Jump to content

Dilation (operator theory): Difference between revisions

From Wikipedia, the free encyclopedia
Content deleted Content added
No edit summary
Replaced page with 'butt'
Line 1: Line 1:
butt
In [[operator theory]], a '''dilation''' of an operator ''T'' on a [[Hilbert space]] ''H'' is an operator on a larger Hilbert space ''K'' , whose restriction to ''H'' is ''T''.

More formally, let ''T'' be a bounded operator on some Hilbert space ''H'', and ''H'' be a subspace of a larger Hilbert space '' H' ''. A bounded operator ''V'' on '' H' '' is a dilation of T if

:<math>P_H \; V | _H = T</math>

where <math>P_H</math> is projection on ''H''.

''V'' is said to be a '''unitary dilation''' (respectively, normal, isometric, etc) if ''V'' is unitary (respectively, normal, isometric, etc). ''T'' is said to be a '''compression''' of ''V''. If an operator ''T'' has a [[spectral set]] <math>X</math>, we say that ''V'' is a '''normal boundary dilation''' or a '''normal <math>\partial X</math> dilation''' if ''V'' is a normal dilation of ''T'' and <math>\sigma(V)\in\partial X</math>.

It should be noted that some texts impose an additional condition. Namely, that a dilation satisfy the following (calculus) property:

:<math>P_H \; f(V) | _H = f(T)</math>

where ''f(T)'' is some specified [[functional calculus]] (for example, the polynomial or ''H''<sup>&infin;</sup> calculus). The utility of a dilation is that it allows the "lifting" of objects associated to ''T'' to the level of ''V'', where the lifted objects may have nicer properties. See, for example, the [[commutant lifting theorem]].

==Applications==

We can show that every contraction on Hilbert spaces has a unitary dilation. A possible construction of this dilation is as follows. For a contraction ''T'', the operator

:<math>D_T = (I - T^* T)^{\frac{1}{2}}</math>

is positive, where the [[continuous functional calculus]] is used to define the square root. The operator ''D<sub>T</sub>'' is called the '''defect operator''' of ''T''. Let ''V'' be the operator on

:<math>H \oplus H</math>

defined by the matrix

:<math> V =
\begin{bmatrix} T & D_{T^*}\\
\ D_T & -T^*
\end{bmatrix}. </math>
''V'' is clearly a dilation of ''T''. Also, ''T''(''I - T*T'') = (''I - TT*'')''T'' implies

:<math> T D_T = D_{T^*} T.</math>

Using this one can show, by calculating directly, that that ''V'' is unitary, therefore an unitary dilation of ''T''. This operator ''V'' is sometimes called the '''Julia operator''' of ''T''.

Notice that when ''T'' is a real scalar, say <math>T = \cos \theta</math>, we have
:<math> V =
\begin{bmatrix} \cos \theta & \sin \theta \\
\ \sin \theta & - \cos \theta
\end{bmatrix}. </math>

which is just the unitary matrix describing rotation by θ. For this reason, the Julia operator ''V(T)'' is sometimes called the ''elementary rotation'' of ''T''.

We note here that in the above discussion we have not required the calculus property for a dilation. Indeed, direct calculation shows the Julia operator fails to be a "degree-2" dilation in general, i.e. it need not be true that

:<math>T^2 = P_H \; V^2 | _H</math>.

However, it can also be shown that any contraction has a unitary dilation which '''does''' have the calculus property above. This is [[Sz.-Nagy's dilation theorem]]. More generally, if <math>\mathcal{R}(X)</math> is a [[Dirichlet algebra]], any operator ''T'' with <math>X</math> as a spectral set will have a normal <math>\partial X</math> dilation with this property. This generalises Sz.-Nagy's dilation theorem as all contractions have the unit disc as a spectral set.

==References==
*T. Constantinescu, ''Schur Parameters, Dilation and Factorization Problems'', Birkhauser Verlag, Vol. 82, ISBN 3-7643-5285-X, 1996.
*Vern Paulsen, ''Completely Bounded Maps and Operator Algebras'' 2002, ISBN 0-521-81669-6


[[Category:Operator theory]]
[[Category:Unitary operators]]

Revision as of 21:26, 17 March 2008

butt