@article{10_4153_CJM_1969_050_6,
     author = {Kreith, Kurt},
     title = {Lower {Bounds} for the {Essential} {Spectrum} of {Fourth-Order} {Differential} {Operators}},
     journal = {Canadian journal of mathematics},
     pages = {460--465},
     year = {1969},
     volume = {21},
     number = {1},
     doi = {10.4153/CJM-1969-050-6},
     url = {http://geodesic.mathdoc.fr/articles/10.4153/CJM-1969-050-6/}
}