@article{JEMS_2010_12_1_a6,
     author = {Antonio Ca\~nada and Salvador Villegas},
     title = {Lyapunov inequalities for {Neumann} boundary conditions at higher eigenvalues},
     journal = {Journal of the European Mathematical Society},
     pages = {163--178},
     publisher = {mathdoc},
     volume = {12},
     number = {1},
     year = {2010},
     doi = {10.4171/jems/193},
     url = {http://geodesic.mathdoc.fr/articles/10.4171/jems/193/}
}