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%I #27 Dec 10 2022 02:06:20
%S 6,20,272,65792,4295032832
%N Numbers of the form 2^(2^k)*F_k, where F_k is a Fermat prime, A019434.
%C Also numbers with power-spectral basis {F_n,(F_n-1)^2}. The first element of the power-spectral basis of a(n) is A019434, and the second element is A001146.
%F a(n) = A001146(n-1)*A019434(n), n = 1..5. [Corrected by _Georg Fischer_, Dec 09 2022]
%e a(2) = 2^2*(2^2+1) = 20, and the spectral basis of 20 is {5,16}, consisting of primes and powers.
%p F := n -> 2^(2^n)+1;
%p a := proc(n) if isprime(F(n)) then return 2^(2^n)*F(n) fi; end;
%p [seq(a(n),n=0..4)];
%Y Cf. A000215, A001146, A019434.
%K nonn
%O 1,1
%A _Walter Kehowski_, Jan 06 2020
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