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a(n) = (2^(3^n)+1)/3^(n+1).
2

%I #9 Jun 13 2024 07:44:09

%S 1,19,1657009,9950006745799417075771,

%T 19389268200585836264288587113776883575610248525384021488302948711030121

%N a(n) = (2^(3^n)+1)/3^(n+1).

%t Table[(2^(3^n) + 1)/3^(n + 1), {n, 6}] (* _Wesley Ivan Hurt_, Dec 26 2023 *)

%K easy,nonn

%O 1,2

%A _Benoit Cloitre_, May 13 2002

%E One more term from _Benoit Cloitre_, Nov 01 2002