S V Ivanov, On
asphericity of group presentations given by labelled oriented
trees, preprint (1996)
9
T Maeda, Knotted
surfaces in the 4–sphere with no minimal Seifert
manifolds, from: "Combinatorial and geometric group theory
(Edinburgh, 1993)", London Math. Soc. Lecture Note Ser. 204,
Cambridge Univ. Press (1995) 239–246 MR1320286
10
W Magnus, Über
diskontinuierliche Gruppen mit einer definierenden Relation
(der Freiheitssatz), J. Reine Angew. Math. 163 (1930)
141–165
11
D I
Moldavanskiĭ, Certain subgroups of groups with one
defining relation, Sibirsk. Mat. Ž. 8 (1967)
1370–1384 MR0220810
12
E R Strasser,
Knot-like groups, from: "Knots, groups, and
3–manifolds (papers dedicated to the memory of R.H.
Fox)", Princeton Univ. Press (1975) 119–133 MR0440531