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 A285509 #21 May 08 2017 21:07:45 %S A285509 1,2,2,2,3,4,5,5,5,5,6,8,10,10,10,9,10,10,10,10,11,13,18,20,18,15,15, %T A285509 15,20,20,19,18,20,20,20,19,20,20,20,20,21,23,31,38,33,28,20,20,21,30, %U A285509 39,39,38,30,29,25,35,40,40,38,31,33,36,40,38,40,35,40,40,40,39,38,40,40,40,39,40,40,40,40,41,43,54,69 %N A285509 a(1) = 1; a(2) = a(3) = a(4) = 2; a(n) = a(a(n-1)-1) + a(n-a(n-3)) for n > 4. %C A285509 Although sequence is unpredictable with its complex growth characteristic and generational structure, it has various signs of order and there are many temporary and simple patterns on it. For example, values of a(n) such that a(n) = a(n + 1) = a(n + 2) = a(n + 3) are 5, 10, 20, 40, 80, 160, 320, 640, 1280, ... %H A285509 Altug Alkan, <a href="/A285509/b285509.txt">Table of n, a(n) for n = 1..10000</a> %H A285509 Altug Alkan, <a href="/A285509/a285509.png">Alternative scatterplot of A285509</a> %e A285509 a(5) = 3 because a(5) = a(a(4)-1) + a(5-a(2)) = a(1) + a(3) = 2. %t A285509 a[1]=1; a[2]=a[3]=a[4]=2; a[n_] := a[n] = a[a[n-1]-1] + a[n-a[n-3]]; Array[a, 84] (* _Giovanni Resta_, Apr 21 2017 *) %o A285509 (PARI) a=vector(10000); a[1]=1;a[2]=a[3]=a[4]=2; for(n=5, #a, a[n]=a[a[n-1]-1]+a[n-a[n-3]]); va = vector(10000, n, a[n]) %Y A285509 Cf. A005185, A055748, A284523, A285507 %K A285509 nonn,look %O A285509 1,2 %A A285509 _Altug Alkan_, Apr 20 2017