Joseph Malkevitch
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 Apr 21 answered How many faces of a solid can one “see”? Jan 29 awarded Nice Answer Dec 29 comment Past open problems with sudden and easy-to-understand solutions Fisk's proof is based on the fact that a simple plane polygon can be triangulated using diagonals between existing vertices. The fact that this is possible to not that hard (though there were incorrect proofs in the past) but an "easier" proof using the notion of an ear of a polygon was developed by Gary Meisters, who died about a month ago. Meisters' showed that polygons with 4 or more vertices have at least two ears made it possible to use induction to prove that simple polygons can be triangulated: legacy.com/obituaries/coloradoan/… Dec 29 comment Past open problems with sudden and easy-to-understand solutions The original question was asked by Sylvester and years later Erdos asked the question in the dual form - the problem can be thought of as being in the projective plane. Tibor Gallai provided an answer and later Kelly a very elegant metrical proof. However, there was an even simpler proof buried in Deutsche Mathematik, published by the Nazis. This proof by Eberhard Melchior was a combinatorial proof more in keeping with Sylvester's original setting. More details are here: en.wikipedia.org/wiki/Sylvester%E2%80%93Gallai_theorem Dec 24 answered Past open problems with sudden and easy-to-understand solutions Aug 20 awarded Yearling Jul 11 answered Extension of Descartes' “Kissing Circles” Theorem Sep 27 comment Good Reference for Justifying (less well-known fields of) Math? This is a wonderfully rich book which looks at mathematics from many perspectives and, thus, gives a picture of the subject that while having a certain "random" quality is also very nuanced. As one reads more of the specialized essays one comes to see the way different aspects of the subject, from its theory to its applications, to the extraordinary people from many countries and cultures who contributed to it, make mathematics an exciting and wonderfully rich subject. Sep 24 awarded Autobiographer Sep 12 comment Periodicity with irrational numbers You might look here: en.wikipedia.org/wiki/Beatty_sequence Sep 5 awarded Nice Answer Aug 26 answered Knots and graphs Aug 20 awarded Yearling Jul 24 answered Mathematical conjectures believed to be false Jul 19 comment Space-filling polyhedra (or honeycomb) survey? For tetrahedra, have you seen: Senechal, Marjorie. "Which tetrahedra fill space?." Mathematics Magazine (1981): 227-243. Jul 10 answered Why do people lose in chess? Jun 24 answered What is the realization of a graph in $\mathbb{R}^d$? Jun 23 answered Why is the 24-cell (also called Icositetrachoron or Hyperdiamond) the unique regular convex polychoron which has no direct three-dimensional analog? Jun 21 answered Order Types, Point-line duality Jun 7 answered Applications of Finite Projective Planes