A159611 Indices of the Fermat primes in the sequence of primes.

2, 3, 7, 55, 6543

Offset

1,1

Comments

A098006(a(n)) = 0. - Reinhard Zumkeller, Mar 26 2013

Links

Table of n, a(n) for n=1..5.

Index entries for sequences that are related to primes dividing Fermat numbers

Formula

a(n) = A000720(A019434(n)). - Michel Marcus, Apr 29 2021

Example

3, the 1st Fermat prime is the 2nd prime, so a(1) = 2.
17, the 3rd Fermat prime is the 7th prime, so a(3) = 7.

Mathematica

PrimePi/@{3, 5, 17, 257, 65537} (* Harvey P. Dale, Aug 07 2022 *)

Prog

(Haskell)
import Data.List (elemIndices)
a159611 n = a159611_list !! (n-1)
a159611_list = map (+ 2) $ elemIndices 0 a098006_list
-- Reinhard Zumkeller, Mar 26 2013
(PARI) for(i=0, 10, isprime(f=2^2^i+1) & print1(primepi(f), ", ")) \\ Michel Marcus, Apr 28 2016
(PARI) a152155(n) = centerlift(Mod(3, 2^(2^n)+1)^(2^(2^n-1)))
print1(2, ", "); for(x=0, oo, if(a152155(x)==-1, print1(primepi(2^(2^x)+1), ", "))) \\ Felix Fröhlich, Apr 30 2021

Crossrefs

Cf. A000040 (primes), A000720, A019434 (Fermat primes).

Cf. A098006.

Sequence in context: A253574 A343557 A238399 * A156585 A354744 A299923

Adjacent sequences: A159608 A159609 A159610 * A159612 A159613 A159614

Keyword

nonn,more

Author

Walter Nissen, Apr 16 2009

Extensions

Name edited by Felix Fröhlich, Apr 30 2021

Status

approved