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 A271800 #21 Apr 15 2016 14:45:27
%S A271800 0,1,2,3,4,5,4,3,2,1,2,3,4,5,6,5,4,3,2,3,4,5,6,7,6,5,4,3,4,5,6,7,8,7,
%T A271800 6,5,4,5,6,7,8,9,8,7,6,5,6,7,8,9,10,9,8,7,6,7,8,9,10,11,10,9,8,7,8,9,
%U A271800 10,11,12,11,10,9,8,9,10,11,12,13,12,11,10
%N A271800 Five steps forward, four steps back.
%H A271800 Colin Barker, <a href="/A271800/b271800.txt">Table of n, a(n) for n = 0..1000</a>
%H A271800 <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 A271800 a(n) = a(n-1) + a(n-9) - a(n-10) for n>9.
%F A271800 a(n) = Sum_{i=1..n} (-1)^floor((2*i-2)/9).
%F A271800 G.f.: x*(1+x+x^2+x^3+x^4-x^5-x^6-x^7-x^8) / ((1-x)^2*(1+x+x^2)*(1+x^3+x^6)). - _Colin Barker_, Apr 15 2016
%p A271800 A271800:=n->add((-1)^floor((2*i-2)/9), i=1..n): seq(A271800(n), n=0..200);
%t A271800 Table[Sum[(-1)^Floor[(2 i - 2)/9], {i, n}], {n, 0, 100}]
%o A271800 (PARI) concat(0, Vec(x*(1+x+x^2+x^3+x^4-x^5-x^6-x^7-x^8)/((1-x)^2*(1+x+x^2)*(1+x^3+x^6)) + O(x^50))) \\ _Colin Barker_, Apr 15 2016
%Y A271800 Cf. A008611 (one step back, two steps forward).
%Y A271800 Cf. A058207 (three steps forward, two steps back).
%Y A271800 Cf. A260644 (four steps forward, three steps back).
%K A271800 nonn,easy
%O A271800 0,3
%A A271800 _Wesley Ivan Hurt_, Apr 15 2016