Bartuzel, G., and A. Fryszkowski. “Pointwise Estimates in the Filippov Lemma and Filippov-Ważewski Theorem for Fourth Order Differential Inclusions”. Topological Methods in Nonlinear Analysis, vol. 52, no. 2, Aug. 2018, pp. 515-40, https://apcz.umk.pl/TMNA/article/view/TMNA.2018.014.