@article{BUMI_1958_3_13_4_a9,
     author = {Bellman, Richard},
     title = {On a {Liouville} transformation for $u_{xx} + u_{yy} \pm  a^{2}(x,y)u=0$.},
     journal = {Bollettino della Unione matematica italiana},
     pages = {535--538},
     publisher = {mathdoc},
     volume = {Ser. 3, 13},
     number = {4},
     year = {1958},
     zbl = {0087.09601},
     mrnumber = {100712},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/BUMI_1958_3_13_4_a9/}
}