Fundamental isomorphism theorems for quantum groups

The lattice of subgroups of a group is the subject of numerous results revolving around the central theme of decomposing the group into “chunks” (subquotients) that can then be compared to one another in various ways. Examples of results in this class would be the Noether isomorphism theorems, Zassenhaus’ butterfly lemma, the Schreier refinement theorem, the Dedekind modularity law, and last but not least the Jordan–Hölder theorem. We discuss analogues of the above-mentioned results in the context of locally compact quantum groups and linearly reductive quantum groups. The nature of the two cases is different: the former is operator algebraic and the latter Hopf algebraic, hence the corresponding two-part organization of our study. Our intention is that the analytic portion be accessible to the algebraist and vice versa. The upshot is that in the locally compact case one often needs further assumptions (integrability, compactness, discreteness). In the linearly reductive case on the other hand, the quantum versions of the results hold without further assumptions. Moreover the case of compact/discrete quantum groups is usually covered by both the linearly reductive and the locally compact framework, thus providing a bridge between the two.