# [seqfan] Re: Period of final n digits of 2^k

Jean-Paul Allouche jean-paul.allouche at imj-prg.fr
Fri Jul 8 09:22:54 CEST 2022

```Dear Neil, dear all

Quick hint: it suffices to look at 2^k mod 5^n. Mimicking a classical proof
that Z/p^aZ is cyclic for p odd prime, one proves by induction that
2^{4.5^{b-1}}  = 1 + r_b 5^b where r_b is not divisible by 5. This gives
period 4.5{b-1} for mod 5^b. Hence the result.

best
jp

Le ven. 8 juil. 2022 à 08:05, Neil Sloane <njasloane at gmail.com> a écrit :

> Dear Seq Fans,
> Consider the sequence {2^k, k >= 0} mod 10^n. Let a(n) be its period.
>
> 2^k mod 10 is 1 2 4 8 6 2 4 8 6 ... which has period 4 (see A000689).  So
> a(1) = 4.
> a(2) = 20 (A000855).
> a(3) = 100 (A126605) according to Zak Seidov, who also says that
> a(4) = 500.
>
> So 4, 20, 100, 500, ? : how does it continue? Is it A005054? But A005054
>
> Best regards
> Neil
>
> Neil J. A. Sloane, Chairman, OEIS Foundation.
> Also Visiting Scientist, Math. Dept., Rutgers University,
> Email: njasloane at gmail.com
>
> --
> Seqfan Mailing list - http://list.seqfan.eu/
>

```