@article{10_4064_cm_76_1_117_142,
     author = {Weijiu Liu and Graham Williams},
     title = {Exact {Neumann} boundary controllability for second order hyperbolic equations},
     journal = {Colloquium Mathematicum},
     pages = {117--142},
     publisher = {mathdoc},
     volume = {76},
     number = {1},
     year = {1998},
     doi = {10.4064/cm-76-1-117-142},
     language = {en},
     url = {http://geodesic.mathdoc.fr/articles/10.4064/cm-76-1-117-142/}
}