Career
Professor at the Technical University of Denmark (Danmarks Tekniske Højskole, DtH) in the period from 1954 to 1974. Følner published a comprehensive survey of almost periodic functions with Harald Bohr, and continued with further studies on this topic in his doctoral dissertation. Følner is known internationally for the Følner sequence: a Følner sequence for a group is a sequence of sets satisfying a particular condition.
If a group has a Følner sequence with respect to its action on itself, the group is amenable.
A more general notion of Følner nets can be defined analogously, and is suited for the study of uncountable groups. Følner married Harald Bohr"s daughter, Ellen Bohr (1923–1972).