The tetrahedron is the simplest of all polyhedra. Any four points in space that are not all on the same plane mark the corners of four triangles. The triangles in turn are the faces of a tetrahedron.
A toy called a tumbler is a toy with an egg-shaped hemisphere at the bottom, which stands up on its own even if it is tilted thanks to the weight at the bottom. A mathematician has not only ...
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