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 A305887 #17 May 29 2025 12:37:42
%S A305887 4,5,10,29,173,249,19073,71489,1166789,3800333,7021253,15920129,
%T A305887 84551600693,224223772673
%N A305887 The least increasing sequence of numbers where all pairwise sums are semiprimes, with a(1)=4.
%C A305887 All terms > 10 are congruent to {3, 9} mod 10.
%C A305887 Triangle of resulting semiprimes begins:
%C A305887 9
%C A305887 14, 15
%C A305887 33, 34, 39
%C A305887 177, 178, 183, 202
%t A305887 Nest[Append[#, Block[{k = Last[#] + 1}, While[! AllTrue[#, PrimeOmega[k + #] == 2 &], k++]; k]] &, {4}, 7] (* _Michael De Vlieger_, Jun 14 2018 *)
%Y A305887 Cf. A001358, A116656, A164979, A181620.
%K A305887 nonn,more
%O A305887 1,1
%A A305887 _Zak Seidov_, Jun 14 2018
%E A305887 a(13) from _Giovanni Resta_, Jun 14 2018
%E A305887 a(14) from _Jinyuan Wang_, May 29 2025