In mathematics, a universal quadratic form is a quadratic form over a ring that represents every element of the ring.[1] A non-singular form over a field which represents zero non-trivially is universal.[2]

Examples

• Over the real numbers, the form x2 in one variable is not universal, as it cannot represent negative numbers: the two-variable form x2y2 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 + t2u2 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.[4]