OFFSET
1,1
COMMENTS
For n > 1, the prime 2*a(n) + 1 is the smallest prime divisor of (2^a(n) + 1)/3. - Emmanuel Vantieghem, Aug 12 2018
Primes p such that 2*p+1 divides 2^p+1. - Hilko Koning, Sep 21 2021
Subset of Josephus_2 primes {A163782} that are themselves also prime. - Joe Nellis, Dec 27 2022
LINKS
Harvey P. Dale, Table of n, a(n) for n = 1..1000
Luis Henri Gallardo, Bell Numbers Modulo p, Appl. Math. E-Notes (2023) Vol. 23, 40-48. See p. 43.
MAPLE
select(t -> isprime(t) and isprime(2*t+1), [2, seq(4*k+1, k=1..10000)]); # Robert Israel, May 20 2015
MATHEMATICA
Select[Prime[Range[500]], PrimeQ[2#+ 1 ] && Mod[#, 4] != 3 &] (* Harvey P. Dale, Jun 15 2013 *)
Select[4Range[100] + 1, PrimeQ[#] && PrimeQ[2# + 1] &] (* Alonso del Arte, Jun 01 2019 *)
PROG
(PARI) forprime(p=2, 10^4, if((p%4!=3)&&isprime(2*p+1), print1(p, ", "))); \\ Joerg Arndt, Nov 18 2014
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Jonathan Vos Post, Mar 23 2005
EXTENSIONS
More terms from Vladimir Joseph Stephan Orlovsky, Jul 07 2009
STATUS
approved