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 A027138 #14 Oct 23 2019 22:07:52
%S A027138 1,7,26,85,264,803,2422,7281,21860,65599,196818,590477,1771456,
%T A027138 5314395,15943214,47829673,143489052,430467191,1291401610,3874204869,
%U A027138 11622614648,34867843987,104603532006,313810596065,941431788244,2824295364783,8472886094402
%N A027138 a(n) = Sum_{k=0..2n} (k+1) * A027113(n, 2n-k).
%H A027138 Colin Barker, <a href="/A027138/b027138.txt">Table of n, a(n) for n = 1..1000</a>
%H A027138 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (5,-7,3).
%F A027138 For n>1, a(n) = 10*3^(n-2) - n - 1.
%F A027138 For n>4, a(n) = 5*a(n-1) - 7*a(n-2) + 3*a(n-3). - _Colin Barker_, Jul 11 2015
%F A027138 G.f.: -x*(x^3-2*x^2+2*x+1) / ((x-1)^2*(3*x-1)). - _Colin Barker_, Jul 11 2015
%o A027138 (PARI) Vec(-x*(x^3-2*x^2+2*x+1)/((x-1)^2*(3*x-1)) + O(x^50)) \\ _Colin Barker_, Jul 11 2015
%K A027138 nonn,easy
%O A027138 1,2
%A A027138 _Clark Kimberling_