Three-torus

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The three-dimensional torus, or triple torus, is defined as the Cartesian product of three circles,

In contrast, the usual torus is the Cartesian product of two circles only.

The triple torus is a three-dimensional compact manifold with no boundary. It can be obtained by "gluing" the three pairs of opposite faces of a cube, where being "glued" can be intuitively understood to mean that when a particle moving in the interior of the cube reaches a point on a face, it goes through it and appears to come forth from the corresponding point on the opposite face. (After gluing the first pair of opposite faces the cube looks like a thick washer, after gluing the second pair the flat faces of the washer it looks like a hollow torus, the last gluing the inner surface of the hollow torus to the outer surface is physically impossible in three-dimensional space so it has to happen in four dimensions.

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