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 A107986 #23 Jul 05 2024 08:12:42 %S A107986 4,9,15,21,25,33,39,45,49,55,63,69,75,81,85,91,99,105,111,115,129,133, %T A107986 141,153,159,165,169,175,183,195,201,213,225,231,235,243,253,259,265, %U A107986 273,279,285,295,309,315,319,333,339,351,355,361,369,375,381,385,391 %N A107986 Composite numbers of the form p+2 where p is prime. %C A107986 This sequence is analogous to the sequence formed by the Goldbach-Euler conjecture that every even number greater than 2 is the sum of 2 primes. If p + 2 is prime then p and p + 2 are twin primes. The number of terms in this sequence is infinite. This follows immediately from the proof that the number of primes p is infinite. Conjecture: The ratio of the number of terms in this sequence to Pi(n) tends to a limit < 1. %C A107986 The first term in this sequence that is not also in A062721 is 45 = 3^2 * 5. - _Alonso del Arte_, May 03 2014 %F A107986 a(n) = A067774(n) + 2. - _Amiram Eldar_, Jul 05 2024 %t A107986 Select[Range[4, 399], Not[PrimeQ[#]] && PrimeQ[# - 2] &] (* _Alonso del Arte_, May 03 2014 *) %Y A107986 Cf. A062721, A067774, A107987. %K A107986 easy,nonn %O A107986 1,1 %A A107986 _Cino Hilliard_, Jun 13 2005