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An illusion due to Franz Joseph Delboeuf, a Belgian philosopher and expert on hypnosis. If the same circle is placed inside two different concentric circles, its size will ...

A puzzle in which one object is to be converted to another by making a finite number of cuts and reassembling it. The cuts are often, but not always, restricted to straight ...

The eccentric angle of a point on an ellipse with semimajor axes of length a and semiminor axes of length b is the angle t in the parametrization x = acost (1) y = bsint, (2) ...

In two dimensions, the curve known as an "egg' is an oval with one end more pointed than the other.

An even Mathieu function ce_r(z,q) with characteristic value a_r.

Johnson solid J_(16).

Johnson solid J_(41).

A Jordan algebra which is not isomorphic to a subalgebra.

The Cartesian product of a finite or infinite set of modules over a ring with only finitely many nonzero entries in each sequence.

If f:E->B is a fiber bundle with B a paracompact topological space, then f satisfies the homotopy lifting property with respect to all topological spaces. In other words, if ...