@article{10_21136_CMJ_1976_101437,
     author = {Everitt, William Norrie and Giertz, Magnus},
     title = {On the limit-3 classification of the square of a second-order, linear differential expression},
     journal = {Czechoslovak Mathematical Journal},
     pages = {653--665},
     publisher = {mathdoc},
     volume = {26},
     number = {4},
     year = {1976},
     doi = {10.21136/CMJ.1976.101437},
     mrnumber = {0430398},
     zbl = {0363.34016},
     language = {en},
     url = {http://geodesic.mathdoc.fr/articles/10.21136/CMJ.1976.101437/}
}