@article{GT_2000_4_1_a7,
     author = {Goda, Hiroshi and Scharlemann, Martin and Thompson, Abigail},
     title = {Levelling an unknotting tunnel},
     journal = {Geometry & topology},
     pages = {243--275},
     publisher = {mathdoc},
     volume = {4},
     number = {1},
     year = {2000},
     doi = {10.2140/gt.2000.4.243},
     url = {http://geodesic.mathdoc.fr/articles/10.2140/gt.2000.4.243/}
}