C. Williams, B. Thwaites, A. van der Poorten, W. Edwards, and L. Williams, Ulam's Conjecture continued again, 1000 pounds for proof. Rumored proof disclaimed. References, PPC Calculator J., 9 (Sept. 1982) 23--24. Gives some history of the problem. B. Thwaites states he originated the problem in 1952 and offers 1000 pounds for a proof of the Conjecture.
[1] [2] [3]