# [seqfan] Re: successive powers of 2 modulo n

allouche at math.jussieu.fr allouche at math.jussieu.fr
Fri Jun 10 09:29:48 CEST 2011

```Hi, also look at A007733 and its firts comment
best
jean-paul

David Scambler <dscambler at bmm.com> a écrit :

> I am just playing around with this - not sure if it is of interest to others.
>
> Take the successive powers of 2 modulo n and the sequence apparently
>  always enters a cycle.
> Is this true? Proof?
>
> e.g. Successive powers of 2 modulo 7 -> 1 2 4 1 2 4 1 2 4 1 2 4 ...
> - a cycle of length 3.
>
> For some numbers n the cycle length is n-1 e.g. moduli
> 2,3,5,11,13,19,29... have cycle lengths 1,2,4,10,12,18,28...
> respectively.
> Is this the maximum length?
>
> For n >= 1 the cycle lengths are
>
> a(n) = 1,1,2,1,4,2,3,1,6,4,10,2,12,3,4,1,8,6,18,4,6, ...
>
> And finally,
>
> Is there an efficient way to calculate successive 2^k mod n without
> actually evaluating 2^k and dividing by n?
>
>
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>
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