@article{10_1017_fms_2016_20,
     author = {MICHAEL FREEDMAN and VYACHESLAV KRUSHKAL},
     title = {ENGEL {RELATIONS} {IN} {4-MANIFOLD} {TOPOLOGY}},
     journal = {Forum of Mathematics, Sigma},
     publisher = {mathdoc},
     volume = {4},
     year = {2016},
     doi = {10.1017/fms.2016.20},
     language = {en},
     url = {http://geodesic.mathdoc.fr/articles/10.1017/fms.2016.20/}
}