PERSSON, Jan. A Note on Formal and Analytic Cauchy Problems for Systems with Constant Coefficients. Mathematica Scandinavica, Berkeley, CA, United States, v. 15, p. 142–146, 1964. DOI: 10.7146/math.scand.a-10737. Disponível em: https://journals.msp.org/mscand/article/view/2607. Acesso em: 25 jul. 2026.