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 A217779 #13 Oct 02 2023 13:48:07 %S A217779 1,4,15,56,208,768,2821,10320,37639,136972,497652,1805984,6548425, %T A217779 23729916,85953823,311240928,1126753336,4078394080,14760382029, %U A217779 53415642632,193291233367,699417041844,2530731376540,9156839587776,33131242464913,119873850697588 %N A217779 Expansion of (1-4x+4*x^2)/((1-5x+5*x^2)*(1-3x+x^2)). %C A217779 A diagonal of the square array A217770. %H A217779 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (8, -21, 20, -5). %F A217779 a(n) = A217770(n,n+2). %F A217779 a(n) = 8*a(n-1) - 21*a(n-2) + 20*a(n-3) - 5*a(n-4) for n>=4, a(0) = 1, a(1) = 4, a(2) = 15, a(3) = 56. %Y A217779 Cf. A217770, A217777, A217778. %K A217779 nonn,easy %O A217779 0,2 %A A217779 _Philippe Deléham_, Mar 24 2013