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 A328333 #8 Feb 16 2025 08:33:58
%S A328333 1,5,9,49,89,489,889,4889,8889,48889,88889,488889,888889,4888889,
%T A328333 8888889,48888889,88888889,488888889,888888889,4888888889,8888888889,
%U A328333 48888888889,88888888889,488888888889,888888888889,4888888888889,8888888888889,48888888888889,88888888888889
%N A328333 Expansion of (1 + 4*x - 6*x^2) / ((1 - x) * (1 - 10*x^2)).
%C A328333 Number of even palindromes < 10^n.
%H A328333 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/PalindromicNumber.html">Palindromic Number</a>
%H A328333 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (1,10,-10).
%t A328333 nmax = 28; CoefficientList[Series[(1 + 4 x - 6 x^2)/((1 - x) (1 - 10 x^2)), {x, 0, nmax}], x]
%t A328333 LinearRecurrence[{1, 10, -10}, {1, 5, 9}, 29]
%o A328333 (PARI) Vec((1 + 4*x - 6*x^2) / ((1 - x) * (1 - 10*x^2)) + O(x^30)) \\ _Michel Marcus_, Oct 13 2019
%Y A328333 Cf. A002113, A029951, A050250, A070199, A328332.
%K A328333 nonn,base,easy
%O A328333 0,2
%A A328333 _Ilya Gutkovskiy_, Oct 12 2019