Landau's converse to H\" older's inequality

By H\" older's inequality, if $\mathbf{x} \in \ell^p$, then $\mathbf{x}\mathbf{y} \in \ell^1$ for all $\mathbf{y} \in \ell^q$. Landau proved the converse result: If $\mathbf{x}\mathbf{y} \in \ell^1$ for all $\mathbf{y} \in \ell^q$, then $\mathbf{x} \in \ell^p$. This paper proves Landau's theorem and considers a related problem for Hilbert's inequality.