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a(n) is the largest prime factor of 2^n + 3^n.
7

%I #20 Sep 08 2022 08:45:13

%S 5,13,7,97,11,61,463,401,577,4621,35839,5521,4057,369181,241,14177,

%T 7039,181,555029,3121,92233,2414250301,13249,41432641,513101,13761229,

%U 377604937,3001769,148466603,24001,22674269,1607133116929,22270249,1282861452271981,392943879511

%N a(n) is the largest prime factor of 2^n + 3^n.

%H Amiram Eldar, <a href="/A094474/b094474.txt">Table of n, a(n) for n = 1..546</a> (terms 1..120 from Vincenzo Librandi)

%F a(n) = A006530(A007689(n)). - _Amiram Eldar_, Dec 27 2019

%e n = 100, 2^100 + 3^100 = 41 * 97 * 281 * 3121 * 742801 * 20017001 * 9937984196743741414107401 so a(100) = 9937984196743741414107401.

%t Table[Part[Reverse[Flatten[FactorInteger[2^w + 3^w]]], 2], {w, 1, 100}]

%o (Magma) [Max(PrimeFactors(2^n + 3^n)):n in[1..68]]; // _Vincenzo Librandi_, Dec 25 2019

%Y Cf. A006530, A007689.

%K nonn

%O 1,1

%A _Labos Elemer_, Jun 01 2004

%E a(34)-a(35) from _Vincenzo Librandi_, Dec 25 2019