Kelevedjiev, Petio, and Nedyu Popivanov. 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 (December):55-70. https://annual.uni-sofia.bg/index.php/fmi/article/view/190.