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Oberseminar SoSe 2010 Zusammenfassung des Vortrags von N. D. Elkies (Harvard, USA)

On the areas of rational triangles

By a "rational triangle" we mean a plane triangle whose sides are rational numbers. By Heron's formula, there exists such a triangle of area  a  if and only if a > 0 and x y z (x + y + z) = a  for some rationals x, y, z. In a 1749 letter to Goldbach, Euler constructed infinitely many such (x, y, z) for any rational a (positive or not), remarking that it cost him much effort, but not explaining his method. We suggest one approach, using only tools available to Euler, that he might have taken, and use this approach to construct several other infinite families of solutions. We then reconsider the problem as a question in arithmetic geometry: x y z (x + y + z) = a  gives a K3 surface, and each family of solutions is a singular rational curve on that surface defined over Q. The structure of that Neron-Severi group of that K3 surface explains why the problem is unusually hard. Along the way we also encounter the Niemeier lattices (the even unimodular lattices in R24) and the non-Hamiltonian Petersen graph.