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Integrable actions and quantum subgroups

We study homomorphisms of locally compact quantum groups from the point of view of integrability of the associated action. For a given homomorphism of quantum groups $Π\colon\mathbb{H}\to\mathbb{G}$ we introduce quantum groups $\mathbb{H}/\!\kerΠ$ and $\overline{\mathrm{im}\,Π}$ corresponding to the classical quotient by kernel and closure of image. We show that if the action of $\mathbb{H}$ on $\mathbb{G}$ associated to $Π$ is integrable then $\mathbb{H}/\!\kerΠ\cong\overline{\mathrm{im}\,Π}$ and characterize such $Π$. As a particular case we consider an injective continuous homomorphism $Π\colon{H}\to{G}$ between locally compact groups $H$ and $G$. Then $Π$ yields an integrable action of $H$ on $L^\infty\;\!\!(G)$ if and only if its image is closed and $Π$ is a homeomorphism of $H$ onto $\mathrm{im}\,Π$. We also give characterizations of open quantum subgroups and of compact quantum subgroups in terms of integrability and show that a closed quantum subgroup always gives rise to an integrable action. Moreover we prove that quantum subgroups closed in the sense of Woronowicz whose associated homomorphism of quantum groups yields an integrable action are closed in the sense of Vaes.