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 2011-05-30, 19:04 #1 CyD   May 2011 102 Posts Fermat number and Modulo for searching divisors Hello, I try to find somebody who will be able to answer me about the following: I hope it is not too much trouble. May be this property can be used for searching Fermat numbers divisors. I know this forum is not for Fermat numbers, but may be, somebody is able to answer. If you know a forum like this one where you think somebody is able to answer, please, let me know. I demonstrate the following property (All numbers are natural numbers) For a composite Fermat number , I suppose it is semi-prim (even if it is not semi-prim). For example of semi-prim, I use a little number N, let it be equal to 105. $N = 3*5*7=105$ Here, N is not semi-prim because it has 3 divisors. I choose to considerate N like a semi-prim event if it is not. $N=D_1*D_2$ Let $D_1$ and $D_2$ be $D_1=3$ and $D_2 =35$ or $D_1 = 5$ and $D_2 = 21$ or $D_1=7$ and $D_2 = 15$ About Fermat numbers : Let define the 2 divisors of $F_m$ by $D_{m,1}$ and $D_{m,2}$ , and $X_m$ and $T_m$ by: $D_{m,1} = X_m.2^{m+2} +1$ and $D_{m,2} = T_m.2^{m+2} +1$ So, we have the following properties (for $i \leq i_{max}$ : $2^{2^{n}-i.(m+2)} = - (-X)^i mod D_{m,1}$ and in an equivalent way : $2^{2^{n}-i.(m+2)} = - (-T)^i mod D_{m,2}$ I try to find on the Internet some information about this property but I find nothing. Do you know some internet sites or books about this property ? Do you think this property can be used for searching Fermat numbers divisors? If I'm not clear, please, let me know. Many thanks by advance, Best Regards, Cyril Delestre
2011-05-30, 21:01   #2
R.D. Silverman

Nov 2003

22×5×373 Posts

Quote:
 Originally Posted by CyD Hello, I try to find somebody who will be able to answer me about the following: I hope it is not too much trouble. May be this property can be used for searching Fermat numbers divisors. I know this forum is not for Fermat numbers, but may be, somebody is able to answer. If you know a forum like this one where you think somebody is able to answer, please, let me know. I demonstrate the following property (All numbers are natural numbers) For a composite Fermat number , I suppose it is semi-prim (even if it is not semi-prim). For example of semi-prim, I use a little number N, let it be equal to 105. $N = 3*5*7=105$ Here, N is not semi-prim because it has 3 divisors. I choose to considerate N like a semi-prim event if it is not. $N=D_1*D_2$ Let $D_1$ and $D_2$ be $D_1=3$ and $D_2 =35$ or $D_1 = 5$ and $D_2 = 21$ or $D_1=7$ and $D_2 = 15$ About Fermat numbers : Let define the 2 divisors of $F_m$ by $D_{m,1}$ and $D_{m,2}$ , and $X_m$ and $T_m$ by: $D_{m,1} = X_m.2^{m+2} +1$ and $D_{m,2} = T_m.2^{m+2} +1$ So, we have the following properties (for $i \leq i_{max}$ : $2^{2^{n}-i.(m+2)} = - (-X)^i mod D_{m,1}$ and in an equivalent way : $2^{2^{n}-i.(m+2)} = - (-T)^i mod D_{m,2}$ I try to find on the Internet some information about this property but I find nothing. Do you know some internet sites or books about this property ? Do you think this property can be used for searching Fermat numbers divisors? If I'm not clear, please, let me know. Many thanks by advance, Best Regards, Cyril Delestre
It is trivially known that any divisor p of 2^(2^n) + 1 must equal 1 mod
(2^(n+2)). I have given proofs on previous occasions. The proof
might be given as a homework problem in a first year number theory class.

This property is useful for trial division. It is often used to find small
divisors for large n. It isn't useful for much of anything else.

Last fiddled with by R.D. Silverman on 2011-05-30 at 21:01 Reason: typo

2011-05-31, 08:16   #3
xilman
Bamboozled!

"πΊππ·π·π­"
May 2003
Down not across

1049610 Posts

Quote:
 Originally Posted by R.D. Silverman It is trivially known that any divisor p of 2^(2^n) + 1 must equal 1 mod (2^(n+2)). I have given proofs on previous occasions. The proof might be given as a homework problem in a first year number theory class. This property is useful for trial division. It is often used to find small divisors for large n. It isn't useful for much of anything else.
It's also of historical interest because it was used to speed the factorization of F_7 by Pollard's rho algorithm.

Pollard's rho isn't really of much use these days now that ECM is available.

Paul

 2011-05-31, 10:52 #4 CyD   May 2011 2 Posts I didn't try to prove that any divisor of $2^{2^{n}}+1$ is like $X.2^{n+2}+1$. I know it's known. I used it in order to demonstrate the following (with the same notation than my previous message) $2^{2^{n}-i.(m+2)} = - (-X_m)^i mod D_{m,1}$ and for example, if $2^{n} = 0 mod (m+2)$ and with $i_{max} = \frac{2^{n}}{m+2}$ then $(-X_m)^{i_{max}} = -1 mod D_{m,1}$ and if you have already prove it and if you know some internet site or book, I am interested by that. Cyril
2011-05-31, 11:24   #5
R.D. Silverman

Nov 2003

11101001001002 Posts

Quote:
 Originally Posted by CyD I didn't try to prove that any divisor of $2^{2^{n}}+1$ is like $X.2^{n+2}+1$. I know it's known. I used it in order to demonstrate the following (with the same notation than my previous message) $2^{2^{n}-i.(m+2)} = - (-X_m)^i mod D_{m,1}$ and for example, if $2^{n} = 0 mod (m+2)$ and with $i_{max} = \frac{2^{n}}{m+2}$ then $(-X_m)^{i_{max}} = -1 mod D_{m,1}$ and if you have already prove it and if you know some internet site or book, I am interested by that. Cyril
I will quote Serge Lang.

I can't be bothered wading through it. If you clean it up and repost
your comments, I will take a look.

Note, however, that trivially m+2 itself is a power of 2.

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