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单词 ProofOfRadiusOfConvergence
释义

proof of radius of convergence


According to Cauchy’s root testMathworldPlanetmath a power seriesMathworldPlanetmath is absolutely convergent if

lim supk|ak(x-x0)k|k=|x-x0|lim supk|ak|k<1.

This is obviously true if

|x-x0|<1lim supk|ak|k=lim infk1|ak|k=.

In the same way we see that the series is divergent if

|x-x0|>lim infk1|ak|k,

which means that the right hand side is the radius of convergenceMathworldPlanetmath of the power series.Now from the ratio testMathworldPlanetmath we see that the power series is absolutely convergent if

limk|ak+1(x-x0)k+1ak(x-x0)k|=|x-x0|limk|ak+1ak|<1.

Again this is true if

|x-x0|<limk|akak+1|.

The series is divergent if

|x-x0|>limk|akak+1|,

as follows from the ratio test in the same way. So we see that in this way too we can the radius of convergence.

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更新时间:2025/5/4 3:14:44