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The 25 pages that link to Quadratic form, each with the reason it gives.
Cauchy–Schwarz inequalityRelated: The proof studies the nonnegative quadratic ‖x−ty‖² as t varies.
Metric tensorRelated: Evaluating the metric twice on a vector gives its squared length.
Hessian matrixRelated: The Hessian defines the quadratic curvature term in a local expansion.
Symmetric matrixRelated: The symmetric part of a matrix alone determines its quadratic form.
Geometric algebraRelated: Its values set the metric relations built into a Clifford algebra.