# Documentation

Mathlib.Analysis.InnerProductSpace.OfNorm

# Inner product space derived from a norm #

This file defines an InnerProductSpace instance from a norm that respects the parallellogram identity. The parallelogram identity is a way to express the inner product of x and y in terms of the norms of x, y, x + y, x - y.

## Main results #

• InnerProductSpace.ofNorm: a normed space whose norm respects the parallellogram identity, can be seen as an inner product space.

## Implementation notes #

We define inner_

$$\langle x, y \rangle := \frac{1}{4} (‖x + y‖^2 - ‖x - y‖^2 + i ‖ix + y‖ ^ 2 - i ‖ix - y‖^2)$$

and use the parallelogram identity

$$‖x + y‖^2 + ‖x - y‖^2 = 2 (‖x‖^2 + ‖y‖^2)$$

to prove it is an inner product, i.e., that it is conjugate-symmetric (inner_.conj_symm) and linear in the first argument. add_left is proved by judicious application of the parallelogram identity followed by tedious arithmetic. smul_left is proved step by step, first noting that $\langle λ x, y \rangle = λ \langle x, y \rangle$ for $λ ∈ ℕ$, $λ = -1$, hence $λ ∈ ℤ$ and $λ ∈ ℚ$ by arithmetic. Then by continuity and the fact that ℚ is dense in ℝ, the same is true for ℝ. The case of ℂ then follows by applying the result for ℝ and more arithmetic.

## TODO #

Move upstream to Analysis.InnerProductSpace.Basic.

• [Jordan, P. and von Neumann, J., On inner products in linear, metric spaces][Jordan1935]
• https://math.stackexchange.com/questions/21792/norms-induced-by-inner-products-and-the-parallelogram-law
• https://math.dartmouth.edu/archive/m113w10/public_html/jordan-vneumann-thm.pdf

## Tags #

inner product space, Hilbert space, norm