HELGASON, Sigurdur. Some problems in the theory of almost periodic functions. Mathematica Scandinavica, Berkeley, CA, United States, v. 3, p. 49–67, 1955. DOI: 10.7146/math.scand.a-10425. Disponível em: https://journals.msp.org/mscand/article/view/2894. Acesso em: 25 aug. 2026.