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@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/} }
Neuman, František. Extremal property of the equation $y''=-k^2y$. Archivum mathematicum, Tome 03 (1967) no. 4, pp. 161-164. http://geodesic.mathdoc.fr/item/ARM_1967__3_4_a0/