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单词 FunctionOfNotBoundedVariation
释义

function of not bounded variation


Example.  We show that the function

f:x{xcosπxwhenx0,0whenx=0,

which is continuousMathworldPlanetmath in the whole , is not of bounded variationMathworldPlanetmath on any interval containing the zero.

Let us take e.g. the interval  [0,a].  Chose a positive integer m such that   1m<a and the partition of the interval with the points   1m,1m+1,1m+2,,1n  into the subintervals[0,1n],[1n,1n-1],,[1m+1,1m],[1m,a].  For each positive integer ν we have (see this (http://planetmath.org/CosineAtMultiplesOfStraightAngle))

f(1ν)=1νcosνπ=(-1)νν.

Thus we see that the total variationMathworldPlanetmath of f in all partitions of  [0,a]  is at least

1n+(1n+1n-1)++(1m+1+1m)=1m+2ν=m+1n1ν.

Since the harmonic series diverges, the above sum increases to as  n.  Accordingly, the total variation must be infinite, and the function f is not of bounded variation on  [0,a].

It is not difficult to justify that f is of bounded variation on any finite interval that does not contain 0.

References

  • 1 E. Lindelöf: Differentiali- ja integralilaskuja sen sovellutukset III. Toinen osa.  Mercatorin Kirjapaino Osakeyhtiö, Helsinki (1940).
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