Lebesgue's universal covering problem

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For other uses of "universal cover" or "universal covering", see Covering space § Universal covers.
An equilateral triangle of diameter 1 doesn’t fit inside a circle of diameter 1

Lebesgue's universal covering problem is an unsolved problem in geometry that asks for the convex shape of smallest area that can cover every planar set of diameter one. The diameter of a set by definition is the least upper bound of the distances between all pairs of points in the set. A shape covers a set if it contains a congruent subset. In other words the set may be rotated, translated or reflected to fit inside the shape.

Unsolved problem in mathematics:

What is the minimum area of a convex shape that can cover every planar set of diameter one?

(more unsolved problems in mathematics)

Formulation and early research

The problem was posed by Henri Lebesgue in a letter to Gyula Pál in 1914. It was published in a paper by Pál in 1920 along with Pál's analysis.[1] He showed that a cover for all curves of constant width one is also a cover for all sets of diameter one and that a cover can be constructed by taking a regular hexagon with an inscribed circle of diameter one and removing two corners from the hexagon to give a cover of area

The shape outlined in black is Pál's solution to Lebesgue's universal covering problem. Within it, planar shapes with diameter one have been included: a circle (in blue), a Reuleaux triangle (in red) and a square (in green).

In 1936, Roland Sprague showed that a part of Pál's cover could be removed near one of the other corners while still retaining its property as a cover.[2] This reduced the upper bound on the area to .

Current bounds

After a sequence of improvements to Sprague's solution, each removing small corners from the solution,[3][4] a 2018 preprint of Philip Gibbs claimed the best upper bound known, a further reduction to area 0.8440935944.[5][6]

The best known lower bound for the area was provided by Peter Brass and Mehrbod Sharifi using a combination of three shapes in optimal alignment, proving that the area of an optimal cover is at least 0.832.[7]

See also

  • Moser's worm problem, what is the minimum area of a shape that can cover every unit-length curve?
  • Moving sofa problem, the problem of finding a maximum-area shape that can be rotated and translated through an L-shaped corridor
  • Kakeya set, a set of minimal area that can accommodate every unit-length line segment (with translations allowed, but not rotations)
  • Blaschke selection theorem, which can be used to prove that Lebesgue's universal covering problem has a solution.

References

  1. ^ Pál, J. (1920). "'Über ein elementares Variationsproblem". Danske Mat.-Fys. Meddelelser III. 2.
  2. JSTOR 24530328
    .
  3. S2CID 122081393
    .
  4. S2CID 20752239
    .
  5. arXiv:1810.10089 [math.MG
    ].
  6. ^ "Amateur mathematician finds smallest universal cover". Quanta Magazine. Archived from the original on 2019-01-14. Retrieved 2018-11-16.
  7. MR 2176049
    .
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