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 A139689 #12 Feb 16 2025 08:33:08 %S A139689 0,1,1,0,1,0,1,0,0,1,0,0,1,0,0,1,0,1,0,0,1,1,0,0,0,0,1,0,1,0,0,1,0,1, %T A139689 1,0,0,1,0,1,0,0,0,1,1,0,0,1,0,0,1,0,0,0,0,0,0,1,1,0,0,1,0,0,0,0,0,0, %U A139689 0,1,0,0,0,1,0,0,0,1,1,0,1,0,0,1,0,0,0,0,0,0,0,0,0,0,1,0,1,1,0,0,1,0,0,1,0 %N A139689 If (n-th Chen prime) + 2 is a prime then 1 else 0. %H A139689 Reinhard Zumkeller, <a href="/A139689/b139689.txt">Table of n, a(n) for n = 1..1000</a> %H A139689 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/ChenPrime.html">Chen Prime</a> %H A139689 <a href="/index/Ch#char_fns">Index entries for characteristic functions</a> %F A139689 a(n) = 2 - A001222(A139690(n)). %F A139689 a(n) = A010051(A139690(n)) = 1 - A064911(A139690(n)). %Y A139689 Cf. A001222, A010051, A064911, A139690. %K A139689 nonn %O A139689 1,1 %A A139689 _Reinhard Zumkeller_, Apr 29 2008