# Infimum of integral of open set

• Dec 1st 2012, 05:59 PM
kanezila
Infimum of integral of open set
I have already done part a and b. Part a is easy, for part b, i let the anti-derivative of f to be k(t)+c and arrive at the answer that F(f)= 1/2+ 2*k(1/2) - k(1). But i don't know how to do the next part, can anyone give me a hint? the question c ask me to show that the infimum of F is 0 and it is never attained on A.
• Dec 1st 2012, 09:45 PM
GJA
Re: Infimum of integral of open set
Hi kanezila,

In part b, to prove that $F$ is continuous at $f$ we need to show that for $\epsilon>0$ there is $\delta>0$ such that whenever $\|f-g\|_{\infty}<\delta$, we have $|F(f)-F(g)|<\epsilon.$ Perhaps looking at an antiderivative will go somewhere, but this is probably best done by using the definition.

To prove part c looking at the following sequence of functions $\{f_{n}\}$ in $A$, where $f_{n}(x)=0$ for $x\in [0,1/2]$, $f_{n}(x)=(n+2)(x-1/2)$ for $x\in [1/2, 1/2+1/(n+2)]$ and $f_{n}(x)=1$ for $x\in [1/2+1/(n+2), 1].$ Then we should be able to show that $F(f_{n})=\frac{1}{2}(\frac{1}{n+2}).$ Since this goes to 0 as n goes to infinity the infimum must be 0, because each of the $f_{n}$'s belong to $A.$

Does this clear things up? Good luck!
• Dec 2nd 2012, 11:28 AM
kanezila
Re: Infimum of integral of open set
thank you very much for this, i have been trying to do this question for a whole day already.
• Dec 2nd 2012, 11:39 AM
cummings123321
Re: Infimum of integral of open set
could you tell me how to prove the first one???many thanks