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a(n)=16^n - 3*2^(2*n - 1) - 1.
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%I #5 May 01 2013 21:16:01

%S 9,231,3999,65151,1047039,16771071,268410879,4294868991,68719083519,

%T 1099510054911,17592179752959,281474951544831,4503599526707199,

%U 72057593635274751,1152921502996234239,18446744067267100671

%N a(n)=16^n - 3*2^(2*n - 1) - 1.

%C Idea resulted from seqfan posts by _Artur Jasinski_.

%F a(n)=16^n - 3*2^(2*n - 1) - 1=(2^(2*n - 1) - 1)*(2^(2*n + 1) + 1).

%t b[n_] := 16^n - 3*2^(2*n - 1) - 1. Table[b[n], {n, 1, 30}]

%K nonn

%O 1,1

%A _Roger L. Bagula_, Nov 24 2008

%E Definition corrected by _N. J. A. Sloane_, Dec 22 2009 (these are not semiprimes).