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Examples

• Over the real numbers, the form x2 in one variable is not universal, as it cannot represent negative numbers: the two-variable form x2 − y2 over R is universal.
• Lagrange's four-square theorem states that every positive integer is the sum of four squares. Hence the form x2 + y2 + z2 + t2 − u2 over Z is universal.
• Over a finite field, any non-singular quadratic form of dimension 2 or more is universal.³

Forms over the rational numbers

The Hasse–Minkowski theorem implies that a form is universal over Q if and only if it is universal over Qp for all p (where we include p = ∞, letting Q∞ denote R).⁴ A form over R is universal if and only if it is not definite; a form over Qp is universal if it has dimension at least 4.⁵ One can conclude that all indefinite forms of dimension at least 4 over Q are universal.⁶

See also

• The 15 and 290 theorems give conditions for a quadratic form to represent all positive integers.

• Lam, Tsit-Yuen (2005). Introduction to Quadratic Forms over Fields. Graduate Studies in Mathematics. Vol. 67. American Mathematical Society. ISBN 0-8218-1095-2. MR 2104929. Zbl 1068.11023.
• Rajwade, A. R. (1993). Squares. London Mathematical Society Lecture Note Series. Vol. 171. Cambridge University Press. ISBN 0-521-42668-5. Zbl 0785.11022.
• Serre, Jean-Pierre (1973). A Course in Arithmetic. Graduate Texts in Mathematics. Vol. 7. Springer-Verlag. ISBN 0-387-90040-3. Zbl 0256.12001.

References

1. Lam (2005) p.10 ↩

2. Rajwade (1993) p.146 ↩

3. Lam (2005) p.36 ↩

4. Serre (1973) p.43 ↩

5. Serre (1973) p.37 ↩

6. Serre (1973) p.43 ↩