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 A128687 #15 Nov 21 2021 07:37:48 %S A128687 1,9,15,39,105,731,2805,59535,2658795,78060135,67554224745 %N A128687 Smallest odd nonprime such that every subset of a(1), ..., a(n) adds to a nonprime. %C A128687 The first 8 terms are from Rivera's puzzle 84. %C A128687 The sequences is infinite [Chris Nash]. - _N. J. A. Sloane_, Jan 20 2017 %H A128687 Chris Nash, <a href="/A052349/a052349.txt">Proof that A052349, A128687, and A128688 are infinite</a> [Cached copy of proof, from The Prime Puzzles and Problems website] %H A128687 Carlos Rivera, <a href="http://www.primepuzzles.net/puzzles/puzz_084.htm">Puzzle 84. Non-primes adding up to non-primes</a>, The Prime Puzzles and Problems Connection. %t A128687 a[1]=1; a[n_]:=a[n]=(s=Subsets[Array[a,n-1],n-1]; c=a[n-1]; While[d=1;While[!PrimeQ[Total[s[[d]]]+c]&&d<Length@s,d++];d!=Length@s||PrimeQ[Total[s[[d]]]+c]||PrimeQ@c,c=c+2];c); Array[a,8] (* _Giorgos Kalogeropoulos_, Nov 19 2021 *) %Y A128687 Cf. A052349 (no restrictions on even or odd), A128688 (restricted to even numbers). %K A128687 hard,nonn %O A128687 1,2 %A A128687 _T. D. Noe_, Mar 20 2007 %E A128687 a(11) from _Donovan Johnson_, Apr 18 2010