In elementary geometry
, the parallelogram law
states that the sum of the squares of the lengths of the fours sides of a parallelogram equals the sum of the squares of the lengths of the two diagonals. In case the parallelogram is a rectangle, the two diagonals are of equal lengths and the statement reduces to the Pythagorean theorem
. But in general, the square of the length of neither
diagonal is the sum of the squares of the lengths of two sides.
In inner product spaces, the statement of the parallelogram law reduces to the algebraic identity
Most normed vector spaces do not have inner products, but all normed vector spaces have norms (hence the name), and thus one can evaluate the expressions on both sides of "=" in the identity above. A remarkable fact is that the identity above holds only
if the norm is one that arises in the usual way from an inner product, because, if the identity above holds, then the function
is an inner product whose norm is precisely this one.