Factorization in Haar system Hardy spaces

Abstract: A Haar system Hardy space is the completion of the linear span of the Haar system $(h_I)I$, either under a rearrangement-invariant norm $|\cdot |$ or under the associated square function norm \begin{equation*} \Bigl| \sum_Ia_Ih_I \Bigr|{} = \Bigl| \Bigl( \sum_I a_I^2 h_I^2 \Bigr)^{1/2} \Bigr|. \end{equation} Apart from $L^p$, $1\le p<\infty$, the class of these spaces includes all separable rearrangement-invariant function spaces on $[0,1]$ and also the dyadic Hardy space $H^1$. Using a unified and systematic approach, we prove that a Haar system Hardy space $Y$ with $Y\ne C(\Delta)$ ($C(\Delta)$ denotes the continuous functions on the Cantor set) has the following properties, which are closely related to the primariness of $Y$: For every bounded linear operator $T$ on $Y$, the identity $I_Y$ factors either through $T$ or through $I_Y - T$, and if $T$ has large diagonal with respect to the Haar system, then the identity factors through $T$. In particular, we obtain that \begin{equation*} \mathcal{M}_Y = { T\in \mathcal{B}(Y) : I_Y \ne ATB\text{ for all } A, B\in \mathcal{B}(Y) } \end{equation*} is the unique maximal ideal of the algebra $\mathcal{B}(Y)$ of bounded linear operators on $Y$. Moreover, we prove similar factorization results for the spaces $\ell^p(Y)$, $1\le p \leq \infty$, and use them to show that they are primary.