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 A228138 #12 Jun 13 2015 00:54:43 %S A228138 4,13,20,50,63,111,130,196,221,305,336,438,475,595,638,776,825,981, %T A228138 1036,1210,1271,1463,1530,1740,1813,2041,2120,2366,2451,2715,2806, %U A228138 3088,3185,3485,3588,3906,4015,4351,4466,4820,4941,5313,5440,5830,5963,6371,6510 %N A228138 Number of blocks in a Steiner system S(2, 4, A228137(n+1)). %H A228138 Colin Barker, <a href="/A228138/b228138.txt">Table of n, a(n) for n = 1..1000</a> %H A228138 <a href="/index/Rec#order_05">Index entries for linear recurrences with constant coefficients</a>, signature (1,2,-2,-1,1). %F A228138 a(n) = (1-(-1)^n+(-4+6*(-1)^n)*n+12*n^2)/4. %F A228138 a(n) = a(n-1)+2*a(n-2)-2*a(n-3)-a(n-4)+a(n-5). %F A228138 G.f.: x*(3*x^5-3*x^4-12*x^3+x^2-9*x-4) / ((x-1)^3*(x+1)^2). %o A228138 (PARI) Vec(x*(3*x^5-3*x^4-12*x^3+x^2-9*x-4)/((x-1)^3*(x+1)^2) + O(x^99)) %Y A228138 Cf. A228137. %K A228138 nonn,easy %O A228138 1,1 %A A228138 _Colin Barker_, Aug 12 2013