A243979 Indices of Wagstaff primes.

2, 5, 14, 124, 399, 4552, 15898, 203095, 37029521, 105973558438, 19140185454656173, 3827634977577891833517

Offset: 1

Author

Omar E. Pol, Jun 18 2014

Keywords

• nonn
• hard
• more

Examples

			For n = 3 the third Wagstaff prime is A000979(3) = 43 and 43 is also the 14th prime number, so a(3) = 14.
		

Links

• Andrew R. Booker, The Nth Prime Page.
• Chris K. Caldwell, Wagstaff, The Top Twenty, The PrimePages.
• Xavier Gourdon and Pascal Sebah, Counting primes.
• Tomás Oliveira e Silva, Tables of values of pi(x) and of pi2(x).
• Samuel S. Wagstaff, Jr., The Cunningham Project.
• Kim Walisch, Fast C++ prime counting function implementation (primecount).
• Wikipedia, Wagstaff prime.

Crossrefs

Cf. A000040, A000720, A000978, A000979, A059305, A123176, A126614, A194810.

Programs

PARI

default(primelimit, 10^9); forprime(p=3, 31, q=(2^p+1)/3; if(isprime(q), print1(primepi(q)", "))) \\ Jens Kruse Andersen, Jun 22 2014

Formula

a(n) = A000720(A000979(n)).

A000040(a(n)) = A000979(n).

Extensions

a(11) from Jens Kruse Andersen, Jun 22 2014

a(12) calculated using Kim Walisch's primecount and added by Amiram Eldar, Sep 05 2024