Difference between revisions of "Closure condition"
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| − | The condition in the geometry of webs (cf. [[Webs, geometry of|Webs, geometry of]]) according to which certain incidences of points and lines of a web imply new incidences. For example, Thomsen's closure condition (see Fig.a) is as follows. The first and the second family of lines of a web are represented by parallel straight lines, the third by curves; the closure condition implies that if | + | {{TEX|done}} |
| + | The condition in the geometry of webs (cf. [[Webs, geometry of|Webs, geometry of]]) according to which certain incidences of points and lines of a web imply new incidences. For example, Thomsen's closure condition (see Fig.a) is as follows. The first and the second family of lines of a web are represented by parallel straight lines, the third by curves; the closure condition implies that if $A,B,C,D$ lie on curves of the third family, then so do $E$ and $F$. | ||
<img style="border:1px solid;" src="https://www.encyclopediaofmath.org/legacyimages/common_img/c022610a.gif" /> | <img style="border:1px solid;" src="https://www.encyclopediaofmath.org/legacyimages/common_img/c022610a.gif" /> | ||
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Figure: c022610a | Figure: c022610a | ||
| − | If | + | If $x,y$ and $z$ are the parameters defining the lines of the three families, the closure condition can be presented in abstract form as a system of defining equations, regarding $z$ as the "product" of $x$ and $y$: $z=xy$, |
| − | + | $$x_0y_1=x_1y_0,x_0y_2=x_2y_0\succeq x_1y_2=x_2y_1.$$ | |
| − | If | + | If $z=xy$ is regarded as a quasi-group operation, Thomsen's closure condition is equivalent to the condition that the quasi-group be isotopic to an Abelian group. |
<img style="border:1px solid;" src="https://www.encyclopediaofmath.org/legacyimages/common_img/c022610b.gif" /> | <img style="border:1px solid;" src="https://www.encyclopediaofmath.org/legacyimages/common_img/c022610b.gif" /> | ||
Revision as of 19:48, 7 July 2014
The condition in the geometry of webs (cf. Webs, geometry of) according to which certain incidences of points and lines of a web imply new incidences. For example, Thomsen's closure condition (see Fig.a) is as follows. The first and the second family of lines of a web are represented by parallel straight lines, the third by curves; the closure condition implies that if $A,B,C,D$ lie on curves of the third family, then so do $E$ and $F$.
Figure: c022610a
If $x,y$ and $z$ are the parameters defining the lines of the three families, the closure condition can be presented in abstract form as a system of defining equations, regarding $z$ as the "product" of $x$ and $y$: $z=xy$,
$$x_0y_1=x_1y_0,x_0y_2=x_2y_0\succeq x_1y_2=x_2y_1.$$
If $z=xy$ is regarded as a quasi-group operation, Thomsen's closure condition is equivalent to the condition that the quasi-group be isotopic to an Abelian group.
Figure: c022610b
Figure: c022610c
Fig.b and Fig.c illustrate the Reidemeister closure condition and the hexagonality condition (all three conditions are equivalent for a plane three-web, even without a differentiability assumption). In abstract form these conditions yield different classes of quasi-groups and loops; in the multi-dimensional geometry of webs, they yield different classes of webs.
Some theorems in projective geometry are essentially closure conditions (such as the theorems of Desargues and Pappus).
References
| [1] | W. Blaschke, "Einführung in die Geometrie der Waben" , Birkhäuser (1955) |
| [2] | V.D. Belousov, V.V. Ryzhkov, "Geometry of webs" J. Soviet Math. , 2 (1974) pp. 331–348 Itogi Nauk. i Tekhn. Alg. Topol. Geom. , 10 (1972) pp. 159–188 |
| [3] | V.D. Belousov, "Algebraic nets and quasi-groups" , Stiintsa , Kishinev (1971) (In Russian) |
Comments
For the notion of isotopic groupoids cf. Isotopy.
Closure condition. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Closure_condition&oldid=13146