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单词 WielandtTheoremForUnitalNormedAlgebras
释义

Wielandt theorem for unital normed algebras


Theorem. (Wielandt (1949)) Let A be a normed unital algebra (with unit e). If x,yA then xy-yxe.

Proof.

Assume there are x,yA such that xy-yx=e. Then for all n we have

xny-yxn=nxn-10

We prove this by inductionMathworldPlanetmath over n. It holds for n=1 by assumptionPlanetmathPlanetmath. Assume it is valid for n. Then xn0 and

xn+1y-yxn+1=xn(xy-yx)+(xny-yxn)x
=xne+nxn-1x=xne+nxn=(n+e)xn

From this identity it follows that

nxn-1=xny-yxn2xny2xn-1xy

It follows that n2||x||||y|| for all n which is impossible.∎

Corollary. The identity operator on a Hilbert spaceMathworldPlanetmath cannot be expressed as a commutator of two bounded linear operators in ().

Remark. The above can be understood as a version of the uncertainty principle in one dimension. Let H=L2(). Let q:HH be q(f)(x):=xf(x) with D(q)={fL2():xxf(x)L2()}, the coordinate operator and p:HH,p(f)(x)=-if(x) the momentum operator with D(p):={fL2():fabsolutely  continuous,fL2()}. It follows that

pq-qp=-iidDonD=D(q)D(p)

According to the corollary D=L2() can never be the case.

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更新时间:2025/5/4 18:44:07