The 3x + 1 Problem: some relevant sites
  1. Proceedings of the International Conference on the Collatz Problem and Related Topics (the 3x + 1 problem is also known as the Collatz Problem)
  2. On the 3x + 1 problem By Eric Roosendaal  
  3. Peter Schorer's papers on the 3x + 1 Problem  
  4. Computational verification of the 3x+1 conjecture from Tomás Oliveira e Silva
  5. The 3x+1 problem and its generalizations by Jeff Lagarias of AT&T Bell Laboratories  
  6. The 3n+1 Problem by Darrell Cox