Translation surface (differential geometry)

For two space curves ${\displaystyle \ c_{1}:\;{\vec {x}}=\gamma _{1}(u)\ }$ and ${\displaystyle \ c_{2}:\;{\vec {x}}=\gamma _{2}(v)\ }$ with ${\displaystyle \gamma _{1}(0)=\gamma _{2}(0)={\vec {0}}}$ the translation surface ${\displaystyle \Phi }$ can be represented by:[5]

(TS) ${\displaystyle \quad {\vec {x}}=\gamma _{1}(u)+\gamma _{2}(v)\;}$

and contains the origin. Obviously this definition is symmetric regarding the curves ${\displaystyle c_{1}}$ and ${\displaystyle c_{2}}$. Therefore, both curves are called generatrices (one: generatrix). Any point ${\displaystyle X}$ of the surface is contained in a shifted copy of ${\displaystyle c_{1}}$ and ${\displaystyle c_{2}}$ resp.. The tangent plane at ${\displaystyle X}$ is generated by the tangentvectors of the generatrices at this point, if these vectors are linearly independent.

If the precondition ${\displaystyle \gamma _{1}(0)=\gamma _{2}(0)={\vec {0}}}$ is not fulfilled, the surface defined by (TS) may not contain the origin and the curves ${\displaystyle c_{1},c_{2}}$. But in any case the surface contains shifted copies of any of the curves ${\displaystyle c_{1},c_{2}}$ as parametric curves ${\displaystyle {\vec {x}}(u_{0},v)}$ and ${\displaystyle {\vec {x}}(u,v_{0})}$ respectively.

The two curves ${\displaystyle c_{1},c_{2}}$ can be used to generate the so called corresponding midchord surface. Its parametric representation is

(MCS) ${\displaystyle \quad {\vec {x}}={\frac {1}{2}}(\gamma _{1}(u)+\gamma _{2}(v))\;.}$

Helicoid as translation surface with identical generatrices ${\displaystyle c_{1},c_{2}}$

Helicoid as translation surface: any parametric curve is a shifted copy of the purple helix.

A helicoid is a special case of a generalized helicoid and a ruled surface. It is an example of a minimal surface and can be represented as a translation surface.

The helicoid with the parametric representation

${\displaystyle {\vec {x}}(u,v)=(u\cos v,u\sin v,kv)}$

has a turn around shift (German: Ganghöhe) ${\displaystyle 2\pi k}$. Introducing new parameters ${\displaystyle \alpha ,\varphi }$[6] such that

${\displaystyle u=2a\cos \left({\frac {\alpha -\varphi }{2}}\right)\ ,\ \ v={\frac {\alpha +\varphi }{2}}}$

and ${\displaystyle a}$ a positive real number, one gets a new parametric representation

• ${\displaystyle {\vec {X}}(\alpha ,\varphi )=\left(a\cos \alpha +a\cos \varphi \;,\;a\sin \alpha +a\sin \varphi \;,\;{\frac {k\alpha }{2}}+{\frac {k\varphi }{2}}\right)}$

${\displaystyle =(a\cos \alpha ,a\sin \alpha ,{\frac {k\alpha }{2}})\ +\ (a\cos \varphi ,a\sin \varphi ,{\frac {k\varphi }{2}})\ ,}$

which is the parametric representation of a translation surface with the two identical (!) generatrices

${\displaystyle c_{1}:\;\gamma _{1}={\vec {X}}(\alpha ,0)=\left(a+a\cos \alpha ,a\sin \alpha ,{\frac {k\alpha }{2}}\right)\quad }$ and

${\displaystyle c_{2}:\;\gamma _{2}={\vec {X}}(0,\varphi )=\left(a+a\cos \varphi ,a\sin \varphi ,{\frac {k\varphi }{2}}\right)\ .}$

The common point used for the diagram is ${\displaystyle P={\vec {X}}(0,0)=(2a,0,0)}$. The (identical) generatrices are helices with the turn around shift ${\displaystyle k\pi \;,}$ which lie on the cylinder with the equation ${\displaystyle (x-a)^{2}+y^{2}=a^{2}}$. Any parametric curve is a shifted copy of the generatrix ${\displaystyle c_{1}}$ (in diagram: purple) and is contained in the right circular cylinder with radius ${\displaystyle a}$, which contains the z-axis.

The new parametric representation represents only such points of the helicoid that are within the cylinder with the equation ${\displaystyle x^{2}+y^{2}=4a^{2}}$.

Helicoid as midchord surface of two identical generatrices (green helix).

From the new parametric representation one recognizes, that the helicoid is a midchord surface, too:

${\displaystyle {\begin{aligned}{\vec {X}}(\alpha ,\varphi )&=\left(a\cos \alpha ,a\sin \alpha ,{\frac {k\alpha }{2}}\right)\ +\ \left(a\cos \varphi ,a\sin \varphi ,{\frac {k\varphi }{2}}\right)\\[5pt]&={\frac {1}{2}}(\delta _{1}(\alpha )+\delta _{2}(\varphi ))\ ,\quad \end{aligned}}}$

where

${\displaystyle d_{1}:\ {\vec {x}}=\delta _{1}(\alpha )=(2a\cos \alpha ,2a\sin \alpha ,k\alpha )\ ,\quad }$ and

${\displaystyle d_{2}:\ {\vec {x}}=\delta _{2}(\varphi )=(2a\cos \varphi ,2a\sin \varphi ,k\varphi )\ ,\quad }$

are two identical generatrices.

In diagram: ${\displaystyle P_{1}:\delta _{1}(\alpha _{0})}$ lies on the helix ${\displaystyle d_{1}}$ and ${\displaystyle P_{2}:\delta _{2}(\varphi _{0})}$ on the (identical) helix ${\displaystyle d_{2}}$. The midpoint of the chord is ${\displaystyle \ M:{\frac {1}{2}}(\delta _{1}(\alpha _{0})+\delta _{2}(\varphi _{0}))={\vec {X}}(\alpha _{0},\varphi _{0})\ }$.

Architecture

A surface (for example a roof) can be manufactured using a jig for curve ${\displaystyle c_{2}}$ and several identical jigs of curve ${\displaystyle c_{1}}$. The jigs can be designed without any knowledge of mathematics. By positioning the jigs the rules of a translation surface have to be respected only.

Descriptive geometry

Establishing a parallel projection of a translation surface one 1) has to produce projections of the two generatrices, 2) make a jig of curve ${\displaystyle c_{1}}$ and 3) draw with help of this jig copies of the curve respecting the rules of a translation surface. The contour of the surface is the envelope of the curves drawn with the jig. This procedure works for orthogonal and oblique projections, but not for central projections.

Differential geometry

For a translation surface with parametric representation ${\displaystyle {\vec {x}}(u,v)=\gamma _{1}(u)+\gamma _{2}(v)\;}$ the partial derivatives of ${\displaystyle {\vec {x}}(u,v)}$ are simple derivatives of the curves. Hence the mixed derivatives are always ${\displaystyle 0}$ and the coefficient ${\displaystyle M}$ of the second fundamental form is ${\displaystyle 0}$, too. This is an essential facilitation for showing that (for example) a helicoid is a minimal surface.

• G. Darboux: Leçons sur la théorie générale des surfaces et ses applications géométriques du calcul infinitésimal , 1–4 , Chelsea, reprint, 972, pp. Sects. 81–84, 218
• Georg Glaeser: Geometrie und ihre Anwendungen in Kunst, Natur und Technik, Springer-Verlag, 2014, ISBN 364241852X, p. 259
• W. Haack: Elementare Differentialgeometrie, Springer-Verlag, 2013, ISBN 3034869509, p. 140
• C. Leopold: Geometrische Grundlagen der Architekturdarstellung. Kohlhammer Verlag, Stuttgart 2005, ISBN 3-17-018489-X, p. 122
• D.J. Struik: Lectures on classical differential geometry , Dover, reprint ,1988, pp. 103, 109, 184

• Encyclopedia of Mathematics