Gabriel's problem for harmonic Hardy spaces

We obtain inequalities of the form $${\int }_C\mathrm{\mid }f(z){\mathrm{\mid }}^p\mathrm{\mid }dz\mathrm{\mid }\le A(p){\int }_{\mathbb{T}}\mathrm{\mid }f(z){\mathrm{\mid }}^p\mathrm{\mid }dz\mathrm{\mid },(p>1)$$ where $f$ is harmonic in the unit disk $\mathbb{D}$, $\mathbb{T}$ is the unit circle, and $C$ is any convex curve in $\mathbb{D}$. Such inequalities were originally studied for analytic functions by R. M. Gabriel [Proc. London Math. Soc. 28(2), 1928]. We show that these results, unlike in the case of analytic functions, cannot be true in general for $0<p\le 1$. Therefore, we produce an inequality of a slightly different type, which deals with the case $0<p<1$. An example is given to show that this result is “best possible”, in the sense that an extension to $p=1$ fails. Then we consider the special case when $C$ is a circle, and prove a refined result which surprisingly holds for $p=1$ as well. We conclude with a maximal theorem which has potential applications.