Cotlar–Stein lemma

The Cotlar–Stein almost orthogonality lemma is a mathematical lemma in the field of functional analysis. It may be used to obtain information on the operator norm on an operator, acting from one Hilbert space into another, when the operator can be decomposed into almost orthogonal pieces.

The original version of this lemma (for self-adjoint and mutually commuting operators) was proved by Mischa Cotlar in 1955{{harvnb|Cotlar|1955}} and allowed him to conclude that the Hilbert transform is a continuous linear operator in L^2 without using the Fourier transform. A more general version was proved by Elias Stein.{{harvnb|Stein|1993}}

Statement of the lemma

Let E,\,F be two Hilbert spaces. Consider a family of operators T_j, j\geq 1, with each T_j a bounded linear operator from E to F.

Denote

: a_{jk}=\Vert T_j T_k^\ast\Vert,

\qquad b_{jk}=\Vert T_j^\ast T_k\Vert.

The family of operators T_j:\;E\to F, j\ge 1, is almost orthogonal if

:A=\sup_{j}\sum_{k}\sqrt{a_{jk}}<\infty,

\qquad B=\sup_{j}\sum_{k}\sqrt{b_{jk}}<\infty.

The Cotlar–Stein lemma states that if T_j are almost orthogonal, then the series \sum_{j}T_j converges in the strong operator topology, and

:\Vert \sum_{j}T_j\Vert \le\sqrt{AB}.

Proof

If T_1,\ldots,T_n is a finite collection of bounded operators, then{{harvnb|Hörmander|1994}}

:\displaystyle{\sum_{i,j} |(T_i v,T_jv)| \le \left(\max_i \sum_j \|T_i^*T_j\|^{1\over 2}\right)\left(\max_i \sum_j \|T_iT_j^*\|^{1\over 2}\right)\|v\|^2.}

So under the hypotheses of the lemma,

:\displaystyle{\sum_{i,j} |(T_i v,T_jv)| \le AB\|v\|^2.}

It follows that

:\displaystyle{\|\sum_{i=1}^n T_iv\|^2 \le AB \|v\|^2,}

and that

:\displaystyle{\|\sum_{i=m}^n T_iv\|^2 \le \sum_{i,j\ge m} |(T_iv,T_jv)|.}

Hence, the partial sums

:\displaystyle{s_n=\sum_{i=1}^n T_iv}

form a Cauchy sequence.

The sum is therefore absolutely convergent with the limit satisfying the stated inequality.

To prove the inequality above set

:\displaystyle{R=\sum a_{ij}T_i^*T_j}

with |aij| ≤ 1 chosen so that

:\displaystyle{(Rv,v)=|(Rv,v)|=\sum |(T_iv,T_jv)|.}

Then

:\displaystyle{\|R\|^{2m} =\|(R^*R)^m\|\le \sum \|T_{i_1}^* T_{i_2} T_{i_3}^* T_{i_4} \cdots T_{i_{4m}}\| \le \sum \left(\|T_{i_1}^*\|\|T_{i_1}^*T_{i_2}\|\|T_{i_2}T_{i_3}^*\|\cdots \|T_{i_{4m-1}}^* T_{i_{4m}}\|\|T_{i_{4m}}\|\right)^{1\over 2}.}

Hence

:\displaystyle{\|R\|^{2m} \le n \cdot \max \|T_i\| \left(\max_i \sum_j \|T_i^*T_j\|^{1\over 2}\right)^{2m}\left(\max_i \sum_j \|T_iT_j^*\|^{1\over 2}\right)^{2m-1}.}

Taking 2mth roots and letting m tend to ∞,

:\displaystyle{\|R\|\le \left(\max_i \sum_j \|T_i^*T_j\|^{1\over 2}\right)\left(\max_i \sum_j \|T_iT_j^*\|^{1\over 2}\right),}

which immediately implies the inequality.

Generalization

The Cotlar-Stein lemma has been generalized, with sums being replaced by integrals.{{harvnb|Knapp|Stein|1971}}{{cite journal|last1=Calderon|first1=Alberto|last2=Vaillancourt|first2=Remi|title=On the boundedness of pseudo-differential operators|journal=Journal of the Mathematical Society of Japan|date=1971|volume=23|issue=2|pages=374–378|doi=10.2969/jmsj/02320374|doi-access=free}} Let X be a locally compact space and μ a Borel measure on X. Let T(x) be a map from X into bounded operators from E to F which is uniformly bounded and continuous in the strong operator topology. If

:\displaystyle{A= \sup_x \int_X \|T(x)^*T(y)\|^{1\over 2} \, d\mu(y),\,\,\, B= \sup_x \int_X \|T(y)T(x)^*\|^{1\over 2}\, d\mu(y),}

are finite, then the function T(x)v is integrable for each v in E with

:\displaystyle{\|\int_X T(x)v\, d\mu(x)\| \le \sqrt{AB} \cdot \|v\|.}

The result can be proven by replacing sums with integrals in the previous proof, or by utilizing Riemann sums to approximate the integrals.

Example

Here is an example of an orthogonal family of operators. Consider the infinite-dimensional matrices.

:

T=\left[

\begin{array}{cccc}

1&0&0&\vdots\\0&1&0&\vdots\\0&0&1&\vdots\\\cdots&\cdots&\cdots&\ddots\end{array}

\right]

and also

:

\qquad

T_1=\left[

\begin{array}{cccc}

1&0&0&\vdots\\0&0&0&\vdots\\0&0&0&\vdots\\\cdots&\cdots&\cdots&\ddots\end{array}

\right],

\qquad

T_2=\left[

\begin{array}{cccc}

0&0&0&\vdots\\0&1&0&\vdots\\0&0&0&\vdots\\\cdots&\cdots&\cdots&\ddots\end{array}

\right],

\qquad

T_3=\left[

\begin{array}{cccc}

0&0&0&\vdots\\0&0&0&\vdots\\0&0&1&\vdots\\\cdots&\cdots&\cdots&\ddots\end{array}

\right],

\qquad

\dots.

Then \Vert T_j\Vert=1 for each j, hence the series \sum_{j\in\mathbb{N}}T_j does not converge in the uniform operator topology.

Yet, since

\Vert T_j T_k^\ast\Vert=0

and

\Vert T_j^\ast T_k\Vert=0

for j\ne k,

the Cotlar–Stein almost orthogonality lemma tells us that

:T=\sum_{j\in\mathbb{N}}T_j

converges in the strong operator topology and is bounded by 1.

Notes

{{reflist|2}}

References

  • {{citation|first=Mischa|last= Cotlar|title=A combinatorial inequality and its application to L2 spaces|journal= Math. Cuyana|volume= 1|year=1955|pages= 41–55}}
  • {{citation|first=Lars|last=Hörmander|title=Analysis of Partial Differential Operators III: Pseudodifferential Operators|publisher=Springer-Verlag|year=1994|edition=2nd|isbn=978-3-540-49937-4|pages=165–166}}
  • {{citation|first1=Anthony W.|last1=Knapp|first2=Elias|last2=Stein|title=Intertwining operators for semisimple Lie groups|journal=Ann. Math.|volume=93|year=1971|pages=489–579|doi=10.2307/1970887 |jstor=1970887 }}
  • {{citation|first=Elias|last= Stein|title=Harmonic Analysis: Real-variable Methods, Orthogonality and Oscillatory Integrals|publisher= Princeton University Press|year= 1993|isbn= 0-691-03216-5}}

{{Functional analysis}}

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Category:Hilbert spaces

Category:Harmonic analysis

Category:Operator theory

Category:Inequalities (mathematics)

Category:Theorems in functional analysis

Category:Lemmas in mathematical analysis