Mathematical identities | Abstract algebra | Functional analysis | Linear algebra | Norms (mathematics) | Vectors (mathematics and physics)
Polarization identity
In linear algebra, a branch of mathematics, the polarization identity is any one of a family of formulas that express the inner product of two vectors in terms of the norm of a normed vector space. If a norm arises from an inner product then the polarization identity can be used to express this inner product entirely in terms of the norm. The polarization identity shows that a norm can arise from at most one inner product; however, there exist norms that do not arise from any inner product. The norm associated with any inner product space satisfies the parallelogram law: In fact, as observed by John von Neumann, the parallelogram law characterizes those norms that arise from inner products. Given a normed space , the parallelogram law holds for if and only if there exists an inner product on such that for all in which case this inner product is uniquely determined by the norm via the polarization identity. (Wikipedia).
