The Arithmetic of Quadratic Forms
We have already determined the integers which can be represented as a sum of two squares. Similarly, one may ask which integers can be represented in the form x 2 + 2y 2 or, more generally, in the form ax 2 + 2bxy + cy 2, where a, b, c are given integers. The arithmetic theory of binary quadratic forms, which had its origins in the work of Fermat, was extensively developed during the 18th century by Euler, Lagrange, Legendre and Gauss. The extension to quadratic forms in more than two variables, which was begun by them and is exemplified by Lagrange’s theorem that every positive integer is a sum of four squares, was continued during the 19th century by Dirichlet, Hermite, H.J.S. Smith, Minkowski and others. In the 20th century Hasse and Siegel made notable contributions. With Hasse’s work especially it became apparent that the theory is more perspicuous if one allows the variables to be rational numbers, rather than integers. This opened the way to the study of quadratic forms over arbitrary fields, with pioneering contributions by Witt (1937) and Pfister (1965–67).
KeywordsQuadratic Form Hyperbolic Plane Isotropic Subspace Quadratic Space Isotropic Vector
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