I know that $F(x)=\int^x_a f(x)dx$ is derivable in $[a,b]$ if $f(x)$ is continuous. Is there a function $f(x)$ which is not continuous, but $F(x)=\int^x_a f(x)dx$ is derivable ?
Find a primitive function that is derivable
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calculus
1 Answers
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Let $f(x) = 0$ if $x$ is irrational, and $f(x) = \frac{1}{q}$ if $x = \frac{p}{q}$, where $x$ is a rational and $p$,$q$ are co-prime integers.
This is the classical example of a function which is Riemann integrable, but not continuous everywhere (it is continuous at the irrationals).
In fact, there is a general result called the Riemann-Lebesgue theorem which states that:
If $f:[a,b]\to \mathbb{R}$ is a bounded function, then $f$ is Riemann integrable on $[a,b]$ if and only if the set of discontinuities of $f$ is of measure $0$.
So you can pick any continuous function, change its value at a finite number of points, and you have your example.
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0@RagibZaman: That wiki does not talk about bounded a.e. In fact, now I remember! If $f$ is Riemann Integrable, then $f$ is bounded. See here: http://books.google.com/books?id=5hM0DcdEGf4C&pg=PA5#v=onepage&q&f=false. – 2012-02-23