Sieving by very thin sets of primes, and Pratt trees with missing primes
  Suppose P is a set of primes, such that for every p in P, every prime factor of p-1 is also in P. If P does not contain all primes, we apply a new sieve method to show that the counting function of P is O(x^{1-c}) for some c>0, where c depends only on the smallest prime not in P. Our proof makes use of results connected with Artin's primitive root conjecture. 
Kevin Ford
math.NT