PERSSON, J. A Note on Formal and Analytic Cauchy Problems for Systems with Constant Coefficients. MATHEMATICA SCANDINAVICA, [S. l.], v. 15, p. 142–146, 1964. DOI: 10.7146/math.scand.a-10737. Disponível em: https://www.mscand.dk/article/view/10737. Acesso em: 29 apr. 2024.