@article{CRMATH_2003__337_9_587_0,
     author = {Perthame, Beno{\i}̂t and Vega, Luis},
     title = {Energy concentration and {Sommerfeld} condition for {Helmholtz} and {Liouville} equations},
     journal = {Comptes Rendus. Math\'ematique},
     pages = {587--592},
     publisher = {Elsevier},
     volume = {337},
     number = {9},
     year = {2003},
     doi = {10.1016/j.crma.2003.09.006},
     language = {en},
     url = {http://archive.numdam.org/articles/10.1016/j.crma.2003.09.006/}
}