Abstract
Let \(\Omega \subset \mathbb {R}^2\) and let \(\mathcal {L} \subset \Omega \) be a one-dimensional set with finite length \(L =|\mathcal {L}|\). We are interested in minimizers of an energy functional that measures the size of a set projected onto itself in all directions: we are thus asking for sets that see themselves as little as possible (suitably interpreted). Obvious minimizers of the functional are subsets of a straight line but this is only possible for \(L \le \text{ diam }(\Omega )\). The problem has an equivalent formulation: the expected number of intersections between a random line and \(\mathcal {L}\) depends only on the length of \(\mathcal {L}\) (Crofton’s formula). We are interested in sets \(\mathcal {L}\) that minimize the variance of the expected number of intersections. We solve the problem for convex \(\Omega \) and slightly less than half of all values of L: there, a minimizing set is the union of copies of the boundary and a line segment.
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References
Asimov, D., Gerver, J.: Minimum opaque manifolds. Geom. Dedicata. 133, 67–82 (2008)
Bagemihl, F.: Some opaque subsets of a square. Michigan Math. J. 6, 99–103 (1959)
Brakke, K.A.: The opaque cube problem. Amer. Math. Mon. 99, 866–871 (1992)
Chang, A., Dabrowski, D., Orponen, T., Villa, M.: Structure of sets with nearly maximal Favard length, arXiv:2203.01279
Dumitrescu, A., Jiang, M.: The opaque square. In: Proceedings 30th Annual Symposium on Computational Geometry (SoCG’14), Association for Computing Machinery, (2014), pp. 529–538,
Dumitrescu, A., Jiang, M., Toth, C.: Computing opaque interior barriers a la Shermer. SIAM J. Discrete Math. 29(3), 1372–1386 (2015)
Faber, V., Mycielski, J., Pedersen, P.: On the shortest curve which meets all the lines which meet a circle. Ann. Polon. Math. 44, 249–266 (1984)
Faber, V., Mycielski, J.: The shortest curve that meets all the lines that meet a convex body. Amer. Math. Mon. 93, 796–801 (1986)
Finch, S.: Beam detection constant, Mathematical Constants, Encyclopedia of Mathematics and its Applications. Cambridge University Press, Cambridge, pp. 515–519
Izumi, T.: Improving the lower bound on opaque sets for equilateral triangle. Discret. Appl. Math. 213, 130–138 (2016)
Jones, R.E.D.: Opaque sets of degree \(\alpha \). Amer. Math. Mon. 71, 535–537 (1964)
Kawamura, A., Moriyama, S., Otachi, Y., Pach, J.: A lower bound on opaque sets. Comput. Geom. 80, 13–22 (2019)
Kawohl, B.: The opaque square and the opaque circle. In: Bandle, Catherine, Everitt, William N., Losonczi, Laszlo, Walter, Wolfgang (eds.) General inequalities, 7 (Oberwolfach, 1995), International Series of Numerical Mathematics, vol. 123, pp. 339–346. Birkhauser, Basel (1997)
Mazurkiewicz, S.: Sur un ensemble ferme, punctiforme, qui rencontre toute droite passant par un certain domaine. Prace Mat.-Fiz. (in Polish and French) 27, 11–16 (1916)
Santaló, L.: Introduction to integral geometry. Publ. Inst. Math. Univ. Nancago, II. Actualites Scientifiques et Industrielles No. 1198 Hermann & Cie, Paris, (1953)
Santaló, L.: Integral geometry and geometric probability, pp. 25–85. Cambridge University Press, Cambridge (2004)
Sen Gupta, H.M., Basu Mazumdar, N.C.: A note on certain plane sets of points. Bull. Calcutta Math. Soc. 47, 199–201 (1955)
Steinerberger, S.: An inequality characterizing convex domains, arXiv:2209.14153
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The author is grateful to Alan Chang for pointing out [4] and valuable discussions. The author is also grateful to two anonymous referees whose careful work greatly increased the readability of the manuscript.
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Communicated by Monika Ludwig.
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S.S. is supported by the NSF (DMS-2123224) and the Alfred P. Sloan Foundation
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Steinerberger, S. Quadratic Crofton and sets that see themselves as little as possible. Monatsh Math (2024). https://doi.org/10.1007/s00605-023-01934-y
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DOI: https://doi.org/10.1007/s00605-023-01934-y