Milusheva, S. D., and Baĭnov D. (Dimitŭr). “Justification of the Averaging Method for Multipoint Boundary Value Problems for a Class of Functional-Differential Equations With ‘maximus’”. Collectanea Mathematica, vol. 37, no. 3, Mar. 2007, pp. 297-04, https://raco.cat/index.php/CollectaneaMathematica/article/view/57056.