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 limp||f||p=||f|| we have to show that ||f|| is the supremum, right??




To show that, we assume that ||f||ϵ is the supremum.



From the essential supremum we have that m({|f|>||f||ϵ})=0.



So, we have to show that m({|f|>||f||ϵ})>0.



Let A={|f|>||f||ϵ}.



We have that A|f|p|f|p||f||p.




A|f|p>A(||f||ϵ)p=(||f||ϵ)pm(A)



So, m(A)1/p(||f||ϵ)<||f||p||f||



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



EDIT:



Is it as followed??




We have that 0<||f||ϵ<||f|| for some ϵ>0.



||f|| is the essential supremum. So, from the definition we have that m({|f(x)>||f||ϵ})>0.



Let A={|f|>||f||ϵ}.



We have that A|f|p|f|p||f||p.



A|f|p>A(||f||ϵ)p=(||f||ϵ)pm(A)




So, m(A)1/p(||f||ϵ)<||f||p||f||



Taking the limit p+ we have the following:



limp+m(A)1/p(||f||ϵ)<limp+||f||pm(A)>0limp+m(A)1/p=1||f||ϵ<limp+||f||p



Is this correct?? How do we conclude that limp+||f||p=||f|| ??

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