Hilbert spaces and orthonormal expansions
Research context
postgraduate concept · established · As of 2026-10-10
A Hilbert space is complete in the norm induced by a real or complex inner product.
Key knowledge
A Hilbert space is complete in the norm induced by a real or complex inner product. For a countable orthonormal basis, each vector has a norm-convergent expansion and Parseval equates its squared norm to the sum of squared coefficients. Countability is a separability assumption; an arbitrary Hilbert space need not possess a countable basis.
Values and equations
x = sum_n <x,e_n> e_n; ||x||² = sum_n |<x,e_n>|²
- x
- Vector in a separable Hilbert space
- e_n
- Complete orthonormal basis
- <·,·>
- Inner product linear in its first argument
Conditions and scope
- Inner product convention here is linear in its first argument; expansion coefficients are <x,e-n>.
- Completeness of the orthonormal system, rather than mere orthogonality, is needed for equality in Parseval.
Sources · 2
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Massachusetts Institute of Technology2 sources
Evidence and review
Author checked this English statement, displayed equation where present and stated hypotheses against these source locations: Lecture 14: completeness, inner product and Cauchy–Schwarz; Lecture 17, minimizer theorem, orthogonal projection and Theorem 184. The check is bounded to the recorded claims; no complete proof reconstruction, software proof replay or universal literature completeness is claimed.
2026-10-10 · Author source-body/statement/equation/domain check, with graph associations scoped individually. Separate independent content/source/inventory review is required before branch closure.
Entry details
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- Postgraduate & research reference