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 A221921 #27 Oct 09 2023 02:20:57
%S A221921 2,20,12,28,8,36,20,44,3,52,28,60,16,68,36,76,10,84,44,92,24,100,52,
%T A221921 108,7,116,60,124,32,132,68,140,18,148,76,156,40,164,84,172,11,180,92,
%U A221921 188,48,196,100,204,26,212,108,220,56,228,116,236,15,244,124,252,64
%N A221921 a(n) = 4*n/gcd(4*n,n+4), n >= 4.
%C A221921 This is the fourth column of the triangle A221918.
%H A221921 <a href="/index/Rec#order_32">Index entries for linear recurrences with constant coefficients</a>, signature (0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,2,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,-1).
%F A221921 a(n) = A221918(n,4) = numerator(n*4/(n+4)), n >= 4.
%F A221921 a(n) = 4*n/gcd(16,n+4), n >= 4.
%F A221921 G.f.: x^4*(-12*x^31 - 4*x^30 - 4*x^29 + 4*x^27 + 4*x^26 + 12*x^25 + x^24 + 20*x^23 + 12*x^22 + 28*x^21 + 8*x^20 + 36*x^19 + 20*x^18 + 44*x^17 + 6*x^16 + 76*x^15 + 36*x^14 + 68*x^13 + 16*x^12 + 60*x^11 + 28*x^10 + 52*x^9 + 3*x^8 + 44*x^7 + 20*x^6 + 36*x^5 + 8*x^4 + 28*x^3 + 12*x^2 + 20*x + 2) / (x^32 - 2*x^16 + 1). - _Colin Barker_, Feb 25 2013
%F A221921 Sum_{k=4..n} a(k) ~ (171/128) * n^2. - _Amiram Eldar_, Oct 09 2023
%e A221921 a(8) = numerator(32/12) = numerator(8/3) = 8 = 32/gcd(32,12) = 32/4 = 32/gcd(16,12).
%t A221921 Table[(4n)/GCD[4n,n+4],{n,4,70}] (* _Harvey P. Dale_, May 15 2018 *)
%o A221921 (PARI) a(n)=4*n/gcd(4*n,n+4) \\ _Charles R Greathouse IV_, Apr 18 2013
%Y A221921 Cf. A221918, A000027 (m=1), A145979(m=2), A221920 (m=3).
%K A221921 nonn,easy
%O A221921 4,1
%A A221921 _Wolfdieter Lang_, Feb 21 2013