@article{10_4153_CMB_1977_062_2,
     author = {Duff, G. F. D.},
     title = {Hyperbolic {Differential} {Equations} and {Wave} {Propagation}},
     journal = {Canadian mathematical bulletin},
     pages = {405--428},
     year = {1977},
     volume = {20},
     number = {4},
     doi = {10.4153/CMB-1977-062-2},
     url = {http://geodesic.mathdoc.fr/articles/10.4153/CMB-1977-062-2/}
}