@article{10_2140_agt_2010_10_637,
     author = {Cooper, Daryl and Lackenby, Marc and Purcell, Jessica S},
     title = {The length of unknotting tunnels},
     journal = {Algebraic and Geometric Topology},
     pages = {637--661},
     year = {2010},
     volume = {10},
     number = {2},
     doi = {10.2140/agt.2010.10.637},
     url = {http://geodesic.mathdoc.fr/articles/10.2140/agt.2010.10.637/}
}