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A220814
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The elements of the set P5 in ascending order.
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3
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2, 3, 7, 13, 17, 19, 29, 37, 43, 53, 59, 73, 79, 97, 103, 107, 109, 113, 127, 137, 149, 157, 163, 173, 193, 197, 223, 227, 229, 233, 239, 257, 293, 307, 313, 317, 337, 347, 349, 379, 389, 409, 433, 439, 443, 449, 457, 467, 479, 487
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OFFSET
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1,1
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COMMENTS
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P5 is the largest set of primes satisfying the conditions: (1) 5 is not in P5; (2) a prime p>5 is in P5 iff all prime divisors of p-1 are in P5.
P5 is also the set of all primes p for which the Pratt tree for p has no node labeled 5.
It is conjectured that this sequence is infinite.
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LINKS
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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.
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FORMULA
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EXAMPLE
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7 is in P5, because 7-1 = 2*3 and 2, 3 are in P5.
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MATHEMATICA
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P5 = {2}; For[p = 2, p < 1000, p = NextPrime[p], If[p != 5 && AllTrue[ FactorInteger[p - 1][[All, 1]], MemberQ[P5, #] &], AppendTo[P5, p]]];
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PROG
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(PARI) P(k, n)=if(n<=k, n<k, my(f=factor(n-1)[, 1]); for(i=1, #f, if(!P(k, f[i]), return(0))); 1)
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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