An isometry of a space with a form between subspaces of a finite-dimensional nonsingular classical space extends to an ambient isometry. This includes alternating bilinear forms in all characteristics, symmetric bilinear forms in characteristic different from two, Hermitian forms with nontrivial involution, and quadratic forms with nonsingular polar form. No nonsingularity of the restricted subspace is required. Bare nonalternating symmetric bilinear forms in characteristic two require additional hypotheses and must not be confused with quadratic spaces.
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Witt's theorem is an important result in the theory of quadratic forms in mathematics, specifically in the context of algebraic groups and linear algebra over fields. It provides a characterization of the equivalence of quadratic forms over fields. In simpler terms, Witt's theorem states that any two non-degenerate quadratic forms over a field can be transformed into each other by means of an appropriate change of variables, if and only if they have the same "Witt index" and the same "discriminant".