@incollection{JEDP_1987____A1_0,
     author = {H\"ormander, Lars},
     title = {Remarks on the {Klein-Gordon} equation},
     booktitle = {},
     series = {Journ\'ees \'equations aux d\'eriv\'ees partielles},
     eid = {1},
     pages = {1--9},
     publisher = {Ecole polytechnique},
     year = {1987},
     zbl = {0655.35057},
     language = {en},
     url = {http://www.numdam.org/item/JEDP_1987____A1_0/}
}