@article{ARM_1967__3_4_a0,
     author = {Neuman, Franti\v{s}ek},
     title = {Extremal property of the equation $y''=-k^2y$},
     journal = {Archivum mathematicum},
     pages = {161--164},
     publisher = {mathdoc},
     volume = {03},
     number = {4},
     year = {1967},
     mrnumber = {0236465},
     zbl = {0217.40202},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/ARM_1967__3_4_a0/}
}