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 A117880 #13 Aug 09 2025 22:35:33
%S A117880 4,9,21,46,93,187,377,755,1513,3027,6059,12127,24257,48529,97059,
%T A117880 194127,388257,776515,1553033,3106083,6212177,12424355,24848723,
%U A117880 49697447,99394909,198789819,397579639,795159283,1590318573,3180637153
%N A117880 a(1) = 4; a(n) is smallest semiprime > 2*a(n-1).
%C A117880 a(1)=4, a(n)=2*a(n-1)+k, where k is least positive integer chosen so that a(n) is the product of two primes. Corresponding k's are 1, 3, 4, 1, 1, 3, 1, 3, 1, 5, 9, 3, 15, 1, 9, 3, 1, 3, 17, 11, 1, 13, 1, 15, 1, 1, 5, 7, 7, 11, 5, 5, 15, 1, 3, 9, 9, 5, 7, 8, ... - _Zak Seidov_, Dec 24 2007
%e A117880 a(1)=4, then
%e A117880 k=1, a(2)=2*4+1=9,
%e A117880 k=3, a(3)=2*9+3=21,
%e A117880 k=4, a(4)=2*21+4=46,
%e A117880 k=1, a(5)=2*46+1=93,
%e A117880 k=1, a(6)=2*93+1=187.
%t A117880 a=0;Do[Do[b=2a+n;If[2==Plus@@FactorInteger[b][[All,2]],Print[b];Break[]],{n,1000}];a=b,{40}] (* _Zak Seidov_, Dec 24 2007 *)
%t A117880 ssp[n_]:=Module[{k=2n+1},While[PrimeOmega[k]!=2,k++];k]; NestList[ssp,4,30] (* _Harvey P. Dale_, Apr 14 2022 *)
%Y A117880 Semiprime analog of A055496.
%Y A117880 Cf. A001358, A076656.
%K A117880 easy,nonn,less
%O A117880 1,1
%A A117880 _Jonathan Vos Post_, May 04 2006
%E A117880 Edited by _N. J. A. Sloane_, Jul 01 2008 at the suggestion of _R. J. Mathar_