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The | In [[operator theory]], a discipline within mathematics, a [[bounded operator]] ''T'': ''X'' → ''Y'' between [[normed vector space]]s ''X'' and ''Y'' is said to be a '''contraction''' if its [[operator norm]] ||''T''|| ≤ 1. Every bounded operator becomes a contraction after suitable scaling. The analysis of contractions provides insight into the structure of operators, or a family of operators. The theory of contractions on [[Hilbert space]] is largely due to [[Béla Szőkefalvi-Nagy]] and [[Ciprian Foias]]. | ||
== Contractions on a Hilbert space == | |||
{{redirect|Operator angle||Camera angle}} | |||
If ''T'' is a contraction acting on a [[Hilbert space]] <math>\mathcal{H}</math>, the following basic objects associated with ''T'' can be defined. | |||
The '''defect operators''' of ''T'' are the operators ''D<sub>T</sub>'' = (1 − ''T*T'')<sup>½</sup> and ''D<sub>T*</sub>'' = (1 − ''TT*'')<sup>½</sup>. The square root is the [[square root of a matrix|positive semidefinite one]] given by the [[spectral theorem]]. The '''defect spaces''' <math>\mathcal{D}_T</math> and <math>\mathcal{D}_{T*}</math> are the ranges Ran(''D<sub>T</sub>'') and Ran(''D<sub>T*</sub>'') respectively. The positive operator ''D<sub>T</sub>'' induces an inner product on <math>\mathcal{H}</math>. The inner product space can be identified naturally with Ran(''D''<sub>''T''</sub>). A similar statement holds for <math>\mathcal{D}_{T*}</math>. | |||
The '''defect indices''' of ''T'' are the pair | |||
:<math>(\dim\mathcal{D}_T, \dim\mathcal{D}_{T^*}).</math> | |||
The defect operators and the defect indices are a measure of the non-unitarity of ''T''. | |||
A contraction ''T'' on a Hilbert space can be canonically decomposed into an orthogonal direct sum | |||
:<math>T = \Gamma \oplus U</math> | |||
where ''U'' is a unitary operator and Γ is ''completely non-unitary'' in the sense that it has no [[invariant subspace|reducing subspaces]] on which its restriction is unitary. If ''U'' = 0, ''T'' is said to be a '''completely non-unitary contraction'''. A special case of this decomposition is the [[Wold decomposition]] for an [[isometry]], where Γ is a proper isometry. | |||
Contractions on Hilbert spaces can be viewed as the operator analogs of cos θ and are called '''operator angles''' in some contexts. The explicit description of contractions leads to (operator-)parametrizations of positive and unitary matrices. | |||
==Dilation theorem for contractions== | |||
[[Sz.-Nagy's dilation theorem]], proved in 1953, states that for any contraction ''T'' on a Hilbert space ''H'', there is a [[unitary operator]] ''U'' on a larger Hilbert space ''K'' ⊇ ''H'' such that if ''P'' is the orthogonal projection of ''K'' onto ''H'' then ''T''<sup>''n''</sup> = ''P'' ''U''<sup>''n''</sup> ''P'' for all ''n'' > 0. The operator ''U'' is called a [[dilation (operator theory)|dilation]] of ''T'' and is uniquely determined if ''U'' is mininal, i.e. ''K'' is the smallest closed subspace invariant under ''U'' and ''U''* containing ''H''. | |||
In fact define<ref>{{harvnb|Sz.-Nagy|Foias|Bercovici|Kérchy|2010|pp=10–14}}</ref> | |||
:<math>\displaystyle{\mathcal{H}=H\oplus H\oplus H \oplus \cdots ,}</math> | |||
the orthogonal direct sum of countably many copies of ''H''. | |||
Let ''V'' be the isometry on <math>\mathcal H</math> defined by | |||
:<math>\displaystyle{V(\xi_1,\xi_2,\xi_3,\dots)=(T\xi_1, \sqrt{I-T^*T}\xi_1,\xi_2,\xi_3,\dots).}</math> | |||
Let | |||
:<math>\displaystyle{\mathcal{K}=\mathcal{H} \oplus \mathcal{H}.}</math> | |||
Define a unitary ''W'' on <math>\mathcal K</math> by | |||
:<math>\displaystyle{W(x,y)=(Vx+(I-VV^*)y,-V^*y).}</math> | |||
''W'' is then a unitary dilation of ''T'' with ''H'' considered as the first component of <math>\mathcal{H}\subset \mathcal{K}</math>. | |||
The minimal dilation ''U'' is obtained by taking the restriction of ''W'' to the closed subspace generated by powers of ''W'' applied to ''H''. | |||
==Dilation theorem for contraction semigroups== | |||
There is an alternative proof of Sz.-Nagy's dilation theorem, which allows significant generalisations.<ref>{{harvnb|Sz.-Nagy|Foias|Bercovici|Kérchy|2010|pp=24–28}}</ref> | |||
Let ''G'' be a group, ''U''(''g'') a unitary representation of ''G'' on a Hilbert space ''K'' and ''P'' an orthogonal projection onto a closed subspace ''H'' = ''PK'' of ''K''. | |||
The operator-valued function | |||
:<math>\displaystyle{\Phi(g)=PU(g)P,}</math> | |||
with values in operators on ''K'' satisfies the positive-definiteness condition | |||
:<math> \sum \lambda_i\overline{\lambda_j} \Phi(g_j^{-1}g_i) = PT^*TP\ge 0,</math> | |||
where | |||
:<math>\displaystyle{T=\sum \lambda_i U(g_i).}</math> | |||
Moreover | |||
:<math>\displaystyle{\Phi(1)=P.}</math> | |||
Conversely, every operator-valued positive-definite function arises in this way. Recall that every (continuous) scalar-valued positive-definition function on a topological group is of the form φ(''g'') = 〈''U<sub>g</sub> v'', ''v''〉 where ''U<sub>g</sub>'' is a (strongly continuous) unitary representation (see [[Bochner's theorem]]). Replacing ''v'', a rank-1 projection, by a general projection gives the operator-valued statement. In fact the construction is identical; this is sketched below. | |||
Let <math>\mathcal H</math> be the space of functions on ''G'' of finite support with values in ''H'' with inner product | |||
:<math>\displaystyle{(f_1,f_2)=\sum_{g,h} (\Phi(h^{-1}g)f_1(g),f_2(h)).}</math> | |||
''G'' acts unitarily on <math>\mathcal H</math> by | |||
:<math>\displaystyle{U(g)f(x)=f(g^{-1}x).}</math> | |||
Moreover ''H'' can be identified with a closed subspace of <math>\mathcal H</math> using the isometric embedding | |||
sending ''v'' in ''H'' to ''f''<sub>''v''</sub> with | |||
:<math>f_v(g)=\delta_{g,1} v. \, </math> | |||
If ''P'' is the projection of <math>\mathcal H</math> onto ''H'', then | |||
:<math>\displaystyle{PU(g)P=\Phi(g),}</math> | |||
using the above identification. | |||
When ''G'' is a separable topological group, Φ is continuous in the strong (or weak) operator topology if and only if ''U'' is. | |||
In this case functions supported on a countable dense subgroup of ''G'' are dense in <math>\mathcal H</math>, so that <math>\mathcal H</math> is separable. | |||
When ''G'' = '''Z''' any contraction operator ''T'' defines such a function Φ through | |||
:<math>\displaystyle \Phi(0)=I, \,\,\, \Phi(n)=T^n,\,\,\, \Phi(-n)=(T^*)^n, </math> | |||
for ''n'' > 0. The above construction then yields a minimal unitary dilation. | |||
The same method can be applied to prove a second dilation theorem of Sz._Nagy for a one-parameter strongly continuous contraction semigroup ''T''(''t'') (''t'' ≥ 0) on a Hilbert space ''H''. {{harvtxt|Cooper|1947}} had previously proved the result for one-parameter semigroups of isometries,<ref>{{harvnb|Sz.-Nagy|Foias|Bercovici|Kérchy|2010|pp=28–30}}</ref> | |||
The theorem states that there is a larger Hilbert space ''K'' containing ''H'' and a unitary representation ''U''(''t'') of '''R''' such that | |||
:<math>\displaystyle{T(t)=PU(t)P}</math> | |||
and the translates ''U''(''t'')''H'' generate ''K''. | |||
In fact ''T''(''t'') defines a continuous operator-valued positove-definite function Φ on '''R''' through | |||
:<math>\displaystyle{\Phi(0)=I, \,\,\, \Phi(t)=T(t),\,\,\, \Phi(-t)= T(t)^*,}</math> | |||
for ''t'' > 0. Φ is positive-definite on cyclic subgroups of '''R''', by the argument for '''Z''', and hence on '''R''' itself by continuity. | |||
The previous construction yields a minimal unitary representation ''U''(''t'') and projection ''P''. | |||
The [[Hille-Yosida theorem]] assigns a closed [[unbounded operator]] ''A'' to every contractive one-parameter semigroup ''T'''(''t'') through | |||
:<math>\displaystyle{A\xi=\lim_{t\downarrow 0} {1\over t}(T(t)-I)\xi,}</math> | |||
where the domain on ''A'' consists of all ξ for which this limit exists. | |||
''A'' is called the '''generator''' of the semigroup and satisfies | |||
:<math> \displaystyle{-\Re (A\xi,\xi)\ge 0}</math> | |||
on its domain. When ''A'' is a self-adjoint operator | |||
:<math>\displaystyle{T(t)=e^{At},}</math> | |||
in the sense of the [[spectral theorem]] and this notation is used more generally in semigroup theory. | |||
The '''cogenerator''' of the semigroup is the contraction defined by | |||
:<math> \displaystyle{T=(A+I)(A-I)^{-1}.}</math> | |||
''A'' can be recovered from ''T'' using the formula | |||
:<math>\displaystyle{A=(T+I)(T-I)^{-1}.}</math> | |||
In particular a dilation of ''T'' on ''K'' ⊃ ''H'' immediately gives a dilation of the semigroup.<ref>{{harvnb|Sz.-Nagy|Foias|Bercovici|Kérchy|2010|pp=143, 147}}</ref> | |||
==Functional calculus== | |||
Let ''T'' be totally non-unitary contraction on ''H''. Then the minimal unitary dilation ''U'' of ''T'' on ''K'' ⊃ ''H'' is unitarily equivalent to a direct sum of copies the bilateral shift operator, i.e. multiplication by ''z'' on L<sup>2</sup>(''S''<sup>1</sup>).<ref>{{harvnb|Sz.-Nagy|Foias|Bercovici|Kérchy|2010|pp=87–88}}</ref> | |||
If ''P'' is the orthogonal projection onto ''H'' then for ''f'' in L<sup>∞</sup> = L<sup>∞</sup>(''S''<sup>1</sup>) it follows that the operator ''f''(''T'') can be defined | |||
by | |||
:<math>\displaystyle{f(T)\xi=Pf(U)\xi.}</math> | |||
Let H<sup>∞</sup> be the space of bounded holomorphic functions on the unit disk ''D''. Any such function has boundary values in L<sup>∞</sup> and is uniquely determined by these, so that there is an embedding H<sup>∞</sup> ⊂ L<sup>∞</sup>. | |||
For ''f'' in H<sup>∞</sup>, ''f''(''T'') can be defined | |||
without reference to the unitary dilation. | |||
In fact if | |||
:<math>\displaystyle{f(z)=\sum_{n\ge 0} a_n z^n}</math> | |||
for |''z''| < 1, then for ''r'' < 1 | |||
:<math>\displaystyle{f_r(z))=\sum_{n\ge 0} r^n a_n z^n}</math> | |||
is holomorphic on |''z''| < 1/''r''. | |||
In that case ''f''<sub>''r''</sub>(''T'') is defined by the holomorphic functional calculus and ''f''(''T'') can be defined by | |||
:<math>\displaystyle{f(T)\xi=\lim_{r\rightarrow 1} f_r(T)\xi.}</math> | |||
The map sending ''f'' to ''f''(''T'') defines an algebra homomorphism of H<sup>∞</sup> into bounded operators on ''H''. Moreover if | |||
:<math>\displaystyle{f^\sim(z)=\sum_{n\ge 0} a_n \overline{z}^n,}</math> | |||
then | |||
:<math>\displaystyle{f^\sim(T)=f(T^*)^*.}</math> | |||
This map has the following continuity property: if a uniformly bounded sequence ''f''<sub>''n''</sub> tends almost everywhere to ''f'', then ''f''<sub>''n''</sub>(''T'') tends to ''f''(''T'') in the strong operator topology. | |||
For ''t'' ≥ 0, let ''e''<sub>''t''</sub> be the inner function | |||
:<math>\displaystyle{e_t(z)=\exp t{z+1\over z-1}.}</math> | |||
If ''T'' is the cogenerator of a one-parameter semigroup of completely non-unitary contractions ''T''(''t''), then | |||
:<math>\displaystyle{T(t)=e_t(T)}</math> | |||
and | |||
:<math>\displaystyle{T={1\over 2}I -{1\over 2}\int_0^\infty e^{-t}T(t)\, dt.}</math> | |||
==C<sub>0</sub> contractions== | |||
A completely non-unitary contraction ''T'' is said to belong to the class C<sub>0</sub> if and only if ''f''(''T'') = 0 for some non-zero | |||
''f'' in H<sup>∞</sup>. In this case the set of such ''f'' forms an ideal in H<sup>∞</sup>. It has the form φ ⋅ H<sup>∞</sup> where ''g'' | |||
is an [[inner function]], i.e. such that |φ| = 1 on ''S''<sup>1</sup>: φ is uniquely determined up to multiplication by a complex number of modulus 1 and is called the '''minimal function''' of ''T''. It has properties analogous to the [[minimal polynomial]] of a matrix. | |||
The minimal function φ admits a canonical factorization | |||
:<math>\displaystyle{\varphi(z) = c B(z) e^{-P(z)},}</math> | |||
where |''c''|=1, ''B''(''z'') is a [[Blaschke product]] | |||
:<math>\displaystyle{B(z)=\prod \left[{|\lambda_i|\over \lambda_i} {\lambda_i -z \over 1-\overline{\lambda}_i }\right]^{m_i},}</math> | |||
with | |||
:<math>\displaystyle{\sum m_i(1-|\lambda_i|) <\infty,}</math> | |||
and ''P''(''z'') is holomorphic with non-negative real part in ''D''. By the [[positive harmonic function#Herglotz representation theorem for holomorphic functions|Herglotz representation theorem]], | |||
:<math>\displaystyle{P(z) =\int_0^{2\pi} {1 + e^{-i\theta}z\over 1 -e^{-i\theta}z} \, d\mu(\theta)}</math> | |||
for some non-negative finite measure μ on the circle: in this case, if non-zero, μ must be [[singular measure|singular]] with respect to Lebesgue measure. In the above decomposition of φ, either of the two factors can be absent. | |||
The minimal function φ determines the [[spectrum]] of ''T''. Within the unit disk, the spectral values are the zeros of φ. There are at most countably many such λ<sub>i</sub>, all eigenvalues of ''T'', the zeros of ''B''(''z''). A point of the unit circle does not lie in the spectrum of ''T'' if and only if φ has a holomorphic continuation to a neighbourhood of that point. | |||
φ reduces to a Blaschke product exactly when ''H'' equals the closure of the direct sum (not necessarily orthogonal) of the generalized eigenspaces<ref>{{harvnb|Sz.-Nagy|Foias|Bercovici|Kérchy|2010|p=138}}</ref> | |||
:<math>\displaystyle{H_i=\{\xi:(T-\lambda_i I)^{m_i} \xi=0\}.}</math> | |||
==Quasi-similarity== | |||
Two contractions ''T''<sub>1</sub> and ''T''<sub>2</sub> are said to be '''quasi-similar''' when there are bounded operators ''A'', ''B'' with trivial kernel and dense range such that | |||
:<math>\displaystyle{AT_1=T_2A,\,\,\, BT_2=T_1B.}</math> | |||
The following properties of a contraction ''T'' are preserved under quasi-similarlity: | |||
*being unitary | |||
*being completely non-unitary | |||
*being in the class C<sub>0</sub> | |||
*being '''multiplicity free''', i.e. having a commutative [[commutant]] | |||
Two quasi-similar C<sub>0</sub> contractions have the same minimal function and hence the same spectrum. | |||
The '''classification theorem''' for C<sub>0</sub> contractions states that two multiplicity free C<sub>0</sub> contractions are quasi-similar if and only if they have the same minimal function (up to a scalar multiple).<ref>{{harvnb|Sz.-Nagy|Foias|Bercovici|Kérchy|2010|pp=395–440}}</ref> | |||
A model for multiplicity free C<sub>0</sub> contractions with minimal function φ is given by taking | |||
:<math> \displaystyle{H=H^2\ominus \varphi H^2,}</math> | |||
where H<sup>2</sup> is the [[Hardy space]] of the circle and letting ''T'' be multiplication by ''z''.<ref>{{harvnb|Sz.-Nagy|Foias|Bercovici|Kérchy|2010|p=126}}</ref> | |||
Such operators are called '''Jordan blocks''' and denoted ''S''(φ). | |||
As a generalization of [[Beurling's theorem]], the commutant of such an operator consists exactly of operators ψ(''T'') with ψ in ''H''<sup>≈</sup>, i.e. multiplication operators on ''H''<sup>2</sup> corresponding to functions in ''H''<sup>≈</sup>. | |||
A C<sub>0</sub> contraction operator ''T'' is multiplcity free if and only if it is quasi-similar to a Jordan block (necessarily corresponding the one corresponding to its minimal function). | |||
'''Examples.''' | |||
*If a contraction ''T'' if quasi-similar to an operator ''S'' with | |||
:<math>\displaystyle{Se_i=\lambda_i e_i}</math> | |||
with the λ<sub>i</sub>'s distinct, of modulus less than 1, such that | |||
:<math>\displaystyle{\sum (1-|\lambda_i|) < 1}</math> | |||
and (''e''<sub>''i''</sub>) is an orthonormal basis, then ''S'', and hence ''T'', is C<sub>0</sub> and multiplicity free. Hence ''H'' is the closure of direct sum of the λ<sub>i</sub>-eigenspaces of ''T'', each having multiplicity one. This can also be seen directly using the definition of quasi-similarity. | |||
*The results above can be applied equally well to one-parameter semigroups, since, from the functional calculus, two semigroups are quasi-similar if and only if their cogenerators are quasi-similar.<ref>{{harvnb|Bercovici|1988|p=95}}</ref> | |||
'''Classification theorem for C<sub>0</sub> contractions:''' ''Every C<sub>0</sub> contraction is canonically quasi-similar to a direct sum of Jordan blocks.'' | |||
In fact every C<sub>0</sub> contraction is quasi-similar to a unique operator of the form | |||
:<math>\displaystyle{S=S(\varphi_1)\oplus S(\varphi_1\varphi_2)\oplus S(\varphi_1\varphi_2\varphi_3) \oplus \cdots }</math> | |||
where the φ<sub>''n''</sub> are uniquely determined inner functions, with φ<sub>''1''</sub> the minimal function of ''S'' and hence ''T''.<ref>{{harvnb|Bercovici|1988|pp=35–66}}</ref> | |||
==See also== | |||
*[[Stinespring dilation theorem]] | |||
*[[Hille-Yosida_theorem#Hille-Yosida_theorem_for_contraction_semigroups|Hille-Yosida theorem for contraction semigroups]] | |||
==Notes== | |||
{{reflist|2}} | |||
==References== | |||
*{{citation|last=Bercovici|first= H.|title=Operator theory and arithmetic in H<sup>∞</sup>|series= | |||
Mathematical Surveys and Monographs|volume= 26|publisher= American Mathematical Society|year= 1988|isbn= 0-8218-1528-8}} | |||
*{{citation|last=Cooper|first=J. L. B.|title= One-parameter semigroups of isometric operators in Hilbert space|journal= Ann. of Math. |volume=48|year=1947| pages=827–842|authorlink=Lionel Cooper (mathematician)}} | |||
*{{citation|last=Gamelin|first= T. W.|title=Uniform algebras|publisher=Prentice-Hall|year= 1969}} | |||
*{{citation|last=Hoffman|first= K.|title=Banach spaces of analytic functions|publisher=Prentice-Hall|year= 1962}} | |||
*{{citation|last=Sz.-Nagy|first= B.|last2= Foias|first2= C.|last3= Bercovici|first3= H.|last4= Kérchy|first4= L.| | |||
title=Harmonic analysis of operators on Hilbert space|edition=Second|series= Universitext|publisher= Springer|year= 2010|isbn= 978-1-4419-6093-1}} | |||
*{{citation|last=Riesz|first= F.|last2=Sz.-Nagy|first2= B.|title=Functional analysis. Reprint of the 1955 original|series= Dover Books on Advanced Mathematics|publisher=Dover|year= 1995|pages=466–472|isbn= 0-486-66289-6}} | |||
[[Category:Operator theory]] |
Latest revision as of 16:34, 21 May 2013
In operator theory, a discipline within mathematics, a bounded operator T: X → Y between normed vector spaces X and Y is said to be a contraction if its operator norm ||T|| ≤ 1. Every bounded operator becomes a contraction after suitable scaling. The analysis of contractions provides insight into the structure of operators, or a family of operators. The theory of contractions on Hilbert space is largely due to Béla Szőkefalvi-Nagy and Ciprian Foias.
Contractions on a Hilbert space
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If T is a contraction acting on a Hilbert space , the following basic objects associated with T can be defined.
The defect operators of T are the operators DT = (1 − T*T)½ and DT* = (1 − TT*)½. The square root is the positive semidefinite one given by the spectral theorem. The defect spaces and are the ranges Ran(DT) and Ran(DT*) respectively. The positive operator DT induces an inner product on . The inner product space can be identified naturally with Ran(DT). A similar statement holds for .
The defect indices of T are the pair
The defect operators and the defect indices are a measure of the non-unitarity of T.
A contraction T on a Hilbert space can be canonically decomposed into an orthogonal direct sum
where U is a unitary operator and Γ is completely non-unitary in the sense that it has no reducing subspaces on which its restriction is unitary. If U = 0, T is said to be a completely non-unitary contraction. A special case of this decomposition is the Wold decomposition for an isometry, where Γ is a proper isometry.
Contractions on Hilbert spaces can be viewed as the operator analogs of cos θ and are called operator angles in some contexts. The explicit description of contractions leads to (operator-)parametrizations of positive and unitary matrices.
Dilation theorem for contractions
Sz.-Nagy's dilation theorem, proved in 1953, states that for any contraction T on a Hilbert space H, there is a unitary operator U on a larger Hilbert space K ⊇ H such that if P is the orthogonal projection of K onto H then Tn = P Un P for all n > 0. The operator U is called a dilation of T and is uniquely determined if U is mininal, i.e. K is the smallest closed subspace invariant under U and U* containing H.
In fact define[1]
the orthogonal direct sum of countably many copies of H.
Let V be the isometry on defined by
Let
W is then a unitary dilation of T with H considered as the first component of .
The minimal dilation U is obtained by taking the restriction of W to the closed subspace generated by powers of W applied to H.
Dilation theorem for contraction semigroups
There is an alternative proof of Sz.-Nagy's dilation theorem, which allows significant generalisations.[2]
Let G be a group, U(g) a unitary representation of G on a Hilbert space K and P an orthogonal projection onto a closed subspace H = PK of K.
The operator-valued function
with values in operators on K satisfies the positive-definiteness condition
where
Moreover
Conversely, every operator-valued positive-definite function arises in this way. Recall that every (continuous) scalar-valued positive-definition function on a topological group is of the form φ(g) = 〈Ug v, v〉 where Ug is a (strongly continuous) unitary representation (see Bochner's theorem). Replacing v, a rank-1 projection, by a general projection gives the operator-valued statement. In fact the construction is identical; this is sketched below.
Let be the space of functions on G of finite support with values in H with inner product
Moreover H can be identified with a closed subspace of using the isometric embedding sending v in H to fv with
If P is the projection of onto H, then
using the above identification.
When G is a separable topological group, Φ is continuous in the strong (or weak) operator topology if and only if U is.
In this case functions supported on a countable dense subgroup of G are dense in , so that is separable.
When G = Z any contraction operator T defines such a function Φ through
for n > 0. The above construction then yields a minimal unitary dilation.
The same method can be applied to prove a second dilation theorem of Sz._Nagy for a one-parameter strongly continuous contraction semigroup T(t) (t ≥ 0) on a Hilbert space H. Template:Harvtxt had previously proved the result for one-parameter semigroups of isometries,[3]
The theorem states that there is a larger Hilbert space K containing H and a unitary representation U(t) of R such that
and the translates U(t)H generate K.
In fact T(t) defines a continuous operator-valued positove-definite function Φ on R through
for t > 0. Φ is positive-definite on cyclic subgroups of R, by the argument for Z, and hence on R itself by continuity.
The previous construction yields a minimal unitary representation U(t) and projection P.
The Hille-Yosida theorem assigns a closed unbounded operator A to every contractive one-parameter semigroup T'(t) through
where the domain on A consists of all ξ for which this limit exists.
A is called the generator of the semigroup and satisfies
on its domain. When A is a self-adjoint operator
in the sense of the spectral theorem and this notation is used more generally in semigroup theory.
The cogenerator of the semigroup is the contraction defined by
A can be recovered from T using the formula
In particular a dilation of T on K ⊃ H immediately gives a dilation of the semigroup.[4]
Functional calculus
Let T be totally non-unitary contraction on H. Then the minimal unitary dilation U of T on K ⊃ H is unitarily equivalent to a direct sum of copies the bilateral shift operator, i.e. multiplication by z on L2(S1).[5]
If P is the orthogonal projection onto H then for f in L∞ = L∞(S1) it follows that the operator f(T) can be defined by
Let H∞ be the space of bounded holomorphic functions on the unit disk D. Any such function has boundary values in L∞ and is uniquely determined by these, so that there is an embedding H∞ ⊂ L∞.
For f in H∞, f(T) can be defined without reference to the unitary dilation.
In fact if
for |z| < 1, then for r < 1
is holomorphic on |z| < 1/r.
In that case fr(T) is defined by the holomorphic functional calculus and f(T) can be defined by
The map sending f to f(T) defines an algebra homomorphism of H∞ into bounded operators on H. Moreover if
then
This map has the following continuity property: if a uniformly bounded sequence fn tends almost everywhere to f, then fn(T) tends to f(T) in the strong operator topology.
For t ≥ 0, let et be the inner function
If T is the cogenerator of a one-parameter semigroup of completely non-unitary contractions T(t), then
and
C0 contractions
A completely non-unitary contraction T is said to belong to the class C0 if and only if f(T) = 0 for some non-zero f in H∞. In this case the set of such f forms an ideal in H∞. It has the form φ ⋅ H∞ where g is an inner function, i.e. such that |φ| = 1 on S1: φ is uniquely determined up to multiplication by a complex number of modulus 1 and is called the minimal function of T. It has properties analogous to the minimal polynomial of a matrix.
The minimal function φ admits a canonical factorization
where |c|=1, B(z) is a Blaschke product
with
and P(z) is holomorphic with non-negative real part in D. By the Herglotz representation theorem,
for some non-negative finite measure μ on the circle: in this case, if non-zero, μ must be singular with respect to Lebesgue measure. In the above decomposition of φ, either of the two factors can be absent.
The minimal function φ determines the spectrum of T. Within the unit disk, the spectral values are the zeros of φ. There are at most countably many such λi, all eigenvalues of T, the zeros of B(z). A point of the unit circle does not lie in the spectrum of T if and only if φ has a holomorphic continuation to a neighbourhood of that point.
φ reduces to a Blaschke product exactly when H equals the closure of the direct sum (not necessarily orthogonal) of the generalized eigenspaces[6]
Quasi-similarity
Two contractions T1 and T2 are said to be quasi-similar when there are bounded operators A, B with trivial kernel and dense range such that
The following properties of a contraction T are preserved under quasi-similarlity:
- being unitary
- being completely non-unitary
- being in the class C0
- being multiplicity free, i.e. having a commutative commutant
Two quasi-similar C0 contractions have the same minimal function and hence the same spectrum.
The classification theorem for C0 contractions states that two multiplicity free C0 contractions are quasi-similar if and only if they have the same minimal function (up to a scalar multiple).[7]
A model for multiplicity free C0 contractions with minimal function φ is given by taking
where H2 is the Hardy space of the circle and letting T be multiplication by z.[8]
Such operators are called Jordan blocks and denoted S(φ).
As a generalization of Beurling's theorem, the commutant of such an operator consists exactly of operators ψ(T) with ψ in H≈, i.e. multiplication operators on H2 corresponding to functions in H≈.
A C0 contraction operator T is multiplcity free if and only if it is quasi-similar to a Jordan block (necessarily corresponding the one corresponding to its minimal function).
Examples.
- If a contraction T if quasi-similar to an operator S with
with the λi's distinct, of modulus less than 1, such that
and (ei) is an orthonormal basis, then S, and hence T, is C0 and multiplicity free. Hence H is the closure of direct sum of the λi-eigenspaces of T, each having multiplicity one. This can also be seen directly using the definition of quasi-similarity.
- The results above can be applied equally well to one-parameter semigroups, since, from the functional calculus, two semigroups are quasi-similar if and only if their cogenerators are quasi-similar.[9]
Classification theorem for C0 contractions: Every C0 contraction is canonically quasi-similar to a direct sum of Jordan blocks.
In fact every C0 contraction is quasi-similar to a unique operator of the form
where the φn are uniquely determined inner functions, with φ1 the minimal function of S and hence T.[10]
See also
Notes
43 year old Petroleum Engineer Harry from Deep River, usually spends time with hobbies and interests like renting movies, property developers in singapore new condominium and vehicle racing. Constantly enjoys going to destinations like Camino Real de Tierra Adentro.
References
- Many property agents need to declare for the PIC grant in Singapore. However, not all of them know find out how to do the correct process for getting this PIC scheme from the IRAS. There are a number of steps that you need to do before your software can be approved.
Naturally, you will have to pay a safety deposit and that is usually one month rent for annually of the settlement. That is the place your good religion deposit will likely be taken into account and will kind part or all of your security deposit. Anticipate to have a proportionate amount deducted out of your deposit if something is discovered to be damaged if you move out. It's best to you'll want to test the inventory drawn up by the owner, which can detail all objects in the property and their condition. If you happen to fail to notice any harm not already mentioned within the inventory before transferring in, you danger having to pay for it yourself.
In case you are in search of an actual estate or Singapore property agent on-line, you simply should belief your intuition. It's because you do not know which agent is nice and which agent will not be. Carry out research on several brokers by looking out the internet. As soon as if you end up positive that a selected agent is dependable and reliable, you can choose to utilize his partnerise in finding you a home in Singapore. Most of the time, a property agent is taken into account to be good if he or she locations the contact data on his website. This may mean that the agent does not mind you calling them and asking them any questions relating to new properties in singapore in Singapore. After chatting with them you too can see them in their office after taking an appointment.
Have handed an trade examination i.e Widespread Examination for House Brokers (CEHA) or Actual Property Agency (REA) examination, or equal; Exclusive brokers are extra keen to share listing information thus making certain the widest doable coverage inside the real estate community via Multiple Listings and Networking. Accepting a severe provide is simpler since your agent is totally conscious of all advertising activity related with your property. This reduces your having to check with a number of agents for some other offers. Price control is easily achieved. Paint work in good restore-discuss with your Property Marketing consultant if main works are still to be done. Softening in residential property prices proceed, led by 2.8 per cent decline within the index for Remainder of Central Region
Once you place down the one per cent choice price to carry down a non-public property, it's important to accept its situation as it is whenever you move in – faulty air-con, choked rest room and all. Get round this by asking your agent to incorporate a ultimate inspection clause within the possibility-to-buy letter. HDB flat patrons routinely take pleasure in this security net. "There's a ultimate inspection of the property two days before the completion of all HDB transactions. If the air-con is defective, you can request the seller to repair it," says Kelvin.
15.6.1 As the agent is an intermediary, generally, as soon as the principal and third party are introduced right into a contractual relationship, the agent drops out of the image, subject to any problems with remuneration or indemnification that he could have against the principal, and extra exceptionally, against the third occasion. Generally, agents are entitled to be indemnified for all liabilities reasonably incurred within the execution of the brokers´ authority.
To achieve the very best outcomes, you must be always updated on market situations, including past transaction information and reliable projections. You could review and examine comparable homes that are currently available in the market, especially these which have been sold or not bought up to now six months. You'll be able to see a pattern of such report by clicking here It's essential to defend yourself in opposition to unscrupulous patrons. They are often very skilled in using highly unethical and manipulative techniques to try and lure you into a lure. That you must also protect your self, your loved ones, and personal belongings as you'll be serving many strangers in your home. Sign a listing itemizing of all of the objects provided by the proprietor, together with their situation. HSR Prime Recruiter 2010 - Many property agents need to declare for the PIC grant in Singapore. However, not all of them know find out how to do the correct process for getting this PIC scheme from the IRAS. There are a number of steps that you need to do before your software can be approved.
Naturally, you will have to pay a safety deposit and that is usually one month rent for annually of the settlement. That is the place your good religion deposit will likely be taken into account and will kind part or all of your security deposit. Anticipate to have a proportionate amount deducted out of your deposit if something is discovered to be damaged if you move out. It's best to you'll want to test the inventory drawn up by the owner, which can detail all objects in the property and their condition. If you happen to fail to notice any harm not already mentioned within the inventory before transferring in, you danger having to pay for it yourself.
In case you are in search of an actual estate or Singapore property agent on-line, you simply should belief your intuition. It's because you do not know which agent is nice and which agent will not be. Carry out research on several brokers by looking out the internet. As soon as if you end up positive that a selected agent is dependable and reliable, you can choose to utilize his partnerise in finding you a home in Singapore. Most of the time, a property agent is taken into account to be good if he or she locations the contact data on his website. This may mean that the agent does not mind you calling them and asking them any questions relating to new properties in singapore in Singapore. After chatting with them you too can see them in their office after taking an appointment.
Have handed an trade examination i.e Widespread Examination for House Brokers (CEHA) or Actual Property Agency (REA) examination, or equal; Exclusive brokers are extra keen to share listing information thus making certain the widest doable coverage inside the real estate community via Multiple Listings and Networking. Accepting a severe provide is simpler since your agent is totally conscious of all advertising activity related with your property. This reduces your having to check with a number of agents for some other offers. Price control is easily achieved. Paint work in good restore-discuss with your Property Marketing consultant if main works are still to be done. Softening in residential property prices proceed, led by 2.8 per cent decline within the index for Remainder of Central Region
Once you place down the one per cent choice price to carry down a non-public property, it's important to accept its situation as it is whenever you move in – faulty air-con, choked rest room and all. Get round this by asking your agent to incorporate a ultimate inspection clause within the possibility-to-buy letter. HDB flat patrons routinely take pleasure in this security net. "There's a ultimate inspection of the property two days before the completion of all HDB transactions. If the air-con is defective, you can request the seller to repair it," says Kelvin.
15.6.1 As the agent is an intermediary, generally, as soon as the principal and third party are introduced right into a contractual relationship, the agent drops out of the image, subject to any problems with remuneration or indemnification that he could have against the principal, and extra exceptionally, against the third occasion. Generally, agents are entitled to be indemnified for all liabilities reasonably incurred within the execution of the brokers´ authority.
To achieve the very best outcomes, you must be always updated on market situations, including past transaction information and reliable projections. You could review and examine comparable homes that are currently available in the market, especially these which have been sold or not bought up to now six months. You'll be able to see a pattern of such report by clicking here It's essential to defend yourself in opposition to unscrupulous patrons. They are often very skilled in using highly unethical and manipulative techniques to try and lure you into a lure. That you must also protect your self, your loved ones, and personal belongings as you'll be serving many strangers in your home. Sign a listing itemizing of all of the objects provided by the proprietor, together with their situation. HSR Prime Recruiter 2010 - Many property agents need to declare for the PIC grant in Singapore. However, not all of them know find out how to do the correct process for getting this PIC scheme from the IRAS. There are a number of steps that you need to do before your software can be approved.
Naturally, you will have to pay a safety deposit and that is usually one month rent for annually of the settlement. That is the place your good religion deposit will likely be taken into account and will kind part or all of your security deposit. Anticipate to have a proportionate amount deducted out of your deposit if something is discovered to be damaged if you move out. It's best to you'll want to test the inventory drawn up by the owner, which can detail all objects in the property and their condition. If you happen to fail to notice any harm not already mentioned within the inventory before transferring in, you danger having to pay for it yourself.
In case you are in search of an actual estate or Singapore property agent on-line, you simply should belief your intuition. It's because you do not know which agent is nice and which agent will not be. Carry out research on several brokers by looking out the internet. As soon as if you end up positive that a selected agent is dependable and reliable, you can choose to utilize his partnerise in finding you a home in Singapore. Most of the time, a property agent is taken into account to be good if he or she locations the contact data on his website. This may mean that the agent does not mind you calling them and asking them any questions relating to new properties in singapore in Singapore. After chatting with them you too can see them in their office after taking an appointment.
Have handed an trade examination i.e Widespread Examination for House Brokers (CEHA) or Actual Property Agency (REA) examination, or equal; Exclusive brokers are extra keen to share listing information thus making certain the widest doable coverage inside the real estate community via Multiple Listings and Networking. Accepting a severe provide is simpler since your agent is totally conscious of all advertising activity related with your property. This reduces your having to check with a number of agents for some other offers. Price control is easily achieved. Paint work in good restore-discuss with your Property Marketing consultant if main works are still to be done. Softening in residential property prices proceed, led by 2.8 per cent decline within the index for Remainder of Central Region
Once you place down the one per cent choice price to carry down a non-public property, it's important to accept its situation as it is whenever you move in – faulty air-con, choked rest room and all. Get round this by asking your agent to incorporate a ultimate inspection clause within the possibility-to-buy letter. HDB flat patrons routinely take pleasure in this security net. "There's a ultimate inspection of the property two days before the completion of all HDB transactions. If the air-con is defective, you can request the seller to repair it," says Kelvin.
15.6.1 As the agent is an intermediary, generally, as soon as the principal and third party are introduced right into a contractual relationship, the agent drops out of the image, subject to any problems with remuneration or indemnification that he could have against the principal, and extra exceptionally, against the third occasion. Generally, agents are entitled to be indemnified for all liabilities reasonably incurred within the execution of the brokers´ authority.
To achieve the very best outcomes, you must be always updated on market situations, including past transaction information and reliable projections. You could review and examine comparable homes that are currently available in the market, especially these which have been sold or not bought up to now six months. You'll be able to see a pattern of such report by clicking here It's essential to defend yourself in opposition to unscrupulous patrons. They are often very skilled in using highly unethical and manipulative techniques to try and lure you into a lure. That you must also protect your self, your loved ones, and personal belongings as you'll be serving many strangers in your home. Sign a listing itemizing of all of the objects provided by the proprietor, together with their situation. HSR Prime Recruiter 2010 - Many property agents need to declare for the PIC grant in Singapore. However, not all of them know find out how to do the correct process for getting this PIC scheme from the IRAS. There are a number of steps that you need to do before your software can be approved.
Naturally, you will have to pay a safety deposit and that is usually one month rent for annually of the settlement. That is the place your good religion deposit will likely be taken into account and will kind part or all of your security deposit. Anticipate to have a proportionate amount deducted out of your deposit if something is discovered to be damaged if you move out. It's best to you'll want to test the inventory drawn up by the owner, which can detail all objects in the property and their condition. If you happen to fail to notice any harm not already mentioned within the inventory before transferring in, you danger having to pay for it yourself.
In case you are in search of an actual estate or Singapore property agent on-line, you simply should belief your intuition. It's because you do not know which agent is nice and which agent will not be. Carry out research on several brokers by looking out the internet. As soon as if you end up positive that a selected agent is dependable and reliable, you can choose to utilize his partnerise in finding you a home in Singapore. Most of the time, a property agent is taken into account to be good if he or she locations the contact data on his website. This may mean that the agent does not mind you calling them and asking them any questions relating to new properties in singapore in Singapore. After chatting with them you too can see them in their office after taking an appointment.
Have handed an trade examination i.e Widespread Examination for House Brokers (CEHA) or Actual Property Agency (REA) examination, or equal; Exclusive brokers are extra keen to share listing information thus making certain the widest doable coverage inside the real estate community via Multiple Listings and Networking. Accepting a severe provide is simpler since your agent is totally conscious of all advertising activity related with your property. This reduces your having to check with a number of agents for some other offers. Price control is easily achieved. Paint work in good restore-discuss with your Property Marketing consultant if main works are still to be done. Softening in residential property prices proceed, led by 2.8 per cent decline within the index for Remainder of Central Region
Once you place down the one per cent choice price to carry down a non-public property, it's important to accept its situation as it is whenever you move in – faulty air-con, choked rest room and all. Get round this by asking your agent to incorporate a ultimate inspection clause within the possibility-to-buy letter. HDB flat patrons routinely take pleasure in this security net. "There's a ultimate inspection of the property two days before the completion of all HDB transactions. If the air-con is defective, you can request the seller to repair it," says Kelvin.
15.6.1 As the agent is an intermediary, generally, as soon as the principal and third party are introduced right into a contractual relationship, the agent drops out of the image, subject to any problems with remuneration or indemnification that he could have against the principal, and extra exceptionally, against the third occasion. Generally, agents are entitled to be indemnified for all liabilities reasonably incurred within the execution of the brokers´ authority.
To achieve the very best outcomes, you must be always updated on market situations, including past transaction information and reliable projections. You could review and examine comparable homes that are currently available in the market, especially these which have been sold or not bought up to now six months. You'll be able to see a pattern of such report by clicking here It's essential to defend yourself in opposition to unscrupulous patrons. They are often very skilled in using highly unethical and manipulative techniques to try and lure you into a lure. That you must also protect your self, your loved ones, and personal belongings as you'll be serving many strangers in your home. Sign a listing itemizing of all of the objects provided by the proprietor, together with their situation. HSR Prime Recruiter 2010 - Many property agents need to declare for the PIC grant in Singapore. However, not all of them know find out how to do the correct process for getting this PIC scheme from the IRAS. There are a number of steps that you need to do before your software can be approved.
Naturally, you will have to pay a safety deposit and that is usually one month rent for annually of the settlement. That is the place your good religion deposit will likely be taken into account and will kind part or all of your security deposit. Anticipate to have a proportionate amount deducted out of your deposit if something is discovered to be damaged if you move out. It's best to you'll want to test the inventory drawn up by the owner, which can detail all objects in the property and their condition. If you happen to fail to notice any harm not already mentioned within the inventory before transferring in, you danger having to pay for it yourself.
In case you are in search of an actual estate or Singapore property agent on-line, you simply should belief your intuition. It's because you do not know which agent is nice and which agent will not be. Carry out research on several brokers by looking out the internet. As soon as if you end up positive that a selected agent is dependable and reliable, you can choose to utilize his partnerise in finding you a home in Singapore. Most of the time, a property agent is taken into account to be good if he or she locations the contact data on his website. This may mean that the agent does not mind you calling them and asking them any questions relating to new properties in singapore in Singapore. After chatting with them you too can see them in their office after taking an appointment.
Have handed an trade examination i.e Widespread Examination for House Brokers (CEHA) or Actual Property Agency (REA) examination, or equal; Exclusive brokers are extra keen to share listing information thus making certain the widest doable coverage inside the real estate community via Multiple Listings and Networking. Accepting a severe provide is simpler since your agent is totally conscious of all advertising activity related with your property. This reduces your having to check with a number of agents for some other offers. Price control is easily achieved. Paint work in good restore-discuss with your Property Marketing consultant if main works are still to be done. Softening in residential property prices proceed, led by 2.8 per cent decline within the index for Remainder of Central Region
Once you place down the one per cent choice price to carry down a non-public property, it's important to accept its situation as it is whenever you move in – faulty air-con, choked rest room and all. Get round this by asking your agent to incorporate a ultimate inspection clause within the possibility-to-buy letter. HDB flat patrons routinely take pleasure in this security net. "There's a ultimate inspection of the property two days before the completion of all HDB transactions. If the air-con is defective, you can request the seller to repair it," says Kelvin.
15.6.1 As the agent is an intermediary, generally, as soon as the principal and third party are introduced right into a contractual relationship, the agent drops out of the image, subject to any problems with remuneration or indemnification that he could have against the principal, and extra exceptionally, against the third occasion. Generally, agents are entitled to be indemnified for all liabilities reasonably incurred within the execution of the brokers´ authority.
To achieve the very best outcomes, you must be always updated on market situations, including past transaction information and reliable projections. You could review and examine comparable homes that are currently available in the market, especially these which have been sold or not bought up to now six months. You'll be able to see a pattern of such report by clicking here It's essential to defend yourself in opposition to unscrupulous patrons. They are often very skilled in using highly unethical and manipulative techniques to try and lure you into a lure. That you must also protect your self, your loved ones, and personal belongings as you'll be serving many strangers in your home. Sign a listing itemizing of all of the objects provided by the proprietor, together with their situation. HSR Prime Recruiter 2010 - Many property agents need to declare for the PIC grant in Singapore. However, not all of them know find out how to do the correct process for getting this PIC scheme from the IRAS. There are a number of steps that you need to do before your software can be approved.
Naturally, you will have to pay a safety deposit and that is usually one month rent for annually of the settlement. That is the place your good religion deposit will likely be taken into account and will kind part or all of your security deposit. Anticipate to have a proportionate amount deducted out of your deposit if something is discovered to be damaged if you move out. It's best to you'll want to test the inventory drawn up by the owner, which can detail all objects in the property and their condition. If you happen to fail to notice any harm not already mentioned within the inventory before transferring in, you danger having to pay for it yourself.
In case you are in search of an actual estate or Singapore property agent on-line, you simply should belief your intuition. It's because you do not know which agent is nice and which agent will not be. Carry out research on several brokers by looking out the internet. As soon as if you end up positive that a selected agent is dependable and reliable, you can choose to utilize his partnerise in finding you a home in Singapore. Most of the time, a property agent is taken into account to be good if he or she locations the contact data on his website. This may mean that the agent does not mind you calling them and asking them any questions relating to new properties in singapore in Singapore. After chatting with them you too can see them in their office after taking an appointment.
Have handed an trade examination i.e Widespread Examination for House Brokers (CEHA) or Actual Property Agency (REA) examination, or equal; Exclusive brokers are extra keen to share listing information thus making certain the widest doable coverage inside the real estate community via Multiple Listings and Networking. Accepting a severe provide is simpler since your agent is totally conscious of all advertising activity related with your property. This reduces your having to check with a number of agents for some other offers. Price control is easily achieved. Paint work in good restore-discuss with your Property Marketing consultant if main works are still to be done. Softening in residential property prices proceed, led by 2.8 per cent decline within the index for Remainder of Central Region
Once you place down the one per cent choice price to carry down a non-public property, it's important to accept its situation as it is whenever you move in – faulty air-con, choked rest room and all. Get round this by asking your agent to incorporate a ultimate inspection clause within the possibility-to-buy letter. HDB flat patrons routinely take pleasure in this security net. "There's a ultimate inspection of the property two days before the completion of all HDB transactions. If the air-con is defective, you can request the seller to repair it," says Kelvin.
15.6.1 As the agent is an intermediary, generally, as soon as the principal and third party are introduced right into a contractual relationship, the agent drops out of the image, subject to any problems with remuneration or indemnification that he could have against the principal, and extra exceptionally, against the third occasion. Generally, agents are entitled to be indemnified for all liabilities reasonably incurred within the execution of the brokers´ authority.
To achieve the very best outcomes, you must be always updated on market situations, including past transaction information and reliable projections. You could review and examine comparable homes that are currently available in the market, especially these which have been sold or not bought up to now six months. You'll be able to see a pattern of such report by clicking here It's essential to defend yourself in opposition to unscrupulous patrons. They are often very skilled in using highly unethical and manipulative techniques to try and lure you into a lure. That you must also protect your self, your loved ones, and personal belongings as you'll be serving many strangers in your home. Sign a listing itemizing of all of the objects provided by the proprietor, together with their situation. HSR Prime Recruiter 2010