Celebratio Mathematica

K. I. Ap­pel and F. M. Djorup: “On the group gen­er­ated by a free semig­roup,” Proc. Am. Math. Soc. 15 : 5 (1964), pp. 838–​840. MR 168682 Zbl 0126.​04701 article

K. I. Ap­pel and F. M. Djorup: “On the equa­tion \( z_1^n z_2^n\cdots z_k^n = y^n \) in a free semig­roup,” Trans. Am. Math. Soc. 134 : 3 (December 1968), pp. 461–​470. MR 230827 Zbl 0169.​02703 article