Yang, Z., and D. O’Regan. “Positive Solutions for a $2n$th-Order Boundary Value Problem Involving All Derivatives of Odd Orders”. Topological Methods in Nonlinear Analysis, vol. 37, no. 1, Apr. 2011, pp. 87-101, https://apcz.umk.pl/TMNA/article/view/TMNA.2011.004.