# [seqfan] Re: Counting problem

Robert G. Wilson, v rgwv at rgwv.com
Wed Jun 17 06:25:44 CEST 2009

```Dear David,

Here is what I get with the Mmca coding. This is for all n-digit numbers. Let me have some time for just the odd digit numbers.

Sincerely yours, Bob.

{1, 3, 7, 14, 28, 57, 115, 230, 460, 921, 1843, 3686, 7372, 14745, 29491, 58982, 117964, 235929, 471859, 943718, 1887436, 3774873, 7549747, 15099494, 30198988, 60397977, 120795955, 241591910, 483183820, 966367641}
a(1)=1 because 5 divides only 5,
a(2)=3 because 25 divides {25, 50 & 75},
a(3)=7 because 125 divides {125, 250, 375, 500, 625, 750 & 925}, etc.
Lim n -> inf. a(n+1}/a{n) = 2.
f[n_] := Floor[(10^n - 1)/5^n] - Floor[10^(n - 1)/5^n]; Array[f, 35]

----- Original Message -----
From: "David Wilson" <davidwwilson at comcast.net>
To: "Sequence Fans" <seqfan at seqfan.eu>
Sent: Tuesday, June 16, 2009 11:22 PM
Subject: [seqfan] Counting problem

> How many n-digit numbers with all odd digits are divisible by 5^n?
>
>
>
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```