1. Nonpositively curved metrics on 2-polyhedra;Ballmann;Math. Z.,1996
2. Whitehead's asphericity question and its relation to other open problems;Berrick,2019
3. J. H. C. Whitehead's asphericity question;Bogley,1993
4. Edge-disjoint branchings;Edmonds,1973
5. Reducible diagrams and equations over groups;Gersten,1987