A180640 Primes P such that P < (largest prime factor of (P-1)) * (largest prime factor of (P+1)).
2, 3, 5, 11, 13, 23, 29, 37, 43, 47, 59, 61, 67, 73, 83, 103, 107, 113, 131, 137, 139, 149, 157, 167, 173, 179, 193, 211, 223, 227, 229, 233, 263, 269, 277, 281, 283, 293, 311, 313, 317, 331, 347, 353, 359, 367, 373, 383, 389, 397, 409, 421, 439, 443, 457, 467
Offset: 1
Keywords
Examples
For n = 3, a(3)=11. The prime P = 11 P-1 = 10 (largest prime factor of 10 is 5) P+1 = 12 (largest prime factor of 12 is 3) 11 < 5*3.
Programs
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Mathematica
Select[Prime[Range[100]],#<(FactorInteger[#-1][[-1,1]] FactorInteger[#+1][[-1,1]])&] (* Harvey P. Dale, Feb 22 2011 *)
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PARI
isok(p) = (p==2) || (isprime(p) && (p < vecmax(factor(p-1)[, 1]) * vecmax(factor(p+1)[, 1]))); \\ Michel Marcus, Oct 29 2022
Extensions
Initial term, i.e., 2, added by Harvey P. Dale, Feb 22 2011