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 A213823 #12 Jun 17 2017 02:59:44 %S A213823 4,41,147,358,710,1239,1981,2972,4248,5845,7799,10146,12922,16163, %T A213823 19905,24184,29036,34497,40603,47390,54894,63151,72197,82068,92800, %U A213823 104429,116991,130522,145058,160635,177289 %N A213823 Principal diagonal of the convolution array A213822. %H A213823 Clark Kimberling, <a href="/A213823/b213823.txt">Table of n, a(n) for n = 1..1000</a> %H A213823 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (4,-6,4,-1). %F A213823 a(n) = (-n - 3n^2 + 12*n^3)/2. %F A213823 a(n) = 4*a(n-1) - 6*a(n-2) + 4*a(n-3) - a(n-4). %F A213823 G.f.: f(x)/g(x), where f(x) = x*(4 + 25*x + 7*x^2) and g(x) = (1-x)^4. %t A213823 (See A213822.) %Y A213823 Cf. A213822. %K A213823 nonn,easy %O A213823 1,1 %A A213823 _Clark Kimberling_, Jul 04 2012