1. W. Banks, I.E. Shparlinski, Smooth values of the Euler function, preprint 2002, 1–11.
2. On the prime power divisors of the iterates of the Euler-φ function;Bassily;Publ. Math. Debrecen,1999
3. C. Batut, K. Belabas, D. Bernardi, H. Cohen, M. Olivier, GP/PARI CALCULATOR Version 2.1.4. UltraSparc (MicroSparc kernel) 32-bit version, 2002.
4. Prime Numbers: A Computational Perspective;Crandall,2001
5. Multiplicative Number Theory;Davenport,1980