@article{10_1017_S0017089599970581,
     author = {LIU, WEIJIU and WILLIAMS, GRAHAM H.},
     title = {Exact {Neumann} boundary controllability for problems of transmission of the wave equation},
     journal = {Glasgow mathematical journal},
     pages = {125--139},
     year = {1999},
     volume = {41},
     number = {1},
     doi = {10.1017/S0017089599970581},
     url = {http://geodesic.mathdoc.fr/articles/10.1017/S0017089599970581/}
}