A068497 Primes p such that 2*p+1 and 2*p-1 are composites.

13, 17, 43, 47, 59, 61, 67, 71, 73, 101, 103, 107, 109, 127, 137, 149, 151, 163, 167, 181, 193, 197, 223, 227, 241, 257, 263, 269, 277, 283, 311, 313, 317, 347, 349, 353, 373, 383, 389, 397, 401, 409, 421, 433, 449, 457, 461, 463, 467, 479, 487, 503, 521

Offset

1,1

Comments

Subsequence of A053176. - Michel Marcus, Jan 12 2015

The sequence is infinite. Among others it contains all the primes of the form 15m+/-2. - Emmanuel Vantieghem, Sep 19 2016

Links

G. C. Greubel, Table of n, a(n) for n = 1..10000

Maple

select(p->isprime(p) and not isprime(2*p+1) and not isprime(2*p-1), [$1..530]); # Muniru A Asiru, Oct 16 2018

Mathematica

lst={}; Do[p=Prime[n]; If[ !PrimeQ[2*p-1]&&!PrimeQ[2*p+1], AppendTo[lst, p]], {n, 6!}]; lst (* Vladimir Joseph Stephan Orlovsky, May 12 2010 *)
Select[Prime[Range[500]], ! PrimeQ[2*# - 1] && ! PrimeQ[2*# + 1] &] (* G. C. Greubel, Oct 15 2018 *)

Prog

(PARI) listp(nn) = {forprime(p=2, nn, if (!isprime(2*p-1) && !isprime(2*p+1), print1(p, ", ")); ); } \\ Michel Marcus, Jan 12 2015
(Magma) [p: p in PrimesUpTo(600) | not IsPrime(2*p+1) and not IsPrime(2*p-1)]; // Vincenzo Librandi, Jan 20 2015
(GAP) Filtered([1..530], p->IsPrime(p) and not IsPrime(2*p+1) and not IsPrime(2*p-1)); # Muniru A Asiru, Oct 16 2018

Crossrefs

Sequence in context: A043893 A279392 A239901 * A125524 A156553 A248474

Adjacent sequences: A068494 A068495 A068496 * A068498 A068499 A068500

Keyword

easy,nonn

Author

Benoit Cloitre, Mar 25 2002

Status

approved