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 A289133 #21 Jun 28 2017 02:52:28
%S A289133 0,0,0,1,1,1,1,1,2,2,2,2,3,3,3,3,3,4,4,4,4,5,5,5,5,5,6,6,6,6,7,7,7,7,
%T A289133 7,8,8,8,8,9,9,9,9,9,10,10,10,10,11,11,11,11,11,12,12,12,12,13,13,13,
%U A289133 13,13,14,14,14,14,15,15,15,15,15,16,16,16,16,17,17,17,17,17,18
%N A289133 a(n) is the number of odd integers divisible by 9 in ]2*(n-1)^2, 2*n^2[.
%C A289133 This sequence has the form (0+2k,0+2k,0+2k,1+2k,1+2k,1+2k,1+2k,1+2k,2+2k) for k>=0.
%H A289133 <a href="/index/Rec#order_10">Index entries for linear recurrences with constant coefficients</a>, signature (1,0,0,0,0,0,0,0,1,-1).
%F A289133 a(n + 9*k) = a(n) + 2*k.
%F A289133 G.f.: (x^8+x^3)/(x^10-x^9-x+1). - _Alois P. Heinz_, Jun 26 2017
%t A289133 Table[Count[Mod[Table[2((n-1)^2 +k)-1,{k,1,2 n-1}],9],0],{n,0,50}]
%o A289133 (PARI) a(n) = sum(k=2*(n-1)^2, 2*n^2, ((k % 2) && ((k % 9) == 0))); \\ _Michel Marcus_, Jun 26 2017
%Y A289133 Cf. A289120, A289122, A288156, A004523, A289139.
%K A289133 nonn,easy
%O A289133 0,9
%A A289133 _Ralf Steiner_, Jun 25 2017
%E A289133 More terms from _Michel Marcus_, Jun 26 2017