Saturday, 20 July 2013

integration - Not Riemman integrable derivaitive



Is there a function F:[a,b]R differentiable with F=f, but f is not Riemann integrable on [a,b]. [a,b] is a bounded interval?






Motivation
Rudin page 152 Theorem 7.17: Suppose {fn} a sequence of functions, differentiable on [a,b] and such that {fn(x0)} converges for some point x0[a,b]. If {fn} converges uniformly on [a,b] to g, then {fn} converges uniformly on [a,b] to f where f=g.



In the remark, it says if the continuity of the function fn is assumed in addition to the above hypothesis, then a much shorter proof can be based on fundamental theorem of calculus and the theorem that bafndαbafdα if we have fnf uniformly.




I was thinking that we don't really need fn to be continuous, we just need it to be Riemann integrable so we can apply Fundamental Thm of Calculus. From the remark I guess that there should be a derivative that is not integrable.






EDIT
Dominic Michaelis answered my question immediately. A further thought yields a more difficult question, is there a function as said above with f bounded on [a,b]?


Answer



Take F[1,1]R with
F(x)={x2sin(1x2)x00x=0
The derivative is not a regulated function, and not bounded at 0. I don't know a definition of riemann integrable allowing those things.


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