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单词 OstensiblyDiscontinuousAntiderivative
释义

ostensibly discontinuous antiderivative


The real function

x15-3cosx(1)

is continuousMathworldPlanetmathPlanetmath for any x (the denominator is always positive) and therefore it has an antiderivative, defined for all x.  Using the universal trigonometric substitutionPlanetmathPlanetmath

cosx:=1-t21+t2,dx=2dt1+t2,t=tanx2,

we obtain

5-3cosx=5(1+t2)-3(1-t2)1+t2=2(1+4t2)1+t2,

whence

dx5-3cosx=dt1+4t2=12arctan2t+C=12arctan(2tanx2)+C.

This result is not defined in the odd multiples of π, and it seems that the functionMathworldPlanetmath (1) does not have a continuous antiderivative.

However, one can check that the function

xx4+12arctansinx3-cosx+C(2)

is everywhere continuous and has as its derivativePlanetmathPlanetmath the function (1); one has

|sinx3-cosx|13-1=12<π2.

References

  • 1 Ernst Lindelöf: Johdatus korkeampaan analyysiin. Fourth edition. Werner Söderström Osakeyhtiö, Porvoo ja Helsinki (1956).
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更新时间:2025/5/4 19:59:07