Wednesday, July 24, 2019

real analysis - Convergence from Lp to Linfty




If f is a function such that fLLp0 where L is the space of essentially bounded functions and 0<p0<. Show that ||f||Lp||f||L as p. Where ||f||L is the least MR such that |f(x)|M for almost every xX.



The hint says to use the monotone convergence theorem, but i can't even see any pointwise convergence of functions.
Any help is appreciated.


Answer



Hint: Let M< and consider
\int_{E_M}\left|\frac{f(x)}{M}\right|^p\,\mathrm{d}x
where E_M=\{x:|f(x)|\gt M\}. I believe the Monotone Convergence Theorem works here.



Further Hint: M\lt\|f\|_{L^\infty} implies E_M has positive measure. On E_M, \left|\frac{f(x)}{M}\right|^p tends to \infty pointwise. MCT says that for some p, the integral above exceeds 1.


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