Definition:Quadratic Form

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Let $\mathbb K$ be a field of characteristic $\Char {\mathbb K} \ne 2$.

Let $V$ be a vector space over $\mathbb K$.

A quadratic form on $V$ is a mapping $q : V \mapsto \mathbb K$ such that:

$\forall v \in V : \forall \kappa \in \mathbb K : \map q {\kappa v} = \kappa^2 \map q v$
$b: V \times V \to \mathbb K: \tuple {v, w} \mapsto \map q {v + w} - \map q v - \map q w$ is a bilinear form

Also defined as

A quadratic form is a homogeneous polynomial of degree $2$.


$x^2 + 2 x y - 3 y^2 + 4 x z$

is a quadratic form in the variables $x$, $y$ and $z$.

Also see