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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.

An even number N for which N=0 (mod 4). The first few positive doubly even numbers are 4, 8, 12, 16, ... (OEIS A008586).

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 ...

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.

The system of partial differential equations u_t = 3ww_x (1) w_t = 2w_(xxx)+2uw_x+u_xw. (2)