Bourbaki

Proposition 5

Tag 0DN7, permanent. Cite it as [Bourbaki, Tag 0DN7].

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

*We suppose that A is a maximal ordered field. Let Q be a non-degenerate quadratic form of finite dimension over A, and (s,t)(s, t) its signature (§7§ 7, no 2, def. 2). Then the type of Q is (s−t).T1(s - t).T_1, and the group W\mathfrak{W} of types of quadratic forms over A is an infinite monogenic group generated by T1T_1.

Indeed, since A∗/(A∗)2A^*/(A^*)^2 is of order 2 and (−1).T1=−T1(-1).T_1 = -T_1, W\mathfrak{W} is generated by T1T_1 and is therefore monogenic. For every n>0n > 0, n.T1n.T_1 is the type of the positive non-degenerate quadratic forms of dimension nn; since these forms are not neutral, we have n.T1≠0n.T_1 \neq 0, which shows that W\mathfrak{W} is infinite. Finally, a form of signature (s,t)(s, t) is isomorphic, with the notation of prop. 4, to the direct sum of ss forms Q1Q_1 and of tt forms −Q1- Q_1 (§ 7, n° 2, th. 1); it follows that its type is (s−t).T1(s - t). T_1.

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

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