A023228 Numbers k such that k and 8*k + 1 are both prime.

2, 5, 11, 17, 29, 71, 101, 107, 131, 137, 149, 179, 239, 269, 347, 401, 431, 449, 479, 491, 509, 557, 599, 617, 659, 677, 761, 809, 821, 857, 929, 941, 947, 977, 1151, 1187, 1229, 1289, 1307, 1361, 1367, 1409, 1487, 1559, 1571, 1601, 1619, 1667, 1697, 1811, 1871

Offset

1,1

Links

Vincenzo Librandi, Table of n, a(n) for n = 1..1000

Samuel S. Wagstaff, Jr., Sum of Reciprocals of Germain Primes, Journal of Integer Sequences, Vol. 24, No. 2 (2021), Article 21.9.5.

Formula

Sum_{n>=1} 1/a(n) is in the interval (1.151956749, 1.4207187) (Wagstaff, 2021). - Amiram Eldar, Nov 04 2021

Mathematica

Select[Prime[Range[2000]], PrimeQ[8# + 1]&] (* Vincenzo Librandi, Feb 02 2014 *)

Prog

(Magma) [ p: p in PrimesUpTo(1900) | IsPrime(8*p+1) ]; // Klaus Brockhaus, Dec 21 2008
(PARI) list(lim)=my(v=List()); forprime(p=2, lim, if(isprime(8*p+1), listput(v, p))); Vec(v) \\ Charles R Greathouse IV, Oct 20 2021

Crossrefs

Cf. A007519 (primes of form 8n+1), A005123 ((( primes == 1 mod 8 ) - 1)/8). - Klaus Brockhaus, Dec 21 2008

Sequence in context: A228353 A055499 A014424 * A027429 A336376 A059987

Adjacent sequences: A023225 A023226 A023227 * A023229 A023230 A023231

Keyword

nonn,easy

Author

David W. Wilson

Status

approved