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Old 2007-06-28, 00:20   #1
vtai
 
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Default help with a proof

let p be a prime. show that 2^Mp = 2(mod Mp). (where Mp is the Mersenne number for p). i was able to show that the congruence is true for fermat numbers (ie. 2^Fn = 2(mod Fn)). but i just can't seem to figure it out for this question. any help with this proof would be appreciated.
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Old 2007-06-28, 11:11   #2
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http://primes.utm.edu/glossary/page....sLittleTheorem
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Old 2007-06-28, 11:19   #3
R.D. Silverman
 
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Quote:
Originally Posted by paulunderwood View Post
And? Note that while p is prime, M_p usually isn't. We are working mod
M_p, not mod p.
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Old 2007-06-28, 12:22   #4
alpertron
 
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Let p be a prime number.

2^p \equiv 1 \,\pmod {2^p-1}

Using Fermat's Little Theorem we get:

2^p \equiv 2 \,\pmod p so 2^p-1 \equiv 1 \,\pmod p

This means that 2^p-1 = kp+1 so we finally get:

2^{2^p-1} \equiv 2^{kp+1} \equiv 2^{kp}*2^1 \equiv (2^p)^k*2 \equiv 1^k*2 \equiv 2\,\pmod {2^p-1}

Last fiddled with by alpertron on 2007-06-28 at 12:28
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Old 2007-06-28, 12:23   #5
R. Gerbicz
 
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Mp=2^p-1, where p is prime. By Fermat's Little Theorem 2^p==2 mod p, so
Mp==1 mod p, so Mp=k*p+1 for some positive integer k, using this:
2^Mp=2^(k*p+1)=2*2^(k*p)=2*(2^p)^k=2*(Mp+1)^k==2*1^k==2 mod Mp, what is needed.
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Old 2007-06-28, 12:37   #6
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Quote:
And?
As this is not the homework section...

Quote:
Note that while p is prime, M_p usually isn't. We are working mod
M_p, not mod p.
Thanks for pointing this out, Bob.

2^p==2 (mod p) by Fermat's Little Theorem since gcd(2,2^p-1)==1
2^p-2==0 (mod p) by subtracting "2" from each side of the congruence equation

By the definition, p divides 2^p-2. That is 2^p-2 is a multiple of p. So 2^p-2 =N*p for some "N".
2^p-1=N*p+1 by adding "1" to each side of the equation.

2^p-1==0 (mod Mp) by definition
2^p==1 (mod Mp) by adding "1" to each side of the congruence equation[*]

Therefore, 2^Mp==2^(N*p+1) (mod Mp)
2^Mp==(2^(N*p))*2 (mod Mp)
2^Mp==((2^p)^N)*2 (mod Mp)
2^Mp==(1^N)*2 (mod Mp) by[*]
2^Mp==2 (mod Mp)

Q.E.D.

This demonstrates why a base 2 Fermat test is useless for Mersenne primes.

(I see that by the time I have written this out two other people have given the proof )

Last fiddled with by paulunderwood on 2007-06-28 at 13:12
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Old 2007-06-28, 13:35   #7
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Quote:
Originally Posted by paulunderwood View Post
2^p==2 (mod p) by Fermat's Little Theorem since gcd(2,2^p-1)==1
It should be ...since gcd(2,p)==1
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Old 2007-06-28, 13:50   #8
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Old 2007-06-28, 14:04   #9
vtai
 
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Thank your for your replies...
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Old 2007-06-28, 14:36   #10
vtai
 
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i have another question that is related...what if p was not prime, how would we prove the congruence then??
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Old 2007-06-28, 14:50   #11
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Quote:
Originally Posted by vtai View Post
i have another question that is related...what if p was not prime, how would we prove the congruence then??
In that case in general 2^p\not\equiv 2\,\pmod p, so you will not get the number 2.

But since 2^p \eq 1\,\pmod{2^p-1} at least you always get a power of 2.

For example if p=6 you get:
2^{2^p-1} \eq 2^{2^p-1 \bmod p} \eq 2^{63 \bmod 6} \eq 2^3 \eq 8\,\pmod{2^p-1}

Last fiddled with by alpertron on 2007-06-28 at 15:02
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