Kelevedjiev, P., & Popivanov, N. (2002). EXISTENCE OF SOLUTIONS OF BOUNDARY VALUE PROBLEMS FOR THE EQUATION $f(t,x,x’,x’’) = 0$ WITH FULLY NONLINEAR BOUNDARY CONDITIONS. Ann. Sofia Univ. Fac. Math. And Inf., 94, 55–70. Retrieved from https://annual.uni-sofia.bg/index.php/fmi/article/view/190