@article{RSMUP_1981__65__223_0,
     author = {Zanolin, Fabio},
     title = {Periodic solutions for second order differential systems with damping},
     journal = {Rendiconti del Seminario Matematico della Universit\`a di Padova},
     pages = {223--234},
     publisher = {Seminario Matematico of the University of Padua},
     volume = {65},
     year = {1981},
     mrnumber = {653296},
     zbl = {0487.34033},
     language = {en},
     url = {http://archive.numdam.org/item/RSMUP_1981__65__223_0/}
}