 mersenneforum.org > Math It seems to work, but why ?
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 Register FAQ Search Today's Posts Mark Forums Read  2005-10-09, 19:50 #1 T.Rex   Feb 2004 France 3·5·61 Posts It seems to work, but why ? With the following, I'm able to find divisors of Mersenne numbers. It is surely not useful, because it is MUCH slower than dividing Mersenne numbers by candidate divisors. But I do not understand why it works. Do you have an idea ? Tony Look at the following PARI/gp code. LLT(p,m) is a routine that computes several values (up to a maximum m) like the LLT does: x=-5 is the starting value, then it applies : . When , it stops and returns the number of iterations done. LLT2(q) enumerates all prime candidate divisors of Mq and calls LLT(p,q) for finding how many iterations (i) are required before x=0. It is a success if x=0 is found after q-2 iterations. (There are some rare cases where x=0 after a number of iterations different than q-2) I've experimented with the following values of q: 11, 13, 17, 23, 29, 31, 41, 43, 47, 53, 59, 89 (not finished for 89), and the result is always: - if Mq is prime, then LLT2 never succeeds (no divisor p is found). - if Mq is composite, then LLT2 finds divisors of Mq, and only them. It seems to be a miracle (or an evidence, or my mistake ?). As an example, for q=53, it finds the 3 divisors of M_53: 6361, 69431 and 20394401. LLT(p,m)= { x=-5; for(i=1,m, x=(-2*x-3)%p; if(x == 0, return(i)); ); return(0); } LLT2(q)= { print(q); for(i=1, sqrt(2^q-1)/2/q, p=1+2*q*i; if(isprime(p), j=LLT(p,q); if(j==q-2, print(i," ", 1+2*q*i, " ",j), if(j!=0, print("# ", j)) ); ); ); print } LLT2(11) LLT2(13) LLT2(17) LLT2(19) LLT2(23) LLT2(29) LLT2(31) LLT2(41) LLT2(43) LLT2(47) LLT2(53) LLT2(59) LLT2(89) LLT2(101)   2005-10-09, 21:57 #2 R. Gerbicz   "Robert Gerbicz" Oct 2005 Hungary 2·36 Posts This isn't a miracle! Let x(0)=-5 and x(n)=-2*x(n-1)-3 mod p. New variable: let y(n)=x(n)+1. So y(0)=-4 and y(n)-1=-2*(y(n-1)-1)-3 mod p from this: y(n)=-2*y(n-1) mod p so x(n)=y(n)-1=(-2)^n*y(0)-1=-(-2)^(n+2)-1 mod p. It is easy to see that LT(p,m)=i if and only if i<=m and i is the smallest solution of x(n)=0 mod p or i=0 if it hasn't got a solution up to m. Suppose that q is prime then every prime divisor of Mq is of the form p=1+2*q*i; where i>=1 is an integer and we can suppose that p2) . If LLT2 find a value : p=1+2*q*i and LLT(p,q)=q-2 so -(-2)^(q-2+2)-1=0 mod p, but q=1 mod 2 so 2^q=1 mod p from this p is a prime divisor of 2^q-1 and 1 2005-10-09, 23:07 #3 wblipp   "William" May 2003 New Haven 3×787 Posts Study the iteration without the mod. The negative signs are distracting, so take it 2 steps at a time, F2k(x) = 4 * F2k-2(x)+3 It's not hard to show that F2k(x) = 22k(x+1)-1 Hence F2k+1(x) = -22k+1(x+1)-1 When x=-5, this is F2k+1(-5) = 22k+3-1 So the value, without any mod operations, is exactly the Mersenne Number at iteration "q-2." When the prime divides the Mersenne number, the mod is of course zero.   2005-10-10, 17:12 #4 T.Rex   Feb 2004 France 3×5×61 Posts Too stupid I am, sometimes ... Yes, it is not a miracle. I've been a kind of stupid: I was studying LLT-like formula modulo Mq : x=(a*x^2+b*x+c)(mod Mq). And, with a=0, I did not think about looking at the numbers x without the modulo ... which are the Mersenne numbers. Thanks for opening my eyes ! Tony   2005-10-10, 21:06 #5 T.Rex   Feb 2004 France 3·5·61 Posts Some LLT-like tests for Mersenne The following formula seem to provide the same kind of primality test for Mersenne than the LLT does. Why ? . . .   2005-10-10, 21:58 #6 Numbers   Jun 2005 Near Beetlegeuse 1100001002 Posts Tony, I may have mis-read your post but according to your bottom line, if you start with then you get: etc. this last has the factors 2, 2, 97, 97 which does not seem to fit very well with a test for division by a Mersenne number. The LL number 37634 on the other hand has the factors 2, 31, 607 which does fit with Mersennes, so I‘m wondering if maybe there’s a typo in your post.   2005-10-11, 08:44 #7 T.Rex   Feb 2004 France 3×5×61 Posts A family of different LLT-like primality tests for Mersennes ? Hi Numbers, About the third formula, if you say: and , then you see that: . So there is no miracle. Better, if you define: , then you produce the numbers: 7, 97, 31*607, ... that appear with the LLT and f_3. But, if you look at second formula: , which is the same than F(x), but with a different x_0, you get: 7^2, 4801, 31*1487071, 52609*80789839489, 127*769*36810112513*10050007226929279, ... . So this F_5 function generates different numbers but it still acts as a LLT-like test (PARI code): F5(q)=x=2; for(i=1,q,x=(2*x^2-1)%(2^q-1);if(x==0,print(i))) Notice that the function mimics the look of Mersenne numbers: . About the first formula: , it produces different numbers: 2, 2*7, 2*547, 2*7*31*61*271, 2*6883*22434744889, 2*7*19*37*43*127*547*883*2269*2521*550554229, ... and it also looks like it is still a LLT-like primality test for Mersenne numbers. f1(q)=x=1;for(i=1,q,x=(6*x*(x-1)+2)%(2^q-1);if(x==0,print(i))). It should be possible to prove that F_5 and f_1 are really LLT-like primality tests for Mersenne numbers, by using method described by P. Ribenboim in his book. I'll have a look. More comments ? Tony   2005-10-11, 21:06   #8
T.Rex

Feb 2004
France

3·5·61 Posts Quote:
 Originally Posted by T.Rex It should be possible to prove that F_5 and f_1 are really LLT-like primality tests for Mersenne numbers, by using method described by P. Ribenboim in his book. I'll have a look.
The methods used by Lucas and Lehmer do not apply for F_5 and f_1. Tony   2005-10-12, 13:22 #9 Numbers   Jun 2005 Near Beetlegeuse 38810 Posts Tony, I know this is going to sound trivially obvious, but all you have is (x+2-2)^2 = x^2 You have one sequence starts with a 4, and goes (x^2)-2 Then you have another sequence starting with a 6, going: (y-2)^2 So the second sequence is just (x+2-2)^2 which is pretty obviously x^2 I can’t believe you haven’t already seen this, but there doesn’t appear to be a better answer to why it happens.   2005-10-12, 15:00 #10 wblipp   "William" May 2003 New Haven 3·787 Posts FWIW, the closed form solution of the first itereation is Sn = (32[sup]n[/sup])/6 + 1/2 Using the MersenneWiki explication of Lucas-Lehmer as a guide, it looks interesting to investigate the order of 3 mod Q - but I don't how.   2005-10-12, 16:44   #11
T.Rex

Feb 2004
France

3×5×61 Posts Quote:
 Originally Posted by Numbers I can’t believe you haven’t already seen this, but there doesn’t appear to be a better answer to why it happens.
In fact (and this happens too often), I realized my mistake once I have posted and switched off my PC and was ready to sleep. It it what I explain in first part of my post #7 about llt and f3. They are 2 different formula for the same kind of numbers.
About F5 and f1 in #7, I've read again papers. Based on Ribenboim work, they cannot be proved by usual technics because we must have: . This comes from a property of Lucas Sequences: . When n=2k.
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