## Re: Modulo order

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• ... an ... The best way I know is to start with N=p-1 and trial divide by each prime factor q of N. If 2^(N/q) = 1 mod p, then replace N by N/q and
Message 1 of 6 , Jul 7, 2004
wrote:
> Can someone give me some more details on working out the order of
an
> element mod p when we have a factorization of p-1?
> Please use the following example p = 176963110569601 =
> 2^7*3*5^2*151*1367*89303 = 1. The order of 2 is 22120388821200.
> But why? How would I compute this from the factorization of p-1?

The best way I know is to start with N=p-1 and trial divide by each
prime factor "q" of N. If 2^(N/q) = 1 mod p, then replace N by N/q
and continue. In your example, trial division by 2 works the first 3
times. Trial division by the other primes fails.

I do this whenever the ElevenSmooth Special Project finds a new
divisor. These factors are found as divisors of 2^3326400-1, so I
know the order is a divisor of GCD(3326400, p-1). I use this process
to find the order of 2, which tells me for which Mersenne number the
prime is a primitive factor. I then tell Will Edgington and post it
on the ElevenSmooth factors page at
http://ElevenSmooth.com/ElevenFactors.html

William
==================
ElevenSmooth http://ElevenSmooth.com
Distributed Factoring of 2^3326400-1
• Hi, I have two experimental methods under investigation for this problem. One is the reverse algorithm as mentioned a week or so back. A second method
Message 2 of 6 , Jul 7, 2004
Hi,

I have two experimental methods under investigation for this problem.

One is the 'reverse algorithm' as mentioned a week or so back.

A second method involves growing a 'binary tree'. This works from
knowing that the final modulo result in a trial factorisation is 1.
From this you can grow a tree, which will eventually give you the
number that you want.

The trick with the tree is to spot non-fruitful branches and to 'prune
them'. You also need to control the growth of the tree, since there
are many possible results, which are multiples of the one that you
want. The terminal condition for any branch of the tree is to trial
factor the number that you just generated and to check for a modulo 1
result.

Once you have grown a few trees then you will start to see the 1's and
0's of the binary representation of a number as a series of decision
points. From that you can start to appreciate that the final order 2
is as much a result of the spacings of 1's and 0's of the original
number as anything else.

The whole trick with this approach is to grow each branch one level at
a time. This prevents uncontrolled growth of any one branch. Another
thing is to watch out for repeating sequences, these need weeding out
too.

Kevin.

> --- In primenumbers@yahoogroups.com, "joseph_osiecki" <osieckis@n...>
> wrote:
> > Can someone give me some more details on working out the order of
> an
> > element mod p when we have a factorization of p-1?
> > Please use the following example p = 176963110569601 =
> > 2^7*3*5^2*151*1367*89303 = 1. The order of 2 is
22120388821200.
> > But why? How would I compute this from the factorization of p-1?
>
> The best way I know is to start with N=p-1 and trial divide by each
> prime factor "q" of N. If 2^(N/q) = 1 mod p, then replace N by N/q
> and continue. In your example, trial division by 2 works the first 3
> times. Trial division by the other primes fails.
>
> I do this whenever the ElevenSmooth Special Project finds a new
> divisor. These factors are found as divisors of 2^3326400-1, so I
> know the order is a divisor of GCD(3326400, p-1). I use this process
> to find the order of 2, which tells me for which Mersenne number the
> prime is a primitive factor. I then tell Will Edgington and post it
> on the ElevenSmooth factors page at
> http://ElevenSmooth.com/ElevenFactors.html
>
> William
> ==================
> ElevenSmooth http://ElevenSmooth.com
> Distributed Factoring of 2^3326400-1
>
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