PETTERSEN, G. M. Summability Methods and Unbounded Sequences. MATHEMATICA SCANDINAVICA, [S. l.], v. 7, p. 170–176, 1959. DOI: 10.7146/math.scand.a-10570. Disponível em: https://tidsskrift.dk/math/article/view/10570. Acesso em: 14 aug. 2026.