= Witt's theorem
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= Witt extension theorem
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= Witt's lemma
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= Witt lemma
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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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