A025621 Numbers of form 4^i*10^j, with i, j >= 0.

1, 4, 10, 16, 40, 64, 100, 160, 256, 400, 640, 1000, 1024, 1600, 2560, 4000, 4096, 6400, 10000, 10240, 16000, 16384, 25600, 40000, 40960, 64000, 65536, 100000, 102400, 160000, 163840, 256000, 262144, 400000, 409600, 640000, 655360, 1000000, 1024000

Offset: 1

Author

David W. Wilson

Keywords

• easy
• nonn

Links

• Charles R Greathouse IV, Table of n, a(n) for n = 1..10000

Crossrefs

Subsequence of A025612.

Programs

Mathematica

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 *)

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

Formula

Sum_{n>=1} 1/a(n) = (4*10)/((4-1)*(10-1)) = 40/27. - Amiram Eldar, Sep 25 2020

a(n) ~ exp(sqrt(2*log(4)*log(10)*n)) / sqrt(40). - Vaclav Kotesovec, Sep 25 2020

a(n) = 4^A025649(n) * 10^A025686(n). - R. J. Mathar, Jul 06 2025