Equichordal point problem: Difference between revisions

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The theorem solves the Equichordal Point Problem of Fujiwara, originally posed in 1916. The problem was rediscovered in 1917 by Wilhelm Blaschke, Rothe and Weizenböck^[1].

The Theorem

There is no $C$ be a Jordan curve with two equichordal points, with respect to which the curve $C$ would be star-shaped. In particular, there is no convex and closed curve with two equichordal points.

The Published Proof

The proof was published in the hard to read article ^[2] There is also easy to read research announcement ^[3], but it only hints at the ideas used in the proof.

Definition of Excentricity (or Eccentricity)

Let $C$ be the hypothetical convex curve with two equichordal points $O_{1}$ and $O_{2}$ . Let $L$ be the common length of all chords of the curve $C$ passing through $O_{1}$ or $O_{2}$ . Then excentricity is the ratio:

a={\frac {\|O_{1}-O_{2}\|}{L}}

Many results on equichordal points refer to the excentricity. It turns out that the smaller the $a$ , the harder is to disprove the existence of the curves with two equichordal points. It can be shown rigorously, that small excentricity means that the curve must be close to the circle^[4].

The History of the Problem

The problem has been extensively studied, with significant papers published over eight decades preceding its solution:

In 1916 Fujiwara^[5] proved that no convex curves with three equichordal points exist.
In 1917 Blaschke, Rothe and Weitzenböck^[1] formulated the problem again.
In 1923 Süss showed certain symmetries and uniqueness of the curve, if it existed.
In 1953 G. A. Dirac showed some explicit bounds on the curve, if it existed.
In 1958 Wirsing^[6] showed that the curve, if it exists, must be an analytic curve. In this deep paper, he correctly identified the problem as perturbation problem beyond all orders.
In 1966 Ehrhart^[7] proved that there are no equichordal curves with eccentricities > 0.5.
In 1988 Michelacci proved that there are no equichordal curves with eccentricities > .33. The proof is mildly computer-assisted.
In 1992 Shäfke and Volkmer^[4] showed that there is at most a finite number of values of eccentricity for which the curve may exist. They outlined a feasible strategy for a computer-assisted proof.
In 1996 Rychlik^[2] fully solved the problem. The proof does not use a computer.

References

^ ^a ^b W. Blaschke, W. Rothe, and R. Weitzenböck. Aufgabe 552. Arch. Math. Phys., 27:82, 1917
^ ^a ^b Marek R. Rychlik, A complete solution to the equichordal point problem of Fujiwara, Blaschke, Rothe and Weizenböck, Inventiones Mathematicae, 1997, Volume 129, Number 1, Pages 141-212.
^ Marek Rychlik, The Equichordal Point Problem, Electronic Research Announcements of the AMS, 1996, pages 108-123, available on-line at [1]
^ ^a ^b R. Schäfke and H. Volkmer, Asymptotic analysis of the equichordal problem, J. Reine Angew. Math. 425 (1992), 9-60
^ M.~Fujiwara. Über die Mittelkurve zweier geschlossenen konvexen Curven in Bezug auf einen Punkt. Tôhoku Math J., 10:99--103, 1916
^ E.~Wirsing, Zur Analytisität von Doppelspeichkurven, Arch. Math. 9 (1958), 300--307.
^ R.~Ehrhart, Un ovale à deux points isocordes?, Enseignement Math. 13 (1967), 119-124

[Blaschke-1] W. Blaschke, W. Rothe, and R. Weitzenböck. Aufgabe 552. Arch. Math. Phys., 27:82, 1917

[The_Proof-2] Marek R. Rychlik, A complete solution to the equichordal point problem of Fujiwara, Blaschke, Rothe and Weizenböck, Inventiones Mathematicae, 1997, Volume 129, Number 1, Pages 141-212.

[The_Announcement-3] Marek Rychlik, The Equichordal Point Problem, Electronic Research Announcements of the AMS, 1996, pages 108-123, available on-line at [1]

[Volkmer-4] R. Schäfke and H. Volkmer, Asymptotic analysis of the equichordal problem, J. Reine Angew. Math. 425 (1992), 9-60

[Fujiwara-5] M.~Fujiwara. Über die Mittelkurve zweier geschlossenen konvexen Curven in Bezug auf einen Punkt. Tôhoku Math J., 10:99--103, 1916

[Wirsing-6] E.~Wirsing, Zur Analytisität von Doppelspeichkurven, Arch. Math. 9 (1958), 300--307.

[Erhhart-7] R.~Ehrhart, Un ovale à deux points isocordes?, Enseignement Math. 13 (1967), 119-124

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 There is also easy to read research announcement <ref name="The_Announcement">[[Marek Rychlik]], The Equichordal Point Problem, Electronic Research Announcements of the AMS,
 , pages 108-123, available on-line at [http://www.mpim-bonn.mpg.de/era-mirror/1996-03-002/1996-03-002.html] </ref>, but it only hints at the ideas used in the proof.
+== Definition of Excentricity (or Eccentricity) ==
+Let <math>C</math> be the hypothetical [[convex]] [[curve]] with two [[equichordal points]] <math>O_1</math>
+and <math>O_2</math>. Let <math>L</math> be the common length of all chords of the curve <math>C</math> passing
+through <math>O_1</math> or <math>O_2</math>. Then excentricity is the ratio:
+:<math>a = \frac{\|O_1-O_2\|}{L} </math>
+Many results on [[equichordal points]] refer to the excentricity. It turns out that the smaller the <math>a</math>, the harder is to disprove the existence of the curves with two equichordal points. It can be shown rigorously,
+that small excentricity means that the curve must be close to the [[circle]]<ref name="Volkmer"/>.
 == The History of the Problem ==