Lebesgue's universal covering problem

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.

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 $${\displaystyle 2-{\frac {2}{\sqrt {3}}}\approx 0.84529946.}$$

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 ${\displaystyle a\leq 0.844137708436}$.

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]

• 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.