Which platonic solids can form a topological torus? - MathOverflow most recent 30 from http://mathoverflow.net 2013-05-22T10:46:48Z http://mathoverflow.net/feeds/question/53601 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/53601/which-platonic-solids-can-form-a-topological-torus Which platonic solids can form a topological torus? fastforward 2011-01-28T11:44:32Z 2011-01-31T21:05:27Z <p>8 cubes can be joined face-to-face to form a closed ring with a hole in it, with each cube sharing a face with only two others. The same can be done with 8 dodecahedrons. Is the same possible with the other platonic solids? What is the minimum number of each solid needed to form such a loop? </p> <p>Given a convex regular d-polytope, is there a general way to determine if it can form a loop, by gluing their d-1 dimensional faces together? (Assuming the loop has a hole and no two objects intersecting, ie no two objects share a d-volume).<br> And is there a way to compute the minimum number of equal polytopes needed for this?</p> <p><img src="http://i.imgur.com/JfQZU.gif" alt="alt text"></p> <p>Edit: added image</p> <p>I am also looking for software that can be used to check for atleast small N</p> http://mathoverflow.net/questions/53601/which-platonic-solids-can-form-a-topological-torus/53658#53658 Answer by Tracy Hall for Which platonic solids can form a topological torus? Tracy Hall 2011-01-28T22:12:21Z 2011-01-28T22:41:50Z <p>All but the tetrahedron.</p> <p>As noted in a comment, ultimately referencing a 1972 paper, for tetrahedra this cannot be done. I haven't looked at the paper, but the proof may go as follows: Let the vertices of your base tetrahedron be the standard basis vectors in $\mathbb R^4$ times 4, so their center point is the all-ones vector. Construct the four matrices which reflect through the faces of your tetrahedron (while fixing the sum-to-4 hyperplane). Observe that some entries of these matrices equal $\frac23$. Apply a reduced word in the reflection group to the all-ones vector, and prove by induction that the first generator in the word is always indicated by which entry has the lowest power of 3 in the denominator, with a predictable pattern mod 3 in the numerators. Conclude that no nontrivial product of reflections takes the center point back to itself.</p> <p>For the octahedron, attaching a pair along opposite faces allows you to continue in the pattern of carbon atoms in the diamond crystal structure, where each atom has tetrahedral bonds in directions all four of which are the negatives of any neighbor's bonds. It is thus possible to glue together 12 octahedra positioned like the carbons and C-C bonds of the "chair" conformation of cyclohexane. Since eight of the twenty faces of an icosahedron are inclined like the faces of an octahedron, the exact same arrangement is possible with icosahedra.</p> <p>EDIT: In fact, you can do better. Given any polytope that has two pairs of opposite parallel facets such that reflected polytopes may be attached to all four facets simultaneously without overlapping, a thickened parallelogram may be constructed by attaching eight identical polytopes along facets. This works because a pair of parallel reflections amounts to a pure translation, making the same possible attachment directions available at both ends of the double reflection. </p> http://mathoverflow.net/questions/53601/which-platonic-solids-can-form-a-topological-torus/53916#53916 Answer by Dave Rusin for Which platonic solids can form a topological torus? Dave Rusin 2011-01-31T21:05:27Z 2011-01-31T21:05:27Z <p>A long time ago a similar question arose in the sci.math newsgroup. I worked it out that in order to make a loop out of dodecahedra, one has to use sets of three dodecahedra in a row, as shown (four times) in the picture above -- otherwise one introduces a set of rotations in R^3 which has no relations which we can use to get the first and last faces to line up. See <a href="http://www.math-atlas.org/98/dodec_prf" rel="nofollow">http://www.math-atlas.org/98/dodec_prf</a></p> <p>dave</p>