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The many proofs of an identity on the norm of oblique projections
Author(s) -
Daniel B. Szyld
Publication year - 2006
Publication title -
numerical algorithms
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.981
H-Index - 64
eISSN - 1572-9265
pISSN - 1017-1398
DOI - 10.1007/s11075-006-9046-2
Subject(s) - mathematics , mathematical proof , theory of computation , hilbert space , oblique case , norm (philosophy) , identity (music) , algebra over a field , pure mathematics , calculus (dental) , algorithm , geometry , epistemology , linguistics , dentistry , acoustics , medicine , philosophy , physics
Given an oblique projector P on a Hilbert space, i.e., an operator satisfying P 2=P, which is neither null nor the identity, it holds that ||P|| = ||I –P||. This useful equality, while not widely-known, has been proven repeatedly in the literature. Many published proofs are reviewed, and simpler ones are presented.

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