1. The 3x+1 semigroup;Applegate;J. Number Theory,2006
2. Variants of the 3N+1 problem and multiplicative semigroups;Farkas,2005
3. Artin's conjecture for primitive roots;Heath-Brown;Q. J. Math. Oxford Ser. (2),1986
4. On Artin's conjecture;Hooley;J. Reine Angew. Math.,1967