Kelevedjiev, P., and N. Popivanov. “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., vol. 94, Dec. 2002, pp. 55-70, https://annual.uni-sofia.bg/index.php/fmi/article/view/190.