Mureşan, A. S. “On the Uniqueness of the Solution of Dirichlet’s Problem Relative to a Strong Elliptic System of Second Order Partial Differential Equations”. Anal. Numér. Théor. Approx., vol. 9, no. 2, Aug. 1980, pp. 245-50, https://ictp.acad.ro/jnaat/journal/article/view/1980-vol9-no2-art9.