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%I #6 Oct 01 2013 17:57:40
%S 2,5,257,46657,16777217,10000000001,8916100448257,11112006825558017,
%T 18446744073709551617,39346408075296537575425,
%U 104857600000000000000000001
%N (2n)^(2n) + 1.
%C a(n) is never a perfect square because (2n)^(2n) is a positive square and the only squares that differ by 1 are 0 and 1. Sierpinski numbers are n^n+1. Hence this sequence is a subset of the Sierpinski numbers (A014566). - _T. D. Noe_, Mar 31 2006
%o (PARI) forstep(x=0,20,2,print1(x^x+1" "))
%K easy,nonn
%O 0,1
%A _Cino Hilliard_, Jul 07 2003