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 A216092 #25 Mar 14 2025 09:20:20
%S A216092 4,24,624,624,90624,890624,2890624,12890624,212890624,8212890624,
%T A216092 18212890624,918212890624,9918212890624,59918212890624,
%U A216092 259918212890624,6259918212890624,56259918212890624,256259918212890624
%N A216092 a(n) = 2^(2*5^(n-1)) mod 10^n.
%C A216092 a(n) is the unique positive integer less than 10^n such that a(n) is divisible by 2^n and a(n) + 1 is divisible by 5^n. - _Eric M. Schmidt_, Sep 01 2012
%H A216092 Robert Israel, <a href="/A216092/b216092.txt">Table of n, a(n) for n = 1..996</a>
%F A216092 a(n) = (5^(2^n) mod 10^n) - 1.
%F A216092 a(n)^3 == a(n) (mod 10^n).
%F A216092 a(n-1) == a(n) (mod 10^(n-1)). - _Robert Israel_, Mar 13 2025
%p A216092 f:= n -> 2&^(2*5^(n-1)) mod 10^n:
%p A216092 map(f, [$1..100]); # _Robert Israel_, Mar 13 2025
%t A216092 Table[PowerMod[5,2^n,10^n],{n,20}]-1 (* _Harvey P. Dale_, Dec 17 2017 *)
%o A216092 (Sage) def A216092(n) : return crt(0, -1, 2^n, 5^n) # _Eric M. Schmidt_, Sep 01 2012
%Y A216092 Cf. A007185, A016090, A216093, A091664, A018247.
%K A216092 nonn
%O A216092 1,1
%A A216092 _V. Raman_, Sep 01 2012