A025617 Numbers of form 4^i*5^j, with i, j >= 0.

1, 4, 5, 16, 20, 25, 64, 80, 100, 125, 256, 320, 400, 500, 625, 1024, 1280, 1600, 2000, 2500, 3125, 4096, 5120, 6400, 8000, 10000, 12500, 15625, 16384, 20480, 25600, 32000, 40000, 50000, 62500, 65536, 78125, 81920, 102400, 128000, 160000, 200000, 250000

Offset: 1

Author

David W. Wilson

Keywords

• easy
• nonn

Links

• Amiram Eldar, Table of n, a(n) for n = 1..10000

Crossrefs

Subsequence of A003592.

Programs

Mathematica

n = 10^5; Flatten[Table[4^i*5^j, {i, 0, Log[4, n]}, {j, 0, Log[5, n/4^i]}]] // Sort (* Amiram Eldar, Sep 24 2020 *)

Formula

Sum_{n>=1} 1/a(n) = (4*5)/((4-1)*(5-1)) = 5/3. - Amiram Eldar, Sep 24 2020

a(n) ~ exp(sqrt(2*log(4)*log(5)*n)) / sqrt(20). - Vaclav Kotesovec, Sep 24 2020

a(n) = 4^A025646(n) * 5^A025650(n). - R. J. Mathar, Jul 06 2025