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单词 ProofOfNeumannSeriesInBanachAlgebras
释义

proof of Neumann series in Banach algebras


Let x be an element of a Banach algebraMathworldPlanetmath with identity, x<1. By applying the properties of the Norm in a Banach algebra, we see that the partial sums form a Cauchy sequencePlanetmathPlanetmath: n=lmxnn=lmxn0 for l,m (as is well known from real analysis), so by completeness of the Banach Algebra, the series convergesPlanetmathPlanetmath to some element y=n=0xn.

We observe that for any m,

(1-x)n=0mxn=n=0mxn-n=1m+1xn=1-xm+1(1)

Furthermore, xm+1xm+1, so limmxm+1=0.

Thus, by taking the limit m on both sides of (1), we get

(1-x)y=1

(We can exchange the limit with the multiplication by (1-x), since the multiplication in Banach algebras is continuousMathworldPlanetmath)

Since the Banach algebra generated by a single element is commutative and (1-x) and y are both in the Banach algebra generated by x, we also get y(1-x)=1. Hence, y=(1-x)-1.

As in the first paragraph, the last claim y11-y again follows by applying the geometric series for real numbers.

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更新时间:2025/5/4 4:54:55