Bourbaki

Corollary 1

Tag 0DL4, permanent. Cite it as [Bourbaki, Tag 0DL4].

Algebra, chapter IX, § 5. Propriétés spéciales aux formes bilinéaires alternées

*Let AA be a commutative field, EE a finite-dimensional vector space of dimension nn over AA, and Φ\Phi an alternating bilinear form on E. There then exists a basis (ei)1≤i≤n(e_i)_{1 \leq i \leq n} of E and a pair integer 2r≤n2r \leq n such that

Φ(∑i=1nξiei,∑i=1nηiei)=∑j=1r(ξ2j−1η2j−ξ2jη2j−1). \Phi\left( \sum_{i=1}^n \xi_i e_i, \sum_{i=1}^n \eta_i e_i \right) = \sum_{j=1}^r (\xi_{2j-1} \eta_{2j} - \xi_{2j} \eta_{2j-1}).

In particular Φ\Phi is of pair rank 2r2r.

A basis satisfying (2) is called a symplectic basis for Φ\Phi.

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

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