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Concept

Constant width

A convex shape that measures the same across between parallel supporting lines whatever direction they take. The circle is not the only one: a Reuleaux triangle rolls as smoothly under a plank and is not round.

Named by 6 essays across one field — each of them below, with the objects they name alongside it.

A Reuleaux triangle. A curve of constant width on 3 vertices, with 6 pairs of parallel supporting lines drawn across it. Every pair is 180.1 apart.

Round is not the only way to be the same width

A shape that measures the same in every direction sounds like a description of a circle. It is not — there are infinitely many others, one of them is on a coin in most people's pockets, and a drill built from one cuts a nearly square hole.

geometry · Constant width One perimeter of 300, spent five ways. Regular polygons all of the same perimeter, drawn to scale beside the circle of that perimeter, with the area each encloses and the ratio 4πA/L².

The most area a fence can hold

One length of boundary, and the question of what shape to bend it into. The answer is a circle, everybody knows it, and the argument that convinced the nineteenth century turned out to prove something slightly different.

geometry · Isoperimetric The two numbers that make up a width. A Reuleaux polygon with 3 sides, its centre marked as the origin, and the two supporting lines with normals 33° and 213°. The perpendicular distances from the origin to the two lines are marked; they add to the width.

The shape described from outside

A convex shape can be given by its boundary or by the family of lines that touch it, and the second description turns the constant-width condition into one line of arithmetic — after which the perimeter falls out, and curves with no corners at all can simply be written down.

geometry · Constant width Area at equal width: the triangle least, the circle most. A bar for each curve of constant width the family draws, all at the same width, with the bar's length its enclosed area and the extremes marked.

The least area a width can hold

Barbier's theorem says every curve of constant width has the same perimeter, which removes perimeter as a way of telling the family apart. Area is not like that — the circle holds the most and the Reuleaux triangle the least — and the reason the minimiser has corners is a constraint rather than a preference.

geometry · Constant width The Reuleaux tetrahedron's width runs from 2.83 to 2.9. The largest and smallest width of the intersection of four balls, plotted against the polar angle of the measuring direction, with the constant width it would need to have drawn flat.

The same question in space

Intersect four balls at the corners of a tetrahedron and the result is not of constant width — it misses by two and a half per cent, computed exactly. Repairing it gives a body that is, and whether that body is the smallest of its kind has been open for a century.

geometry · Constant width The largest hexagon of diameter one. Regular hexagon of diameter 1: area 0.649519053; best hexagon: area 0.674981443, 6 pairs at distance 1; vertices (0.3438, -0.3253) (0.5000, 0.2115) (-0.0000, 0.6138) (-0.5000, 0.2115) (-0.3438, -0.3253) (-0.0000, -0.3862).

The largest polygon of diameter one

Among pentagons whose points are never more than one unit apart, the regular pentagon has the most area, and the same is true for every odd number of sides. For six sides it is false: a lopsided hexagon found in 1975 beats the regular one by four per cent. Symmetry wins half the cases and loses the other half, and the reason is parity.

geometry · Isoperimetric

Named alongside it

The objects these essays reach for when they reach for this one.

ConvexityReuleaux triangleSupport functionAreaExtremal problemRegular polygonSymmetryBarbier's theoremCircleConvex hullCurvatureDiameter

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