[1] ERDÖS P.: On the sum $\sum_{d|2^n-1} d^{-1}$. Israel J. Math. 9 (1971), 43-48. MR 0269613

[2] ERDÖS P.-KISS P.-POMERANCE, C: On prime divisors of Mersenne numbers. Acta Arith. 57 (1991), 267-281. MR 1105611 | Zbl 0733.11003

[3] EVEREST G. R.-SHPARLINSKI I.: Divisor sums of generalized exponential polynomials. Canad. Math. Bull. 39 (1996), 35-46. MR 1382488

[4] von zur GATHEN J.-KNOPFMACHER A.-LUCA F.-LUCHT L.-SHPARLINSKI I.: Average order in cyclic groups. J. Theor. Nombres Bordeaux (To appear). Zbl 1079.11003

[5] LUCA F.: Some mean values related to average multiplicative orders of elements in finite fields. Ramanujan J. (To appear). MR 2166376 | Zbl 1155.11344

[6] LUCA F.-SHPARLINSKI I.: Arithmetical functions with linear recurrence sequences. Preprint, 2002. MR 2332599

[7] MONTGOMERY H. L.-VAUGHAN R. C : The large sieve. Mathematika 20 (1973) 119-134. MR 0374060 | Zbl 0296.10023

[8] POMERANCE C.: On primitive divisors of Mersenne numbers. Acta Arith. 46 (1986) 355-367. MR 0871278 | Zbl 0606.10034

[9] SHPARLINSKI I.: The number of prime divisors of recurrence sequences. Mat. Zametki 38 (1985), 29-34. (Russian) MR 0804177

[10] SHPARLINSKI I.: The number of different prime divisors of recurrent sequences. Mat Zametki 42 (1987), 494-507. (Russian) MR 0917803

[11] SHPARLINSKI I.: Some arithmetic properties of recurrence sequences. Mat. Zametki 47 (1990), 124-131 (Russian). [Translation: Math. Notes 47 (1990), 612-617]. MR 1074537

[12] TENENBAUM G.: Introduction to Analytic and Probabilistic Number Theory. (Transl from the 2nd French ed. by C. B. Thomas). Cambridge Stud. Adv. Math. 46, Cambridge University Press, Cambridge, 1995. MR 1342300 | Zbl 0880.11001