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单词 InfinitelydifferentiableFunctionThatIsNotAnalytic
释义

infinitely-differentiable function that is not analytic


If f𝒞, then we can certainly write a Taylor seriesMathworldPlanetmath for f. However, analyticity requires that this Taylor series actually converge (at least across some radius of convergenceMathworldPlanetmath) to f. It is not necessary that the power seriesMathworldPlanetmath for f converge to f, as the following example shows.

Let

f(x)={e-1x2x00x=0.

Then f𝒞, and for any n0, f(n)(0)=0 (see below). So the Taylor series for f around 0 is 0; since f(x)>0 for all x0, clearly it does not converge to f.

Proof that f(n)(0)=0

Let p(x),q(x)[x] be polynomials, and define

g(x)=p(x)q(x)f(x).

Then, for x0,

g(x)=(p(x)+p(x)2x3)q(x)-q(x)p(x)q2(x)e-1x2.

Computing (e.g. by applying L’Hôpital’s rule (http://planetmath.org/LHpitalsRule)), we see that g(0)=limx0g(x)=0.

Define p0(x)=q0(x)=1. Applying the above inductively, we see that we may write f(n)(x)=pn(x)qn(x)f(x). So f(n)(0)=0, as required.

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更新时间:2025/5/4 9:55:07