## anonymous 5 years ago Thanks guys - Find the greatest prime factor of 2^20-1 (I got 41, could someone tell me if that is correct and how they obtained their solution?)

1. amistre64

2^(20-1)?

2. anonymous

(2^20)-2

3. anonymous

Whoops, (2^20)-1

4. anonymous

so you are try to find the largest prime p such that $2^{20}\equiv 1(mod p)$

5. anonymous

Yes. I totally forgot how to use modulus, but yes, I believe that is it.

6. anonymous

yeah my number theory is a bit rusty, I would need to look up some theorems as well...

7. anonymous

Oh darn. =] Anyone care to help?