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On a theorem of Privalov and normal functions
Authors:Daniel Girela
Affiliation:Departamento de Análisis Matemático, Facultad de Ciencias, Universidad de Málaga, 29071 Málaga, Spain
Abstract:A well known result of Privalov asserts that if $f$ is a function which is analytic in the unit disc $Delta ={zin mathbb {C} : vert zvert <1} $, then $f$ has a continuous extension to the closed unit disc and its boundary function $f(esp {itheta })$ is absolutely continuous if and only if $fsp {prime }$ belongs to the Hardy space $Hsp {1}$. In this paper we prove that this result is sharp in a very strong sense. Indeed, if, as usual, $M_{1}(r, fsp {prime })= frac {1}{2pi }int _{-pi }sp {pi }left vert fsp {prime }(resp {itheta }) right vert , dtheta ,$ we prove that for any positive continuous function $phi $ defined in $(0, 1)$ with $phi (r)to infty $, as $rto 1$, there exists a function $f$ analytic in $Delta $ which is not a normal function and with the property that $M_{1}(r, fsp {prime })leq phi (r) $, for all $r$ sufficiently close to $1$.

Keywords:Normal functions   Hardy spaces   integral means   theorem of Privalov
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