an integrable function which does not tend to zero
In this entry, we give an example of a function such that is Lebesgueintegrable on but does not tend to zero as.
Set
Note that every term in this series is positive, hence we may integrateterm-by-term, then make a change of variable andcompute the answer:
However, when is an integer, , so not only does nottend to zero as , it gets arbitrarily large.As we can see from the plot, we have a sequence of peaks which, as they gettaller, also get narrower in such a way that the total area underthe curve stays finite:
integrable-no-tend-zero.gif
By a variation of our procedure, we can produce a function which isdefined almost everywhere on the interval , is Lebesgue integrable,but is unbounded on any subinterval, no matter how small. For instance,define
Making a computation similar to the one above, we find that
Hence the integral is finite.
Now, however, we find that cannot be bounded in any interval,however small. For, in any interval, we can find rationalnumbers. Given a rational number , there are an infinitenumber of ways to express it as a fraction . Foreach of these ways, we have a term in the series which equals 1when , hence diverges to infinity
.
To help in understanding this function, we have made a slide showwhich shows partial sums of the series. As before, the successivepeaks become narrower in such a way that the arae under the curvestays finite but, this time, instead of marching off to infinity,they become dense in the interval.
integrable-everywhere-unbounded_3.gif