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%I #24 Sep 08 2022 08:46:07
%S 4,6,15,221,4294049777
%N Semiprimes of the form (2^n + 1)*(2^n - n + 1).
%C Generated by n: 0, 1, 2, 4, 16, ...
%C The positions of a(n) in A001358: 1, 2, 6, 75, ...
%e 4 is in this sequence because (2^0 + 1)*(2^0 - 0 + 1) = 2*2 = 4 is semiprime for n = 0,
%e 6 is in this sequence because (2^1 + 1)*(2^1 - 1 + 1) = 3*2 = 6 is semiprime for n = 1,
%e 15 is in this sequence because (2^2 + 1)*(2^2 - 2 + 1) = 5*3 = 15 is semiprime for n = 2.
%o (Magma) k := 1;
%o for n in [1..10000] do
%o if IsPrime(k*2^n + 1) and IsPrime(k*2^n - n + 1) then
%o (k*2^n + 1)*(k*2^n - n + 1);
%o end if;
%o end for;
%Y Cf. A019434, A100361, A100362.
%K nonn
%O 1,1
%A _Juri-Stepan Gerasimov_, Mar 15 2014
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