Fei, Guihua. “Multiple Periodic Solutions of Asymptotically Linear Hamiltonian Systems via Conley Index Theory”. Topological Methods in Nonlinear Analysis 23, no. 1 (March 1, 2004): 89–114. Accessed March 29, 2024. https://apcz.umk.pl/TMNA/article/view/TMNA.2004.005.