MULLIKIN, A. L.; SMITH, K. T. Elliptic Differential Problems with High Order Boundary Conditions. Mathematica Scandinavica, Berkeley, CA, United States, v. 12, p. 178–188, 1963. DOI: 10.7146/math.scand.a-10681. Disponível em: https://journals.msp.org/mscand/article/view/2639. Acesso em: 25 jul. 2026.