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 A320468 #37 Aug 28 2019 11:52:36
%S A320468 1,41,61,881,2101,19721,61741,456161,1690981,10814201,44633821,
%T A320468 260917841,1153594261,6371951081,29443836301,156882857921,
%U A320468 745759583941,3883416742361,18798608421181,96466943268401,472439111692021,2401777977060041,11850560210900461,59886119752101281,296897323970110501
%N A320468 a(n) = a(n-1) + 20*a(n-2), n >= 2; a(0)=1, a(1)=41.
%H A320468 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (1,20).
%F A320468 a(n) = (-4)^(n+1) + 5^(n+1).
%F A320468 G.f.: (1+40*x)/((1-5*x)*(1+4*x)).
%F A320468 a(n) == 9*A053428(n)*A224473(n) mod 10^n.
%F A320468 a(n)*A224473(n) == 9*A053428(n) mod 10^n.
%o A320468 (PARI) {a(n) = (-4)^(n+1)+5^(n+1)}
%o A320468 (PARI) N=40; x='x+O('x^N); Vec((1+40*x)/((1-5*x)*(1+4*x)))
%Y A320468 Cf. A053428, A224473 (trimorphic number), A320469, A321133.
%K A320468 nonn,easy
%O A320468 0,2
%A A320468 _Seiichi Manyama_, Aug 27 2019