@article{CLTM_1975_1975_1_a19,
     author = {D{\'\i}az, J. B.},
     title = {Reflection principles for the iterated {Helmholtz} equation.},
     journal = {Collectanea mathematica},
     pages = {253-266},
     publisher = {mathdoc},
     volume = {26},
     number = {3},
     year = {1975},
     zbl = {0333.35023},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/CLTM_1975_1975_1_a19/}
}