• W. W. Boone,
Decision problems about algebraic and logical systems as a whole and recursively enumerable degrees of unsolvability. 1968 Contributions to Math. Logic (Colloquium, Hannover, 1966), North-Holland, Amsterdam. • W. W. Boone,
Roger Lyndon, Frank Cannonito,
Word Problems: Decision Problem in Group Theory, North-Holland, 1973. ==References==