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A133378 a(1)=1. For n >= 2, a(n) = the largest prime divisor of (a(n-1)^a(n-1) + 1).

%I A133378 #21 Jan 28 2023 12:29:14
%S A133378 1,2,5,521
%N A133378 a(1)=1. For n >= 2, a(n) = the largest prime divisor of (a(n-1)^a(n-1) + 1).
%C A133378 a(5) depends on the factorization of the 1408-digit composite number (521^521+1)/(2 * 3^2 * 29 * 375121); see factordb.com link. - _Michael S. Branicky_, Jan 27 2023
%H A133378 factordb.com, <a href="http://factordb.com/index.php?id=1100000000195192576">521^521+1</a>.
%t A133378 NestList[FactorInteger[#^#+1][[-1,1]]&,1,3] (* _Harvey P. Dale_, Jun 21 2012 *)
%Y A133378 Cf. A007571.
%K A133378 more,nonn
%O A133378 1,2
%A A133378 _Leroy Quet_, Oct 28 2007