A cylinder
If we take a long and narrow rectangle and paste together the two shortest sides after having impressed two half a torsions, we get an anomalous cylinder: the boundary is made of two crossed circumpherences, as in a Hopf link. This is not the case of a common cylinder.
© matematita
immagine di Gian Marco Todesco
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From "fragments of topology" (From "XlaTangente")
Further information:
http://www.toonz.com/personal/todesco