@article{RSMUP_1997__97__193_0,
     author = {Matzeu, M. and Girardi, M.},
     title = {On periodic solutions of a class of second order nonautonomous systems with nonhomogeneous potentials indefinite in sign},
     journal = {Rendiconti del Seminario Matematico della Universit\`a di Padova},
     pages = {193--210},
     publisher = {Seminario Matematico of the University of Padua},
     volume = {97},
     year = {1997},
     mrnumber = {1476171},
     zbl = {0891.34050},
     language = {en},
     url = {http://www.numdam.org/item/RSMUP_1997__97__193_0/}
}