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

%I #13 Sep 24 2020 04:09:41

%S 1,4,5,16,20,25,64,80,100,125,256,320,400,500,625,1024,1280,1600,2000,

%T 2500,3125,4096,5120,6400,8000,10000,12500,15625,16384,20480,25600,

%U 32000,40000,50000,62500,65536,78125,81920,102400,128000,160000,200000,250000

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

%H Amiram Eldar, <a href="/A025617/b025617.txt">Table of n, a(n) for n = 1..10000</a>

%F Sum_{n>=1} 1/a(n) = (4*5)/((4-1)*(5-1)) = 5/3. - _Amiram Eldar_, Sep 24 2020

%F a(n) ~ exp(sqrt(2*log(4)*log(5)*n)) / sqrt(20). - _Vaclav Kotesovec_, Sep 24 2020

%t 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 *)

%Y Subsequence of A003592.

%K easy,nonn

%O 1,2

%A _David W. Wilson_