OFFSET
2,1
COMMENTS
If p - 1 is squarefree, the multiplicative order of 2 modulo a(n) is 2p. - Vladimir Shevelev, Jul 15 2008
The prime numbers in this sequence are the Wagstaff primes (A000979). - Omar E. Pol, Nov 05 2013
LINKS
Vincenzo Librandi, Table of n, a(n) for n = 2..200
FORMULA
EXAMPLE
a(2) = (2^prime(2) + 1)/3 = (2^3 + 1)/3 = 9/3 = 3.
a(3) = (2^prime(3) + 1)/3 = (2^5 + 1)/3 = 33/3 = 11.
a(4) = (2^prime(4) + 1)/3 = (2^7 + 1)/3 = 129/3 = 43.
MATHEMATICA
Table[(2^Prime[n] + 1)/3, {n, 2, 20}]
PROG
(Magma) [(2^NthPrime(n)+1)/3: n in [2..20]]; // Vincenzo Librandi, Mar 29 2012
(PARI) a(n)=2^prime(n)\/3 \\ Charles R Greathouse IV, Mar 29 2012
(PARI) vecextract(apply(p->2^p\/3, primes(100)), "2..") \\ Charles R Greathouse IV, Mar 29 2012
CROSSREFS
KEYWORD
nonn
AUTHOR
Artur Jasinski, Feb 09 2007
STATUS
approved