@article{10_4153_CJM_1978_104_2,
     author = {Rival, Ivan and Sands, Bill},
     title = {Planar {Sublattices} of a {Free} {Lattice.} {I}},
     journal = {Canadian journal of mathematics},
     pages = {1256--1283},
     year = {1978},
     volume = {30},
     number = {6},
     doi = {10.4153/CJM-1978-104-2},
     url = {http://geodesic.mathdoc.fr/articles/10.4153/CJM-1978-104-2/}
}