%I #31 Aug 07 2022 13:52:11
%S 2,3,7,55,6543
%N Indices of the Fermat primes in the sequence of primes.
%C A098006(a(n)) = 0. - _Reinhard Zumkeller_, Mar 26 2013
%H <a href="/index/Pri">Index entries for sequences that are related to primes dividing Fermat numbers</a>
%F a(n) = A000720(A019434(n)). - _Michel Marcus_, Apr 29 2021
%e 3, the 1st Fermat prime is the 2nd prime, so a(1) = 2.
%e 17, the 3rd Fermat prime is the 7th prime, so a(3) = 7.
%t PrimePi/@{3,5,17,257,65537} (* _Harvey P. Dale_, Aug 07 2022 *)
%o (Haskell)
%o import Data.List (elemIndices)
%o a159611 n = a159611_list !! (n-1)
%o a159611_list = map (+ 2) $ elemIndices 0 a098006_list
%o -- _Reinhard Zumkeller_, Mar 26 2013
%o (PARI) for(i=0, 10, isprime(f=2^2^i+1) & print1(primepi(f), ", ")) \\ _Michel Marcus_, Apr 28 2016
%o (PARI) a152155(n) = centerlift(Mod(3, 2^(2^n)+1)^(2^(2^n-1)))
%o print1(2, ", "); for(x=0, oo, if(a152155(x)==-1, print1(primepi(2^(2^x)+1), ", "))) \\ _Felix Fröhlich_, Apr 30 2021
%Y Cf. A000040 (primes), A000720, A019434 (Fermat primes).
%Y Cf. A098006.
%K nonn,more
%O 1,1
%A _Walter Nissen_, Apr 16 2009
%E Name edited by _Felix Fröhlich_, Apr 30 2021