Bergman and Szego projections, Extremal Problems, and Square Functions

We study estimates for Hardy space norms of analytic projections. We first find a sufficient condition for the Bergman projection of a function in the unit disc to belong to the Hardy space $H^p$ for $1<p<\mathrm{\infty }$. We apply the result to prove a converse to an extension of Ryabykh’s theorem about Hardy space regularity for Bergman space extremal functions. We also prove that the $H^q$ norm of the Szeg"{o} projection of $F^{p\mathrm{/}2}{\bar{F}}^{(p\mathrm{/}2)-1}$ cannot be too small if $F$ is analytic, for certain values of $p$ and $q$. We apply this to show that the best analytic approximation in $L^p$ of a function in both $L^p$ and $L^q$ will also lie in $L^q$, for certain values of $p$ and $q$.