MARKSJÖ, B.; TEXTORIUS, Björn. On the Stability of Linear Differential Equations in Spaces with an Indefinite Metric. Mathematica Scandinavica, Berkeley, CA, United States, v. 20, p. 177–192, 1967. DOI: 10.7146/math.scand.a-10829. Disponível em: https://journals.msp.org/mscand/article/view/2493. Acesso em: 25 jul. 2026.