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 A224475 #9 Feb 16 2025 08:33:19
%S A224475 4,99,749,6249,31249,281249,781249,75781249,925781249,1425781249,
%T A224475 86425781249,336425781249,4836425781249,69836425781249,19836425781249,
%U A224475 7519836425781249,62519836425781249,12519836425781249,9512519836425781249,34512519836425781249
%N A224475 (2*5^(2^n) + (10^n)/2 - 1) mod 10^n: a sequence of trimorphic numbers ending (for n > 1) in 9.
%C A224475 a(n) is the unique positive integer less than 10^n such that a(n) + 2^(n-1) - 1 is divisible by 2^n and a(n) + 1 is divisible by 5^n.
%H A224475 Eric M. Schmidt, <a href="/A224475/b224475.txt">Table of n, a(n) for n = 1..1000</a>
%H A224475 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/TrimorphicNumber.html">Trimorphic Number</a>
%H A224475 <a href="/index/Ar#automorphic">Index entries for sequences related to automorphic numbers</a>
%F A224475 a(n) = (A224473(n) + 10^n / 2) mod 10^n.
%t A224475 Table[Mod[2*5^2^n+(10^n/2)-1,10^n],{n,20}] (* _Harvey P. Dale_, Sep 08 2024 *)
%o A224475 (Sage) def A224475(n) : return crt(2^(n-1)+1, -1, 2^n, 5^n)
%Y A224475 Cf. A033819. Converges to the 10-adic number A091661. The other trimorphic numbers ending in 9 are included in A002283, A198971, and A224473.
%K A224475 nonn,base
%O A224475 1,1
%A A224475 _Eric M. Schmidt_, Apr 07 2013