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 A220815 The elements of the set P7 in ascending order. 3
 2, 3, 5, 11, 13, 17, 19, 23, 31, 37, 41, 47, 53, 61, 67, 73, 79, 83, 89, 97, 101, 103, 107, 109, 131, 137, 139, 149, 151, 157, 163, 167, 179, 181, 191, 193, 199, 223, 229, 241, 251, 257, 263, 269, 271, 277, 283, 293, 307, 311 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS P7 is the largest set of primes satisfying the conditions: (1) 7 is not in P7; (2) a prime p>7 is in P7 iff all prime divisors of p-1 are in P7. P7 is also the set of all primes p for which the Pratt tree for p has no node labeled 7. It is conjectured that this sequence is infinite. LINKS Ivan Neretin, Table of n, a(n) for n = 1..10000 K. Ford, S. Konyagin and F. Luca, Prime chains and Pratt trees, arXiv:0904.0473 [math.NT], 2009-2010; Geom. Funct. Anal., 20 (2010), pp. 1231-1258. Kevin Ford, Sieving by very thin sets of primes, and Pratt trees with missing primes, arXiv preprint arXiv:1212.3498 [math.NT], 2012-2013. FORMULA Ford proves that a(n) >> n^k for some k > 1. - Charles R Greathouse IV, Dec 26 2012 EXAMPLE 11 is in P7, because 11-1 = 2*5 and 2, 5 are in P7. MATHEMATICA P7 = {2}; For[p = 2, p < 1000, p = NextPrime[p], If[p != 7 && AllTrue[ FactorInteger[p - 1][[All, 1]], MemberQ[P7, #] &], AppendTo[P7, p]]]; P7 (* Jean-François Alcover, Jan 05 2019 *) PROG (PARI) P(k, n)=if(n<=k, n

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