Gheorghiu, C. I., and A. Tămăşan. “On the Existence and Uniqueness of Positive Solutions of Some Mildly Nonlinear Elliptic Boundary Value Problems”. Rev. Anal. Numér. Théor. Approx., vol. 24, no. 1, Aug. 1995, pp. 125-9, https://ictp.acad.ro/jnaat/journal/article/view/1995-vol24-nos1-2-art13.