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 A072021 #22 Oct 30 2019 04:41:02
%S A072021 5,5200,521000,5213750,521875,5218750,52130234375,5734841796875,
%T A072021 57869714843750,526046650390625,5265674365234375,52187008544921875,
%U A072021 526515306396484375,5213023309008789062500,5213596736358642578125,5260466086273193359375,526041911745452880859375
%N A072021 Smallest solution to gcd(x, reverse(x)) = 5^n.
%H A072021 Hiroaki Yamanouchi, <a href="/A072021/b072021.txt">Table of n, a(n) for n = 1..25</a>
%F A072021 a(n) = A069554(5^n).
%e A072021 For n = 4, gcd(521875, 578125) = 3125 = 5^4.
%e A072021 For n = 8, a(8) = 5734841796875 = 5^9*2936239, reverse(a(8)) = 5786971484375 = 5^8*71*208657.
%o A072021 (PARI) a(n) = {my(k = 1); while (gcd(k, fromdigits(Vecrev(digits(k)))) != 5^n, k++); k;} \\ _Michel Marcus_, Jul 13 2018
%Y A072021 Cf. A004086, A055483, A069554, A071686, A072005, A072050, A072016-A072018.
%K A072021 nonn,base
%O A072021 1,1
%A A072021 _Labos Elemer_, Jun 06 2002
%E A072021 a(9)-a(18) from _Hiroaki Yamanouchi_, Sep 10 2014