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 A221904 #24 Feb 13 2024 03:36:08
%S A221904 2,19,181,1729,16561,159049,1531441,14782969,143046721,1387420489,
%T A221904 13486784401,131381059609,1282429536481,12541865828329,
%U A221904 122876792454961,1205891132094649,11853020188851841
%N A221904 a(n) = 9^n + 10^n.
%H A221904 Vincenzo Librandi, <a href="/A221904/b221904.txt">Table of n, a(n) for n = 0..1000</a>
%H A221904 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (19,-90).
%F A221904 G.f.: 1/(1 - 9*x) + 1/(1 - 10*x).
%F A221904 E.g.f.: exp(9*x) + exp(10*x).
%F A221904 a(n) = 19*a(n-1) - 90*a(n-2), a(1)=2, a(2)=19.
%t A221904 CoefficientList[Series[1/(1-9*x) + 1/(1-10*x), {x, 0, 30}], x]
%t A221904 LinearRecurrence[{19,-90},{2,19},30] (* _Harvey P. Dale_, Nov 10 2017 *)
%o A221904 (Magma) [9^n + 10^n: n in [0..30]];
%Y A221904 Cf. A007689, A074611, A074615, A074619, A074622, A074624, A155872.
%K A221904 nonn,easy
%O A221904 0,1
%A A221904 _Vincenzo Librandi_, Feb 06 2013