@article{ARM_1972_8_2_a0,
     author = {Barv{\'\i}nek, Erich},
     title = {Quadratic phases of differential equations $y''=q(t)y$},
     journal = {Archivum mathematicum},
     pages = {63--78},
     year = {1972},
     volume = {8},
     number = {2},
     mrnumber = {0326037},
     zbl = {0283.34011},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/ARM_1972_8_2_a0/}
}