Projection onto a closed Hilbert subspace
Research context
postgraduate concept · established · As of 2026-10-10
For a closed linear subspace M of a Hilbert space H, every x has a unique decomposition x=P-M x+z with P-M x in M and z orthogonal to M.
Key knowledge
For a closed linear subspace M of a Hilbert space H, every x has a unique decomposition x=P-M x+z with P-M x in M and z orthogonal to M. P-M x is the unique nearest point in M; P-M is linear, idempotent and contractive. Closedness and completeness ensure the distance infimum is attained.
Values and equations
H = M ⊕ M-perp; P_M²=P_M; ||P_M x|| ≤ ||x||
- H
- Hilbert space
- M
- Closed linear subspace
- P_M
- Orthogonal projection
Conditions and scope
- M is a closed linear subspace; projection onto an arbitrary nonconvex subset need not be unique.
- For a nonzero M, the projection norm equals one; for M={0} it is zero.
Sources · 1
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Evidence and review
Author checked this English statement, displayed equation where present and stated hypotheses against these source locations: 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
Permanent link to this entry ↗- Record identity
- research-math-closed-subspace-projection
- Revision
- research.1
- Structured reference record
- Postgraduate & research reference