PETTERSEN, G. M. Summability Methods and Unbounded Sequences. Mathematica Scandinavica, Berkeley, CA, United States, v. 7, p. 170–176, 1959. DOI: 10.7146/math.scand.a-10570. Disponível em: https://journals.msp.org/mscand/article/view/2776. Acesso em: 8 sep. 2026.