@article{10_4064_cm_23_1_107_114,
     author = {J. Kisy\'nski},
     title = {On operator-valued solutions of {d'Alembert's} functional equation, {I}},
     journal = {Colloquium Mathematicum},
     pages = {107--114},
     publisher = {mathdoc},
     volume = {23},
     number = {1},
     year = {1971},
     doi = {10.4064/cm-23-1-107-114},
     language = {en},
     url = {http://geodesic.mathdoc.fr/articles/10.4064/cm-23-1-107-114/}
}