In number theory, Meyer's theorem on quadratic forms states that an indefinite quadratic form Q in five or more variables over the field of rational numbers nontrivially represents zero. In other words, if the equation
- Q(x) = 0
has a non-zero real solution, then it has a non-zero rational solution (the converse is obvious). By clearing the denominators, an integral solution x may also be found.
Meyer's theorem is usually deduced from the Hasse–Minkowski theorem (which was proved later) and the following statement:
- A rational quadratic form in five or more variables represents zero over the field Qp of the p-adic numbers for all p.
Meyer's theorem is best possible with respect to the number of variables: there are indefinite rational quadratic forms Q in four variables which do not represent zero. One family of examples is given by
- Q(x1,x2,x3,x4) = x12 + x22 − p(x32 + x42),
where p is a prime number that is congruent to 3 modulo 4. This can be proved by the method of infinite descent using the fact that if the sum of two perfect squares is divisible by such a p then each summand is divisible by p.
- Meyer, A. (1884). "Mathematische Mittheilungen". Vierteljahrschrift der Naturforschenden Gesellschaft in Zürich 29: 209–222.
- Milnor, J.; Husemoller, D. (1973). Symmetric Bilinear Forms. Ergebnisse der Mathematik und ihrer Grenzgebiete. 73. Springer-Verlag. ISBN 3-540-06009-X.
- Serre, Jean-Pierre (1973). A Course in Arithmetic. Graduate Texts in Mathematics. 7. Springer-Verlag. ISBN 0-387-90040-3. https://archive.org/details/courseinarithmet00serr.
- Cassels, J.W.S. (1978). Rational Quadratic Forms. London Mathematical Society Monographs. 13. Academic Press. ISBN 0-12-163260-1.
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