1. N. D. Gupta and K. W. Weston, "On Groups of Exponent Four," J. Algebra 17(1971), 59–66.
2. N. D. Gupta and R. B. Quintana, "On Groups of Exponent Four III," Proc. Amer. Math. Soc. 33(1972), 15–19.
3. M. Hall, "Notes on Groups of Exponent Four," (to appear).
4. G. Higman, "The Orders of Relatively Free Groups," Proc. Internat. Conf. Theory of Groups, Canberra (1965), 153–165.
5. Ī. D. Ivanjuta, "Certain Groups of Exponent Four," Dopovīdī Akad. Nauk Ukraïn. RSR Ser. A(1969), 787–790, 860.