In a previous paper Guo et al. (Fractional integration on mixed norm spaces. I. Preprint, 2022), we characterized the boundedness of fractional integration operators between mixed norm spaces over the unit disk. In this paper, we characterize the boundedness between X and Y, where X,Y∈{Hp(0<p<∞),H∞,BMOA,B,H(p,q,α)}.\documentclass[12pt]{minimal}
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\begin{document}$$\begin{aligned} \begin{aligned} X, Y \in \{ H^p \ (0<p<\infty ),\ H^\infty ,\ \text {BMOA}, \ \mathcal {B}, \ H(p,q,\alpha ) \}. \end{aligned} \end{aligned}$$\end{document}As in Guo et al. (Fractional integration on mixed norm spaces. I. Preprint, 2022), we cover three types of fractional integration: Flett, Hadamard, and Riemann-Liouville, and we consider complex orders t∈C\documentclass[12pt]{minimal}
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\begin{document}$$t \in \mathbb {C}$$\end{document} instead of mere t∈R\documentclass[12pt]{minimal}
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\begin{document}$$t \in \mathbb {R}$$\end{document}. Our findings provide, in particular, a complete answer to a problem started by Flett in 1972 (Theorem C).
