REUTER, G. E. H. A note on contraction semigroups. Mathematica Scandinavica, Berkeley, CA, United States, v. 3, p. 275–280, 1955. DOI: 10.7146/math.scand.a-10446. Disponível em: https://journals.msp.org/mscand/article/view/2877. Acesso em: 8 sep. 2026.