Bourbaki

Proposition 4

Tag 0DN5, permanent. Cite it as [Bourbaki, Tag 0DN5].

Algebra, chapter IX, § 8. Types de formes quadratiques

*Let Q be a non-degenerate quadratic form on a finite-dimensional vector space E over A. Suppose that A is of characteristic ≠2\neq 2, and let (x1,…,xn)(x_1, \ldots, x_n) be an orthogonal basis of V. Let T1T_1 be the type of the quadratic form Q1Q_1 defined on the vector space A and such that Q1(1)=1Q_1(1) = 1. The type of Q is then ∑i=1nQ(xi)⋅T1\sum_{i=1}^n Q(x_i) \cdot T_1.

Indeed the form Q is equivalent to

(Q(x1)Q1)τ…τ(Q(xn)Q1)(Q(x_1)Q_1) \tau \ldots \tau (Q(x_n)Q_1)

Bourbaki puts the proof directly after the statement, so what is above is the statement and its proof together.

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