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 A172256 Primes p such that 2*p+-3 are both nonprimes. 2
 59, 61, 79, 103, 109, 131, 149, 151, 163, 179, 239, 257, 271, 281, 293, 313, 359, 367, 389, 401, 419, 449, 479, 491, 499, 541, 569, 571, 593, 601, 619, 673, 677, 683, 691, 709, 719, 733, 761, 769, 821, 823, 829, 839, 857, 877, 883, 911, 919, 947, 953, 971, 983, 1009 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS In the first 10000 primes there are 5698 terms (~57% of the primes). In the 10000 primes from prime(1,000,000,000) to prime(1,000,010,000) there are 8432 primes in this sequence or ~84%. It seems likely the density of these terms within the primes slowly approaches 100%. This indicates the density of "Prime Septets", as defined in A268593 (which rely upon primes in the complement of this sequence), declines steadily at larger n.  - Richard R. Forberg, Feb 12 2016 LINKS Harvey P. Dale, Table of n, a(n) for n = 1..1000 MATHEMATICA npQ[n_]:=Module[{c=2n}, !PrimeQ[c+3]&&!PrimeQ[c-3]]; Select[Prime[ Range[ 200]], npQ] (* Harvey P. Dale, Jan 21 2013 *) PROG (MAGMA)[p: p in PrimesUpTo(1100)| not IsPrime(2*p+3)and not IsPrime(2*p-3)] // Vincenzo Librandi, Dec 08 2010 (PARI) isok(p) = isprime(p) && !isprime(2*p+3) && !isprime(2*p-3); \\ Michel Marcus, Feb 12 2016 CROSSREFS Cf. A000040, A131426, A141468. Sequence in context: A104916 A172462 A042739 * A172056 A032648 A235808 Adjacent sequences:  A172253 A172254 A172255 * A172257 A172258 A172259 KEYWORD nonn AUTHOR Juri-Stepan Gerasimov, Jan 30 2010 EXTENSIONS Corrected and extended by Vincenzo Librandi, Apr 01 2010 STATUS approved

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Last modified April 19 06:30 EDT 2019. Contains 322237 sequences. (Running on oeis4.)