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 A106667 #12 Sep 17 2018 20:13:50 %S A106667 -1,1,1,-1,1,-1,1,-1,-1,1,-1,-1,1,0,-1,-1,1,0,-1,1,0,0,-1,-1,0,1,0,1, %T A106667 -1,-1,-1,-1,1,-1,1,0,-1,0,-1,0,1,-1,1,0,1,-1,-1,0,1,0,-1,1,0,-1,-1, %U A106667 -1,1,0,0,1,0,-1,-1,1,0,-1,0,-1,1,0,-1,-1,0,0,-1,0,-1,0,-1,-1,1,0,1,0,0,-1,-1,0,1,0,-1,-1,-1,-1 %N A106667 a(n) = 1 if prime(n) + 2 is a prime, a(n) = -1 if prime(n) + 2 is a semiprime, otherwise 0. %H A106667 Robert Israel, <a href="/A106667/b106667.txt">Table of n, a(n) for n = 1..10000</a> %e A106667 a(1) = -1 because prime(1) = 2 and 2 + 2 = 4 is a semiprime; %e A106667 a(2) = 1 because prime(2) = 3 and 3 + 2 = 5 is a prime; %e A106667 a(14) = 0 because prime(14) = 43 and 43 + 2 = 45 is neither prime nor semiprime. %p A106667 p:= 1: %p A106667 for n from 1 to 100 do %p A106667 p:= nextprime(p); %p A106667 if isprime(p+2) then A[n]:=1 %p A106667 elif numtheory:-bigomega(p+2)=2 then A[n]:=-1 %p A106667 else A[n]:= 0 %p A106667 fi %p A106667 od: %p A106667 seq(A[n],n=1..100); # _Robert Israel_, Aug 29 2018 %Y A106667 Cf. A001359, A063637. %K A106667 easy,sign,less %O A106667 1,1 %A A106667 _Giovanni Teofilatto_, May 13 2005 %E A106667 Corrected, and definition clarified, by _Robert Israel_, Aug 29 2018