Geodesic
A particular case of the equation $K(y)u_{xx}+\operatorname{sgn}(xy)K(x)u_{yy}=0$
Trudy seminara po kraevym zadačam, Trudy Seminara po Kraevym Zadacham, Tome 12 (1975) no. 12, pp. 126-133 
Yu. M. Krikunov. A particular case of the equation $K(y)u_{xx}+\operatorname{sgn}(xy)K(x)u_{yy}=0$. Trudy seminara po kraevym zadačam, Trudy Seminara po Kraevym Zadacham, Tome 12 (1975) no. 12, pp. 126-133. http://geodesic.mathdoc.fr/item/KUKZ_1975_12_12_a15/
@article{KUKZ_1975_12_12_a15,
     author = {Yu. M. Krikunov},
     title = {A particular case of the equation $K(y)u_{xx}+\operatorname{sgn}(xy)K(x)u_{yy}=0$},
     journal = {Trudy seminara po kraevym zada\v{c}am},
     pages = {126--133},
     year = {1975},
     volume = {12},
     number = {12},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/KUKZ_1975_12_12_a15/}
}
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