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In [[operator theory]], an area of mathematics, '''Douglas' lemma''' relates [[Matrix decomposition|factorization]], range inclusion, and majorization <!-- [[Majorization]] discusses a different topic, so is not currently appropriate to link to--> of [[Hilbert space]] operators. It is generally attributed to [[Ronald G. Douglas]], although Douglas acknowledges that aspects of the result may already have been known. The statement of the result is as follows: | |||
'''Theorem''': If ''A'' and ''B'' are [[bounded operator]]s on a Hilbert space ''H'', the following are equivalent: | |||
# <math>\text{im}(A)\subseteq\text{im}(B).\,</math> | |||
# <math>AA^*\leq\lambda^2 BB^*</math> for some <math>\lambda\geq 0.\,</math> | |||
# There exists a bounded operator ''C'' on ''H'' such that ''A'' = ''BC''. | |||
Moreover, if these equivalent conditions hold, then there is a unique operator ''C'' such that | |||
* <math>\Vert C \Vert^2=\inf\{\mu :\,AA^*\leq\mu BB^*\}.</math> | |||
* ker(''A'') = ker(''C'') | |||
* <math>\text{im}(C)\subseteq\overline{\text{im}(B^*)}</math> | |||
==See also== | |||
[[Positive operator]] | |||
==References== | |||
*Douglas, R.G.: "On Majorization, Factorization, and Range Inclusion of Operators on Hilbert Space". ''Proceedings of the American Mathematical Society'' '''17''', 413–415 (1966) | |||
{{mathanalysis-stub}} | |||
[[Category:Operator theory]] | |||
Revision as of 18:34, 24 January 2014
In operator theory, an area of mathematics, Douglas' lemma relates factorization, range inclusion, and majorization of Hilbert space operators. It is generally attributed to Ronald G. Douglas, although Douglas acknowledges that aspects of the result may already have been known. The statement of the result is as follows:
Theorem: If A and B are bounded operators on a Hilbert space H, the following are equivalent:
- for some
- There exists a bounded operator C on H such that A = BC.
Moreover, if these equivalent conditions hold, then there is a unique operator C such that
- ker(A) = ker(C)
See also
References
- Douglas, R.G.: "On Majorization, Factorization, and Range Inclusion of Operators on Hilbert Space". Proceedings of the American Mathematical Society 17, 413–415 (1966)