Edwin F. Taylor & John Archibald Wheeler (1966 - first edition only) Spacetime Physics (W.H. Freeman, San Francisco) ISBN0-7167-0336-X, Chapter 1, Exercise 51, pp. 97–98: "Clock paradox III" (pdf).
isfdb.org
Heinlein, Robert A. (1953). Sky Lift. Greenleaf Publishing Company. p. 164.