Geodesic
A boundary value problem for the equation $u_{xx}+\operatorname{sgn}(xy)u_{yy}=0$
Trudy seminara po kraevym zadačam, Trudy Seminara po Kraevym Zadacham, Tome 11 (1974) no. 11, pp. 100-106 
Yu. M. Krikunov. A boundary value problem for the equation $u_{xx}+\operatorname{sgn}(xy)u_{yy}=0$. Trudy seminara po kraevym zadačam, Trudy Seminara po Kraevym Zadacham, Tome 11 (1974) no. 11, pp. 100-106. http://geodesic.mathdoc.fr/item/KUKZ_1974_11_11_a12/
@article{KUKZ_1974_11_11_a12,
     author = {Yu. M. Krikunov},
     title = {A boundary value problem for the equation $u_{xx}+\operatorname{sgn}(xy)u_{yy}=0$},
     journal = {Trudy seminara po kraevym zada\v{c}am},
     pages = {100--106},
     year = {1974},
     volume = {11},
     number = {11},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/KUKZ_1974_11_11_a12/}
}
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