1. On the homotopy type of non-simply connected co-H-spaces;Berstein;Ill. J. Math.,1976
2. Category and generalised Hopf invariants;Berstein;Ill. J. Math.,1968
3. On Hopf invariants;Boardman;Comment. Math. Helv.,1967
4. A note on spaces of category ⩽ 2;Curjel;Math. Z.,1963
5. Cogroups and suspensions;Ganea;Invent. Math.,1970