Sunday, 26 March 2017

real analysis - Show that limptoinfty|f|p=|f|infty

In a space with measure 1, ||f||p is an increasing function with respect of p. To show that lim we have to show that ||f||_{\infty} is the supremum, right??




To show that, we assume that ||f||_{\infty}-\epsilon is the supremum.



From the essential supremum we have that m(\{|f|>||f||_{\infty}-\epsilon\})=0.



So, we have to show that m(\{|f|>||f||_{\infty}-\epsilon\})>0.



Let A=\{|f|>||f||_{\infty}-\epsilon\}.



We have that \int_A |f|^p \leq \int |f|^p \leq ||f||_{\infty}^p.




\int_A |f|^p >\int_A (||f||_{\infty}-\epsilon)^p=(||f||_{\infty}-\epsilon)^p m(A)



So, m(A)^{1/p} (||f||_{\infty}-\epsilon)<||f||_p \leq ||f||_{\infty}



How could we continue to show that m(A)>0??



EDIT:



Is it as followed??




We have that 0<||f||_{\infty}-\epsilon<||f||_{\infty} for some \epsilon>0.



||f||_{\infty} is the essential supremum. So, from the definition we have that m\left ( \{|f(x)>||f||_{\infty}-\epsilon\} \right )>0.



Let A=\{|f|>||f||_{\infty}-\epsilon\}.



We have that \int_A |f|^p \leq \int |f|^p \leq ||f||_{\infty}^p.



\int_A |f|^p >\int_A (||f||_{\infty}-\epsilon)^p=(||f||_{\infty}-\epsilon)^p m(A)




So, m(A)^{1/p} (||f||_{\infty}-\epsilon)<||f||_p \leq ||f||_{\infty}



Taking the limit p \rightarrow +\infty we have the following:



\lim_{p \rightarrow +\infty}m(A)^{1/p} (||f||_{\infty}-\epsilon)<\lim_{p \rightarrow +\infty} ||f||_p \overset{ m(A)>0 \Rightarrow \lim_{p \rightarrow +\infty}m(A)^{1/p}=1}{\Longrightarrow} \\ ||f||_{\infty}-\epsilon<\lim_{p \rightarrow +\infty} ||f||_p



Is this correct?? How do we conclude that \lim_{p \rightarrow +\infty} ||f||_p=||f||_{\infty} ??

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