J. A. Al-hawasy and S. J. M. Al-Qaisi, “The Continuous Classical Boundary Optimal Control of a Couple Nonlinear Elliptic Partial Differential Equations with State Constraints”, Al-Mustansiriyah Journal of Science, vol. 30, no. 1, pp. 143–151, Aug. 2019, doi: 10.23851/mjs.v30i1.464.