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 A027993 #11 Jun 22 2018 11:35:03
%S A027993 1,6,20,54,132,306,688,1518,3308,7146,15336,32742,69604,147426,311264,
%T A027993 655326,1376220,2883546,6029272,12582870,26214356,54525906,113246160,
%U A027993 234880974,486539212,1006632906,2080374728,4294967238
%N A027993 a(n) = 1*T(n,2n) + 2*T(n,2n-1) + ... + (2n+1)*T(n,0), T given by A027926.
%H A027993 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (6,-13,12,-4).
%F A027993 (n+5)*2^n - 2n - 4, a(0)=1. - _Ralf Stephan_, Mar 25 2004
%F A027993 G.f. ( 1-3*x^2 ) / ( (2*x-1)^2*(x-1)^2 ). - _R. J. Mathar_, Jun 12 2013
%t A027993 LinearRecurrence[{6,-13,12,-4},{1,6,20,54},30] (* _Harvey P. Dale_, Jun 22 2018 *)
%Y A027993 Cf. A047860.
%K A027993 nonn
%O A027993 0,2
%A A027993 _Clark Kimberling_