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 A224473 #12 Feb 16 2025 08:33:19 %S A224473 9,49,249,1249,81249,781249,5781249,25781249,425781249,6425781249, %T A224473 36425781249,836425781249,9836425781249,19836425781249, %U A224473 519836425781249,2519836425781249,12519836425781249,512519836425781249,4512519836425781249,84512519836425781249 %N A224473 (2*5^(2^n) - 1) mod 10^n: a sequence of trimorphic numbers ending in 9. %C A224473 a(n) is the unique positive integer less than 10^n such that a(n) - 1 is divisible by 2^n and a(n) + 1 is divisible by 5^n. %H A224473 Eric M. Schmidt, <a href="/A224473/b224473.txt">Table of n, a(n) for n = 1..1000</a> %H A224473 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/TrimorphicNumber.html">Trimorphic Number</a> %H A224473 <a href="/index/Ar#automorphic">Index entries for sequences related to automorphic numbers</a> %F A224473 a(n) = (2 * A007185(n) - 1) mod 10^n. %o A224473 (Sage) def A224473(n) : return crt(1, -1, 2^n, 5^n); %Y A224473 Cf. A033819. Corresponding 10-adic number is A091661. The other trimorphic numbers ending in 9 are included in A002283, A198971 and A224475. %K A224473 nonn,base %O A224473 1,1 %A A224473 _Eric M. Schmidt_, Apr 07 2013