It is well-known that any compact Lie group appears as closed subgroup of a unitary group, $G\subset U_N$. The unitary group $U_N$ has a free analogue $U_N^+$, and the study of the closed quantum subgroups $G\subset U_N^+$ is a problem of general interest. We review here the basic tools for dealing with such quantum groups, with all the needed preliminaries included, and we discuss as well a number of more advanced topics.
"Quantum groups, from a functional analysis perspective." Adv. Oper. Theory 4 (1) 164 - 196, Winter 2019. https://doi.org/10.15352/aot.1804-1342