Bartuzel, G., and A. Fryszkowski. “Stability of Principal Eigenvalue of the Schrödinger Type Problem for Differential Inclusions”. Topological Methods in Nonlinear Analysis, vol. 16, no. 1, Sept. 2000, pp. 181-94, https://apcz.umk.pl/TMNA/article/view/TMNA.2000.037.