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 A025621 #18 Jul 06 2025 10:02:25
%S A025621 1,4,10,16,40,64,100,160,256,400,640,1000,1024,1600,2560,4000,4096,
%T A025621 6400,10000,10240,16000,16384,25600,40000,40960,64000,65536,100000,
%U A025621 102400,160000,163840,256000,262144,400000,409600,640000,655360,1000000,1024000
%N A025621 Numbers of form 4^i*10^j, with i, j >= 0.
%H A025621 Charles R Greathouse IV, <a href="/A025621/b025621.txt">Table of n, a(n) for n = 1..10000</a>
%F A025621 Sum_{n>=1} 1/a(n) = (4*10)/((4-1)*(10-1)) = 40/27. - _Amiram Eldar_, Sep 25 2020
%F A025621 a(n) ~ exp(sqrt(2*log(4)*log(10)*n)) / sqrt(40). - _Vaclav Kotesovec_, Sep 25 2020
%F A025621 a(n) = 4^A025649(n) * 10^A025686(n). - _R. J. Mathar_, Jul 06 2025
%t A025621 n = 10^6; Flatten[Table[4^i*10^j, {i, 0, Log[4, n]}, {j, 0, Log10[n/4^i]}]] // Sort (* _Amiram Eldar_, Sep 25 2020 *)
%o A025621 (PARI) list(lim)=my(v=List(), N); for(n=0, logint(lim\=1, 10), N=10^n; while(N<=lim, listput(v, N); N<<=2)); Set(v) \\ _Charles R Greathouse IV_, Jan 10 2018
%Y A025621 Subsequence of A025612.
%K A025621 easy,nonn
%O A025621 1,2
%A A025621 _David W. Wilson_