Andres, J., L. Malaguti, and M. Pavlačková. “On Second-Order Boundary Value Problems in Banach Spaces: A Bound Sets Approach”. Topological Methods in Nonlinear Analysis, vol. 37, no. 2, Apr. 2011, pp. 303-41, https://apcz.umk.pl/TMNA/article/view/TMNA.2011.016.