Not quite regular polyhedra - MathOverflow most recent 30 from http://mathoverflow.net 2013-06-19T01:34:29Z http://mathoverflow.net/feeds/question/75308 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/75308/not-quite-regular-polyhedra Not quite regular polyhedra Edmund Harriss 2011-09-13T13:03:15Z 2012-05-18T21:24:42Z <p>Take a naive interpretation of regular polyhedra:</p> <p>All vertices (including epsilon ball) congruent</p> <p>All edges congruent</p> <p>All faces congruent</p> <p>We can now find interesting families by removing one requirement. For example the uniform polyhedra have all vertices and edges congruent, but not all faces, and their duals have faces and edges congruent, but not vertices. </p> <p>Are there examples, or interesting families, of polyhedra where every pair of faces is congruent and every pair of vertices, but not every pair of edges?</p> http://mathoverflow.net/questions/75308/not-quite-regular-polyhedra/75311#75311 Answer by Douglas Zare for Not quite regular polyhedra Douglas Zare 2011-09-13T13:44:08Z 2011-09-13T13:44:08Z <p>Rhombic <a href="http://en.wikipedia.org/wiki/Disphenoid" rel="nofollow">disphenoids</a> are examples of polyhedra with identical vertices and faces, but distinguishable edges. These are irregular tetrahedra whose faces are scalene triangles. Their symmetry groups are isomorphic to \$C_2 \oplus C_2\$, and act transitively on the faces and vertices. You can make a disphenoid from an acute triangle by folding along the line segments connecting the midpoints of the sides. </p> <p>All tetrahedra whose sides have equal area are disphenoids. Also, ideal hyperbolic tetrahedra have the same symmetries as a disphenoid.</p> http://mathoverflow.net/questions/75308/not-quite-regular-polyhedra/76355#76355 Answer by Edmund Harriss for Not quite regular polyhedra Edmund Harriss 2011-09-25T18:42:20Z 2012-05-18T20:57:00Z <p>It turns out that these polyhedra that have congruent vertices and faces have a name. They are the <em>Noble Polyhedra</em>. If one insists that they also be convex the Noble polyhedra are the regular polyhedra plus the <a href="http://en.wikipedia.org/wiki/Disphenoid" rel="nofollow">disphenoids</a> mentioned in Douglas Zare's answer. </p> <p>When one allows intersecting faces, however, new collections turn up, such as the stephanoids, originally studied by Max Brüker:</p> <p>Max Brückner <em>Uber die gleichecking-gleichflachigen, diskontinuierlichen und nichtkonvexen Polyheder</em><br> Nova Acta Leop. 86(1906), No. 1, pp. 1 – 348 + 29 plates.<br> <a href="http://bulatov.org/polyhedra/bruckner1906/index.html" rel="nofollow">Images of the plates with pictures of the models.</a></p> <p>These shapes are also discussed and further developed by Branko Grünbaum:</p> <p>Polyhedra with hollow faces<br> <em>Proc. NATO-ASI Conf. on polytopes: abstract, convex and computational</em>, Toronto 1983, Ed. Bisztriczky, T. Et Al., Kluwer Academic (1994), p 43-70.</p> <p>Grünbaum's constructions do use generalisations of the definition of polyhedra. For a thorough discussion of these (including having polygons return to the same vertex, and coplanar faces) see the following paper, which also has a discussion of Noble Polyhedra. </p> <p>Grünbaum, B. Are your polyhedra the same as my polyhedra?<br> <em>Discrete and Computational Geometry: The Goodman-Pollack Festschrift</em> B. Aronov, S. Basu, J. Pach, and Sharir, M., eds. Springer, New York 2003, pp. 461 – 488.<br> <a href="http://www.math.washington.edu/~grunbaum/Your%20polyhedra-my%20polyhedra.pdf" rel="nofollow">http://www.math.washington.edu/~grunbaum/Your%20polyhedra-my%20polyhedra.pdf</a></p> <p>Interestingly the classification of all Noble Polyhedra is still an open problem.</p> http://mathoverflow.net/questions/75308/not-quite-regular-polyhedra/97354#97354 Answer by Patricia Hersh for Not quite regular polyhedra Patricia Hersh 2012-05-18T21:24:42Z 2012-05-18T21:24:42Z <p>Allan Edmonds has a few papers studying "equifacetal simplices", i.e. simplices in which any two facets must be congruent. Here are references:</p> <p>Edmonds, Allan, The center conjecture for equifacetal simplices. Adv. Geom. 9 (2009), no. 4, 563--576.</p> <p>Edmonds, Allan L., The partition problem for equifacetal simplices. Beitrage Algebra Geom. 50 (2009), no. 1, 195-213.</p> <p>Edmonds, Allan, The geometry of an equifacetal simplex. Mathematika 52 (2005), no. 1-2, 31-45.</p>