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 A213556 #12 Jun 17 2017 03:53:56 %S A213556 1,19,118,446,1271,3017,6300,11964,21117,35167,55858,85306,126035, %T A213556 181013,253688,348024,468537,620331,809134,1041334,1324015,1664993, %U A213556 2072852,2556980,3127605,3795831,4573674,5474098,6511051,7699501 %N A213556 Principal diagonal of the convolution array A213555. %H A213556 Clark Kimberling, <a href="/A213556/b213556.txt">Table of n, a(n) for n = 1..1000</a> %H A213556 <a href="/index/Rec#order_06">Index entries for linear recurrences with constant coefficients</a>, signature (6,-15,20,-15,6,-1). %F A213556 a(n) = (9*n^5 + 15*n^4 + 5*n^3 + n)/30. %F A213556 a(n) = 6*a(n-1) - 15*a(n-2) + 20*a(n-3) - 15*a(n-4) + 6*a(n-5) - a(n-6). %F A213556 G.f.: x*(1 + 13*x + 19*x^2 + 3*x^3)/(1 - x)^6. %t A213556 (See A213553.) %Y A213556 Cf. A213553, A213500. %K A213556 nonn,easy %O A213556 1,2 %A A213556 _Clark Kimberling_, Jun 17 2012