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 A306878 #7 Mar 15 2019 12:30:10 %S A306878 0,0,0,0,2,1,2,3,3,3,5,5,6,6,7,6,9,10,8,10,11,10,12,13,13,13,15,14,16, %T A306878 18,15,18,20,16,20,21,20,21,24,21,23,26,24,24,29,25,27,30,26,30,32,29, %U A306878 31,33,31,33,36,33,34,41,34,36,42,35,40,42,40,40,43,42,44,48,44,44 %N A306878 Number of 0 < k < n such that n-k and n+k are both nonprimes. %F A306878 a(n) = n - 1 - A061357(n) - A291564(n). %o A306878 (PARI) a(n)=my(s); for(c=1, n-1, s+=(bigomega(n-c)-1)*(bigomega(n+c)-1)!=0); s %Y A306878 Cf. A061357, A291564. %K A306878 nonn %O A306878 1,5 %A A306878 _Gionata Neri_, Mar 14 2019