1. Rétractions de graphes;Hell;Ph.D. Thesis, Université de Montréal,1972
2. Absolute planar retracts and the four color conjecture;Hell;J. Comb. Theory Ser. B,1974
3. Absolute Retracts of Graphs;Hell,1974
4. Graph retractions;Hell;Atti die convegni lincei 17, Teorie Combinatorie,1976
5. A fixed edge theorem for graphs with loops;Nowakowski;University of Calgary, Research Paper No. 394,1978