On a Problem of J.H.C. Whitehead and a Problem of Alonzo Church. W.W. Boone; H., Jr. Rogers — 1966 Mathematica Scandinavica
Some Unsolvable Problems about Elements and Subgroups of Groups. B.H. Neumann; G. Baumslag; W.W. Boone — 1959 Mathematica Scandinavica