1. A proof of Higman's embedding theorem using Britton extensions of groups, in Word Problems. Studies in Logic and the Foundations of Mathematics;Aanderaa,1973
2. Modular machines, the word problem for finitely presented groups and Collins’ theorem, in Word Problems II: The Oxford Book, Studies in Logic and the Foundations of Mathematics;Aanderaa,1979
3. Finitely presented groups with word problem of arbitrary degrees of unsolvability,;Clapham;Proc. London Math. Soc.,1964
4. An embedding theorem for finitely generated groups,;Clapham;Proc. London Math. Soc.,1967
5. Subgroups of finitely presented groups,;Higman;Proc. Roy. Soc. A,1961