$f:[a,b] \to \mathbb R$ be continuous , differentiable function such that $f'$ is bounded on $[a,b]$ ; then is $f' \in \mathcal R[a,b]$?

125 Views Asked by At

Let $f:[a,b] \to \mathbb R$ be continuous , differentiable function ( having one sided derivatives at $a$ and $b$ ) such that $f'$ is bounded on $[a,b]$ ; then is $f'$ Riemann integrable on $[a,b]$ ?

1

There are 1 best solutions below

7
On

Not true. The first example given appeared in

Vito Volterra, Alcune osservazioni sulle funzioni punteggiate discontinue, Gior- nale di Matematiche, (1881) vol. 19, 76–86.

[Added note:]

Maybe some of you are at a library with inadequate archives ... or don't like reading Italian. So I will try a little harder to help.

All you need to do is take a fat Cantor set and construct your function around that. My friend James Foran included a construction of such a function on page 267 in his book

Foran, James: Fundamentals of Real Analysis. Marcel Dekker lnc., New York 1991, 496 pp.,

I thought this was pretty well known. It was precisely the motivation that Lebesgue gave for developing a new theory of integration. If you look in the early literature of integration theory you find a great deal of attention on "improper" integrals--how best to handle unbounded functions. Volterra's example shows that the Riemann integral doesn't handle even the bounded functions correctly.