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A chord which is a normal at each end. A centrosymmetric set K subset R^d has d double normals through the center (Croft et al. 1991). For a curve of constant width, all ...

The double sphere is the degenerate quartic surface (x^2+y^2+z^2-r^2)^2=0 obtained by squaring the left-hand side of the equation of a usual sphere x^2+y^2+z^2-r^2=0.

A sphere with two handles and two holes, i.e., a genus-2 torus.

y=delta^'(x-a), where delta(x) is the delta function.

A surface that contains two families of rulings. The only three doubly ruled surfaces are the plane, hyperbolic paraboloid, and single-sheeted hyperboloid.

For R[a+b-c-d]<-1 and a and b not integers,

If the lines joining corresponding points of two directly similar figures are divided proportionally, then the locus of the points of the division will be a figure directly ...

The quantity 12 is sometimes referred to as one dozen.

A drafter triangle is a name sometimes used to refer to a 30-60-90 triangle. For this reason, a polyform consisting of multiple 30-60-90 triangles is known as a polydrafter.

The ending of a game in which neither of two players wins, sometimes also called a "tie." A game in which no draw is possible is called a categorical game.