C. Baiocchi and 3. , Tag-systems (Italian), Dipartimento di Matematica dell'Università " La Sapienza " di Roma 98, 1998.

C. Baiocchi and M. Margenstern, Cellular automata about the 3x + 1 problem, Proc. LCCS'2001, pp.37-45, 2001.

M. Margenstern, Frontier between decidability and undecidability: a survey, Proc. MCU'98, pp.141-177, 1998.
DOI : 10.1016/S0304-3975(99)00102-4
URL : http://doi.org/10.1016/s0304-3975(99)00102-4

M. Margenstern, Frontier between decidability and undecidability: a survey, Theoretical Computer Science, vol.231, issue.2, pp.217-251, 2000.
DOI : 10.1016/S0304-3975(99)00102-4
URL : http://doi.org/10.1016/s0304-3975(99)00102-4

P. Michel, Busy beaver competition and Collatz-like problems, Archive for Mathematical Logic, vol.69, issue.4, pp.351-367, 1993.
DOI : 10.1007/BF01409968

P. Michel and M. Margenstern, Generalized 3x+1 functions and the theory of computation, pp.105-128

D. Woods and T. Neary, The complexity of small universal Turing machines: A survey, Theoretical Computer Science, vol.410, issue.4-5, pp.443-450, 1110.
DOI : 10.1016/j.tcs.2008.09.051