See (in French) http://fr.wikipedia.org/wiki/Bourse_de_Fortunatus:

The gold sack of Fortunatus is the name given to a mathematical object, similar to the Möbius strip but with one more dimension (which makes it impossible to construct in our 3-dimensional world). Like the Möbius strip, it is one-sided, the exterior and interior faces of the sack of Fortunatus coincide (we can go from one to the other without crossing the boundary), or, in a more symbolical interpretation, since the world outside the sack is also in it, it contains the whole world.

See 3-dimensional manifold.
 
 
 

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© Robert FERRÉOL  2017