HELGASON, Sigurdur. Some problems in the theory of almost periodic functions. MATHEMATICA SCANDINAVICA, [S. l.], v. 3, p. 49–67, 1955. DOI: 10.7146/math.scand.a-10425. Disponível em: https://tidsskrift.dk/math/article/view/10425. Acesso em: 13 aug. 2026.