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 A381120 #21 Feb 16 2025 21:54:41 %S A381120 63,630,2423,5653,9104,26308,36108,41622,64526,85121,108917,143913, %T A381120 148305,176405,316974,399168,399907,406487,536926,621016,830793, %U A381120 937038,937109,970243,1088629,1480545,1895503,3961587,4651102,5171081,5487450,6219705,7327856,8118740 %N A381120 Numbers k such that both A381019(k) and A381019(k+1) are composite. %C A381120 Initially, A381019 consists of runs of primes separated by single composite numbers. These indices k mark the beginning of two or more consecutive composite numbers in A381019. %o A381120 (PARI) lista(nn) = my(c, f, m=1, q=1, r=0, t, u=List([]), w=vector(nn)); for(n=2, nn, t=0; forprime(p=2, max(m, sqrtint(n)), c=n-1-w[p]; if(c>1&&!w[c], listput(u, c))); listsort(u, 1); for(i=1, #u, c=u[i]; f=factor(c)[, 1]; t=1; for(j=1, #f, if(n-c<=w[f[j]], t=0; break)); if(t, u=u[i+1..#u]; if(r, print1(n-1, ", ")); r=1; w[c]=1; for(j=1, #f, w[f[j]]=n); m=max(m, f[#f]); break)); if(!t, r=0; u=List([]); if(nn>q=nextprime(q+1), w[q]=n))); \\ _Jinyuan Wang_, Feb 16 2025 %Y A381120 Cf. A381019, A379810 (the values of A381019(k)). %K A381120 nonn %O A381120 1,1 %A A381120 _N. J. A. Sloane_, Feb 15 2025 %E A381120 a(7)-a(8) from _Michael De Vlieger_, Feb 15 2025 %E A381120 a(9)-a(34) from _Jinyuan Wang_, Feb 16 2025