This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A239323 #26 May 11 2024 19:12:14
%S A239323 4,6,15,221,4294049777
%N A239323 Semiprimes of the form (2^n + 1)*(2^n - n + 1).
%C A239323 Generated by n: 0, 1, 2, 4, 16, ...
%C A239323 The positions of a(n) in A001358: 1, 2, 6, 75, ...
%e A239323 4 is in this sequence because (2^0 + 1)*(2^0 - 0 + 1) = 2*2 = 4 is semiprime for n = 0,
%e A239323 6 is in this sequence because (2^1 + 1)*(2^1 - 1 + 1) = 3*2 = 6 is semiprime for n = 1,
%e A239323 15 is in this sequence because (2^2 + 1)*(2^2 - 2 + 1) = 5*3 = 15 is semiprime for n = 2.
%t A239323 Select[Table[(2^n+1)(2^n-n+1),{n,0,20}],PrimeOmega[#]==2&] (* _Harvey P. Dale_, May 11 2024 *)
%o A239323 (Magma) k := 1;
%o A239323 for n in [1..10000] do
%o A239323 if IsPrime(k*2^n + 1) and IsPrime(k*2^n - n + 1) then
%o A239323 (k*2^n + 1)*(k*2^n - n + 1);
%o A239323 end if;
%o A239323 end for;
%Y A239323 Cf. A019434, A100361, A100362.
%K A239323 nonn
%O A239323 1,1
%A A239323 _Juri-Stepan Gerasimov_, Mar 15 2014