1. Elwyn R. Berlekamp, John H. Conway, and Richard K. Guy, Winning Ways for your Mathematical Plays, Academic Press, 1982.
2. Letters
3. Don't Try to Solve These Problems!
4. The 3x+ 1 Problem and its Generalizations
5. N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973.