function of not bounded variation
Example. We show that the function
which is continuous in the whole , is not of bounded variation
on any interval containing the zero.
Let us take e.g. the interval . Chose a positive integer such that and the partition of the interval with the points into the subintervals. For each positive integer we have (see this (http://planetmath.org/CosineAtMultiplesOfStraightAngle))
Thus we see that the total variation of in all partitions of is at least
Since the harmonic series diverges, the above sum increases to as . Accordingly, the total variation must be infinite, and the function is not of bounded variation on .
It is not difficult to justify that 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).