## Re: [PrimeNumbers] Re: Cross Base Factorising

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• Actually it is practical. Remember that you don t store the actual number, only the exponent. So in the example of 3^88-1 I know that everything below my
Message 1 of 6 , Jul 1, 2004
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Actually it is practical.

Remember that you don't store the actual number, only the exponent. So in
the example of 3^88-1 I know that everything below my addition point is
2222222..... As such it gets discarded. I only count how many 2's I get in
the string.

Kevin.

At 04:26 PM 2/07/2004, eharsh82 wrote:
>Yes it should work!
>
>"Take a number known to be composite, in this case 2047 and check it
>against the smallest 3^n -1 number that it can divide."
>
>Finding n is really difficult for large numbers. So your algorithm is
>not practical.
>
>
>Harsh
>
>
>
>
>
>--- In primenumbers@yahoogroups.com, Kevin Acres <research@r...>
>wrote:
> > OK, here's a bombproof way to factorize a Mersenne, given that the
> > algorithm is implementable in a computer. Please make me aware of
>any
> > counter-example should you know of one.
> >
> > Remember that we are dealing solely with an exponent of a number to
>a given
> > base. You don't need to keep the actual number itself in computer
>memory
> > in an implementation.
> >
> > Given that the "reverse method" can find the smallest exponent in a
>given
> > base for a given number.
> >
> > And given that p divides (n^(p-1))-1 where p is prime or a base n
> > pseudoprime. Taken another way p+1 divides n^p - 1.
> >
> > Take a number known to be composite, in this case 2047 and check it
>against
> > the smallest 3^n -1 number that it can divide.
> >
> > The reverse method shows us that 2047 divides 2^11-1 (itself) and
>3^88 - 1.
> >
> > We now know for sure that 2047 isn't prime because 88 doesn't
>divide 2046,
> > (2047-1).
> >
> > We also know that x+1 will divide 3^x -1. In this case x+1 is 88+1
>or 89.
> >
> > Now we find that 89 divides 2047 giving us 23, the other factor.
> > Effectively factorizing 2^11-1 without ever having to work in base
>2.
> >
> > Now take another number, this time known to be prime, 8191
> >
> > Applying the reverse algorithm in base 3 we find that 8191 divides
>3^8190 -
> > 1. 8190 divides 8191-1 so we now that 8191 is prime in base 3. The
>reverse
> > algorithm in base 2 shows that 8191 divides 2^13-1 and that 13
>divides
> > 8190, therefore 8191 is base 2 prime as well.
> >
> > So am I wrong or does this method of factorizing work in all cases
>for
> > Mersenne numbers?
> >
> > Kevin.
>
>
>
>
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