Linked from

The 5 pages that link to Parseval's identity, each with the reason it gives.

• Orthonormal basisRelated: It expresses preservation of energy across a complete orthonormal expansion.

• Inner product spaceRelated: It expresses inner-product length through orthonormal coordinates.

• Fuglede's conjectureRelated: It connects completeness of an exponential family to Fourier energy on the set.