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 A213839 #18 Jun 17 2017 02:59:44
%S A213839 1,20,89,240,505,916,1505,2304,3345,4660,6281,8240,10569,13300,16465,
%T A213839 20096,24225,28884,34105,39920,46361,53460,61249,69760,79025,89076,
%U A213839 99945,111664,124265,137780,152241,167680
%N A213839 Principal diagonal of the convolution array A213838.
%H A213839 Clark Kimberling, <a href="/A213839/b213839.txt">Table of n, a(n) for n = 1..1000</a>
%H A213839 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (4,-6,4,-1).
%F A213839 a(n) = (8*n - 21*n^2 + 16*n^3)/3.
%F A213839 a(n) = 4*a(n-1) - 6*a(n-2) + 4*a(n-3) - a(n-4).
%F A213839 G.f.: f(x)/g(x), where f(x) = x*(1 + 16*x + 15*x^2) and g(x) = (1-x)^4.
%t A213839 (See A213838.)
%t A213839 LinearRecurrence[{4,-6,4,-1},{1,20,89,240},40] (* _Harvey P. Dale_, Mar 10 2013 *)
%Y A213839 Cf. A213838.
%K A213839 nonn,easy
%O A213839 1,2
%A A213839 _Clark Kimberling_, Jul 05 2012