@article{ZNSL_1998_248_a12,
     author = {M. N. Yakovlev},
     title = {Solvability of a periodic boundary-value problem for a system of first-order ordinary differential equations 
},
     journal = {Zapiski Nauchnykh Seminarov POMI},
     pages = {231--241},
     publisher = {mathdoc},
     volume = {248},
     year = {1998},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/ZNSL_1998_248_a12/}
}