A387122 - OEIS

%I #20 Nov 20 2025 04:17:10

%S 7,3,13,17,5,29,37,41,7,61,13,11,17,89,97,109,11,5,137,29,149,23,13,

%T 181,197,41,19,31,17,229,257,53,277,281,43,61,317,47,337,349,19,73,11,

%U 389,397,401,17,449,457,461,67,37,97,101,47,23,541,109,557,113,569,31

%N Greatest prime divisor of 2*prime(n) + 3.

%F a(n) = A006530(A129069(n)).

%e a(1) = Gpf(2*2+3) = Gpf(7) = 7.

%e a(2) = Gpf(2*3+3) = Gpf(9) = 3.

%p f:= n -> max(numtheory:-factorset(2*ithprime(n)+3)):

%p map(f, [$1..100]); # _Robert Israel_, Nov 12 2025

%t a[n_] := FactorInteger[2*Prime[n] + 3][[-1, 1]]; Array[a, 65] (* _Amiram Eldar_, Nov 12 2025 *)

%o (PARI) a(n) = vecmax(factor(2*prime(n) + 3)[, 1]);

%Y Cf. A006530, A129069.

%K nonn

%O 1,1

%A _Alain Rocchelli_, Nov 12 2025