernard Morin in 1978. Another parametrization was discovered by Rob Kusner and Robert Bryant.. Boy's surface is one of the two possible immersions of the real projective plane which have only a single triple point. Unlike the Roman surface and the cross-cap, it has no other singularities than self-intersections (that is, it has no pinch-points).


Parametrization

Boy's surface can be parametrized in several ways. One parametrization, discovered by Rob Kusner and Robert Bryant, is the following: given a complex number ''w'' whose magnitude is less than or equal to one ( \, w \, \le 1), let :\begin g_1 &= - \operatorname \left \right\ pt g_2 &= - \operatorname \left \right\ pt g_3 &= \operatorname \left \right- \\ \end and then set :\beginx\\ y\\ z\end = \frac \beging_1\\ g_2\\ g_3\end we then obtain the Cartesian coordinates ''x'', ''y'', and ''z'' of a point on the Boy's surface. If one performs an inversion of this parametrization centered on the triple point, one obtains a complete minimal surface with three ends (that's how this parametrization was discovered naturally). This implies that the Bryant–Kusner parametrization of Boy's surfaces is "optimal" in the sense that it is the "least bent" immersion of a projective plane into three-space.


Property of Bryant–Kusner parametrization

If ''w'' is replaced by the negative reciprocal of its complex conjugate, -, then the functions ''g''1, ''g''2, and ''g''3 of ''w'' are left unchanged. By replacing in terms of its real and imaginary parts , and expanding resulting parameterization, one may obtain a parameterization of Boy's surface in terms of rational functions of and . This shows that Boy's surface is not only an algebraic surface, but even a
rational surface In algebraic geometry, a branch of mathematics Mathematics is a field of study that discovers and organizes methods, Mathematical theory, theories and theorems that are developed and Mathematical proof, proved for the needs of empirical sc ...
. The remark of the preceding paragraph shows that the generic fiber of this parameterization consists of two points (that is that almost every point of Boy's surface may be obtained by two parameters values).


Relation to the real projective plane

Let P(w) = (x(w), y(w), z(w)) be the Bryant–Kusner parametrization of Boy's surface. Then : P(w) = P\left(- \right). This explains the condition \left\, w \right\, \le 1 on the parameter: if \left\, w \right\, < 1, then \left\, - \right\, > 1 . However, things are slightly more complicated for \left\, w \right\, = 1. In this case, one has - = -w . This means that, if \left \, w \right\, = 1, the point of the Boy's surface is obtained from two parameter values: P(w) = P(-w). In other words, the Boy's surface has been parametrized by a disk such that pairs of diametrically opposite points on the
perimeter A perimeter is the length of a closed boundary that encompasses, surrounds, or outlines either a two-dimensional shape or a one-dimensional line. The perimeter of a circle or an ellipse is called its circumference. Calculating the perimet ...
of the disk are equivalent. This shows that the Boy's surface is the image of the real projective plane, RP2 by a smooth map. That is, the parametrization of the Boy's surface is an immersion of the real projective plane into the
Euclidean space Euclidean space is the fundamental space of geometry, intended to represent physical space. Originally, in Euclid's ''Elements'', it was the three-dimensional space of Euclidean geometry, but in modern mathematics there are ''Euclidean spaces ...
.


Symmetries

Boy's surface has 3-fold
symmetry Symmetry () in everyday life refers to a sense of harmonious and beautiful proportion and balance. In mathematics, the term has a more precise definition and is usually used to refer to an object that is Invariant (mathematics), invariant und ...
. This means that it has an axis of discrete rotational symmetry: any 120° turn about this axis will leave the surface looking exactly the same. The Boy's surface can be cut into three mutually congruent pieces.


Applications

Boy's surface can be used in sphere eversion as a half-way model. A half-way model is an immersion of the sphere with the property that a rotation interchanges inside and outside, and so can be employed to evert (turn inside-out) a sphere. Boy's (the case p = 3) and Morin's (the case p = 2) surfaces begin a sequence of half-way models with higher symmetry first proposed by George Francis, indexed by the even integers 2p (for p odd, these immersions can be factored through a projective plane). Kusner's parametrization yields all these.


Models


Model at Oberwolfach

The Oberwolfach Research Institute for Mathematics has a large model of a Boy's surface outside the entrance, constructed and donated by Mercedes-Benz in January 1991. This model has 3-fold rotational symmetry and minimizes the Willmore energy of the surface. It consists of steel strips representing the image of a polar coordinate grid under a parameterization given by Robert Bryant and Rob Kusner. The meridians (rays) become ordinary Möbius strips, i.e. twisted by 180 degrees. All but one of the strips corresponding to circles of latitude (radial circles around the origin) are untwisted, while the one corresponding to the boundary of the unit circle is a Möbius strip twisted by three times 180 degrees — as is the emblem of the institute .


Model made for Clifford Stoll

A model was made in glass by glassblower Lucas Clarke, with the cooperation of Adam Savage, for presentation to
Clifford Stoll Clifford Paul "Cliff" Stoll (born June 4, 1950) is an American astronomer, author and teacher. He is best known for his investigation in 1986, while working as a system administrator at the Lawrence Berkeley National Laboratory, that led to th ...
. It was featured on Adam Savage's
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channel, Tested. All three appeared in the video discussing it.


References


Citations


Sources

* This describes a piecewise linear model of Boy's surface. ** Article on the cover illustration that accompanies the Rob Kirby article. * . * Sanderson, B
''Boy's will be Boy's''
(undated, 2006 or earlier). *


External links


Boy's surface
at MathCurve; contains various visualizations, various equations, useful links and references

– applet from ''Plus Magazine''.

including th
original article
and a
embedding
of a topologist in th
Oberwolfach Boy's surface



A paper model of Boy's surface
– pattern and instructions

in Constructive Solid Geometry together with assembling instructions
''Boy's surface''
visualization video from the Mathematical Institute of the Serbian Academy of the Arts and Sciences
''This Object Should've Been Impossible to Make''
Adam Savage making a museum stand for a glass model of the surface {{DEFAULTSORT:Boy's Surface Surfaces Geometric topology Eponyms in geometry