IDCZAK, D. $M$-periodic problem of order $2k$. Topological Methods in Nonlinear Analysis, [S. l.], v. 11, n. 1, p. 169–185, 1998. Disponível em: https://apcz.umk.pl/TMNA/article/view/TMNA.1998.011. Acesso em: 19 may. 2024.