Fractional Volterra-type operators from Bergman spaces with two-sided doubling weights to Hardy spaces

RESUMO

For \(\omega\in\calD\), we give necessary and sufficient symbol conditions for the boundedness and compactness of the Riemann--Liouville family\(V^\varphi_{\alpha,\beta}:A^p_\omega\to H^q\) for every \(0<p,q<\infty\) and admissible \(\alpha,\beta>0\): without any additional weight assumption when \(\alpha\ge\beta\), and, when \(\alpha<\beta\), under the condition \(p(\beta-\alpha)<d_-(\omega)\), where \(d_-(\omega)\) is the critical reverse-doubling exponent. We prove that this inequality is exactly equivalent to the naturally shifted source weight retaining the two-sided doubling and tail geometry required by the reduction; an integrable logarithmic example shows that the endpoint fails. The extension from power weights is not formal, because the reduction replaces \(\omega\) by \(\omega(z)(1-|z|^2)^{p(\alpha-\beta)}\), and a negative shift may destroy even integrability. Combining moment-induced Littlewood--Paley theory for doubling Bergman spaces with an exact coefficient-multiplier comparison, we establish the required Riemann--Liouville transfer, including the finite-dimensional exceptional modes at positive integral shift orders. A finite-codimensional range decomposition and a fractional \(g\)-function reduction then reduce both questions to a single weighted area-map problem. Within the admissible parameter set, the effects of \(\alpha\) are absorbed by the shifted source weight and bounded model corrections, so the final symbol conditions depend on \(\beta\). The thresholds \(p=2\) and \(p=q\) yield pointwise, Carleson, tent-integral, and non-tangential maximal criteria. In the tent-integral range, boundedness already implies compactness.