1. Amigó, J.M.: Accelerated Collatz dynamics. Trabajos I+D I-2000-5, Operations Research Center of Miguel Hernandez University (available from www.crm.es/Publications/Preprints01.htm, preprint 474) (2000)
2. Andrei S., Masalagiu C. (1998): About the Collatz conjecture. Acta Informatica 35, 167–179
3. Andrei S., Kudlek M., Niculescu R.S. (2000): Some results on the Collatz problem. Acta Inform. 37(2): 145–160
4. Böhm, C., Sonntacchi, G.: On the existence of cycles of a given length in integer sequences like x n+1 = x n /2 if x n even, and x n+1 = 3x n + 1 otherwise, Atti. Accad. Naz. Lincei, VIII Ser., Rend., Cl. Sci. Fis. Mat. Nat. LXIV, 260–264 (1978)
5. Chamberland, M.: An update on the 3x + 1 problem. (available from www.math.grin.edu/~ chamberl/papers.html)