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 A213552 #12 Jul 25 2015 11:03:31 %S A213552 1,15,81,281,756,1722,3486,6462,11187,18337,28743,43407,63518,90468, %T A213552 125868,171564,229653,302499,392749,503349,637560,798974,991530, %U A213552 1219530,1487655,1800981,2164995,2585611,3069186,3622536,4252952 %N A213552 Principal diagonal of the convolution array A213551. %H A213552 Clark Kimberling, <a href="/A213552/b213552.txt">Table of n, a(n) for n = 1..1000</a> %H A213552 <a href="/index/Rec#order_06">Index entries for linear recurrences with constant coefficients</a>, signature (6,-15,20,-15,6,-1). %F A213552 a(n) = n*(1 + n)*(2 + n)(-3 + 7*n + 16*n^2)/120. %F A213552 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 A213552 G.f.: x*(1 + 9*x + 6*x^2)/(1 - x)^6. %t A213552 (See A213551.) %Y A213552 Cf. A213551, A213500. %K A213552 nonn,easy %O A213552 1,2 %A A213552 _Clark Kimberling_, Jun 17 2012