20050921, 12:05  #1 
Sep 2005
127 Posts 
3PRP faster than LL for GIMPS?
import java.math.BigInteger;
public class Mersennef5 { static BigInteger zero = new BigInteger("0"); static BigInteger one = new BigInteger("1"); static BigInteger two = new BigInteger("2"); static BigInteger three = new BigInteger("3"); static BigInteger nine = new BigInteger("9"); static BigInteger tenthousand = new BigInteger("10000"); public static void main (String args[]) throws java.io.IOException { BigInteger n = new BigInteger("1"); BigInteger m = new BigInteger("0"); while (n.compareTo(tenthousand) < 0) { if (isprime(n)) { System.out.print(n.toString() + '\r'); m = mersenne(n); if (fermat2(m)) System.out.println(n.toString()); } n = n.add(one); } } static BigInteger mersenne(BigInteger n) { BigInteger mersenne = new BigInteger("1"); BigInteger i = new BigInteger("0"); while (i.compareTo(n) < 0) { mersenne = mersenne.multiply(two); i = i.add(one); } mersenne = mersenne.subtract(one); return mersenne; } static boolean isprime(BigInteger n) { return n.isProbablePrime(100); } static boolean fermat2(BigInteger n) { boolean prime = true; // workaround  apparent java bug in modPow  JGW if (n.compareTo(one) == 0) return false; if (n.compareTo(two) == 0) return true; if (n.remainder(two).compareTo(zero) == 0) return false; // end workaround if (power(three, (n.add(one)), n).compareTo(nine) == 0) prime = true; else prime = false; return prime; } static BigInteger power(BigInteger x, BigInteger n, BigInteger mod) { // Knuth 4.6.3  computes x^n modulo mod BigInteger N = new BigInteger("0"); BigInteger Y = new BigInteger("0"); BigInteger Z = new BigInteger("0"); N=n; Y=one; Z=x; while (true) { if (N.remainder(two).compareTo(zero) > 0) { N=N.divide(two); Y=Z.multiply(Y).remainder(mod); if (N.compareTo(zero) == 0) return Y; } else { N=N.divide(two); } Z=Z.multiply(Z).remainder(mod); } } } 
20050921, 12:12  #2  
Nov 2003
2^{2}×5×373 Posts 
Quote:
May I suggest that this forum is for the discussion of mathematics. If you wish to discuss an algorithm, please give a mathematical description. It is unreasonable to expect others to translate your code so that we may understand what it does. (2) Please explain why you think determining if a Mersenne prime candidate is a 3prp would be faster than a LL test. Tell us what aspects of the computation would be faster. I will give a hint: It isn't. In fact, it would be slightly slower. I will give a second hint: It won't prove primaility as LL does. 

20050921, 12:39  #3 
Sep 2005
7F_{16} Posts 
At least (naively) seems (at least) as fast as this...
import java.math.BigInteger;
public class Mersenneu3 { static BigInteger zero = new BigInteger("0"); static BigInteger one = new BigInteger("1"); static BigInteger two = new BigInteger("2"); static BigInteger tenthousand = new BigInteger("10000"); public static void main (String args[]) throws java.io.IOException { BigInteger n = new BigInteger("1"); BigInteger m = new BigInteger("0"); while (n.compareTo(tenthousand) < 0) { if (isprime(n)) { System.out.print(n.toString() + '\r'); m = mersenne(n); if (lucaslehmer(n, m)) System.out.println(n.toString()); } n = n.add(one); } } static BigInteger mersenne(BigInteger n) { BigInteger mersenne = new BigInteger("1"); BigInteger i = new BigInteger("0"); while (i.compareTo(n) < 0) { mersenne = mersenne.multiply(two); i = i.add(one); } mersenne = mersenne.subtract(one); return mersenne; } static boolean isprime(BigInteger n) { return n.isProbablePrime(100); } static boolean lucaslehmer(BigInteger n, BigInteger m) { BigInteger i = new BigInteger("1"); BigInteger S = new BigInteger("4"); for (i = one; i.compareTo(n.subtract(one)) < 0; i = i.add(one)) S = ((S.multiply(S)).subtract(two)).remainder(m); if (S.compareTo(zero) == 0) return true; else return false; } } 
20050921, 12:53  #4 
Sep 2005
127 Posts 
...explanation
I posted the two separate competing sections of code first, on its own, and in its entirety, in case anyone fancied repeating my experiment unchanged. Clearly there are other issues (eg FFT multiplication), but maybe these apply equally.
My math reasoning goes like this: First, you will notice I use the "Russian Peasant" method for evaluating modPow in the 3PRP code. Secondly I actually evaluate 3^(n+1) [mod n] (rather than 3^n) and check for equality with nine (rather than 3). This hopefully cunning alteration means the modPow, in the case of a Mersenne number, is being raised to an exact power of two, which should further speed it up. Obviously a 3PRP is not as definitive in _proving_ primality, but _is_ as definitive in _disproving_ it, which is what we want for GIMPS 99% of the time. J 
20050921, 13:24  #5  
Nov 2003
2^{2}·5·373 Posts 
Quote:
Just what we need. Another crank. I requested that the poster describe the algorithm, rather than posting code. So what does he do? He posts yet more code. And people wonder why I lose patience. I suggest to the poster that he show EXACTLY how his powering algorithm reduces the number of multiplications. [Hint: it doesn't; I told him this in private email, but of course this was ignored and followed by another public post] His trick of computing 3^(n+1), rather than 3^n is indeed useful since every iteration is now just a squaring. But it STILL REQUIRES n multiplications! [squarings] Go read the section on "addition chains" in Knuth Vol 2. One can not do better than n squarings. log_2[N] is a lower bound on the length of any addition chain. When N = 2^n, we get a chain of length n. 

20050921, 13:29  #6 
Sep 2005
127 Posts 
As I said in _my_ private email :)
1) With the 3PRP, we no longer need the subtraction of 2 at each step (as per LL), so maybe we gain here?
2) I've done the actual test [at least in Java, up to n=10000], and it seems to work, so I was looking for explanation, as much as anything else... J 
20050921, 14:36  #7  
Nov 2003
2^{2}·5·373 Posts 
Quote:
peasant" powering algorithm has a fair amount of nonconstant overhead per iteration beyond that of the single precision subtraction in LL. (2) "seems to work". What seems to work? Of course one can do a base 3 prp test. And If you wanted an explanation for something, then why did you post CODE??? If you are asking whether 3 is ever a FALSE witness to 2^n1, the answer is certainly yes, but the occurrence will be VERY VERY rare. On rough probabilistic grounds I would expect #{x < N  x = 2^n1, x composite, 3 is a false witness} to be no more than loglog N, i.e. false witnesses should be at least as rare as Mersenne primes themselves and certainly as hard to find. 

20050921, 15:25  #8 
Aug 2004
italy
113 Posts 
Bearnol, can you please put comments in your code, if you want us to understand it ? (no, JAVA is not a self documenting language).
beside that, the entire thread should probably go to the software forum, since it is dealing mostly with implementation details. thanks. Pierpaolo 
20050923, 11:51  #9 
Feb 2003
2·59 Posts 
IMHO, a 3PRP test (or any kPRP) and the LL test for a Mersenne M(n) have the same complexity O(n) if they use the same method for multiplication (squaring). The lack of substraction for each step has almost zero influence on the total time, a substraction takes several orders of magnitude less cycles than a multiplication. Make the following test:
 supose you want to test M(n)  calculate M(n)  2 (n1 times) and measure the total time => this is the maximum gain you can expect 
20051004, 14:47  #10 
Sep 2005
127 Posts 
...so in fact (I think we're now all agreed :) ) PRP would prove (slightly) faster than LL [as it would not need the subtractions, but otherwise be similar]. This is what I have observed in practice. Who's going to tell George Woltman?!

20051004, 14:55  #11 
"Nancy"
Aug 2002
Alexandria
9A3_{16} Posts 
No one will have to tell him, he reads the forum himself. The PRP idea has been suggested many times before, i.e. on the Mersenne mailing list before the forum even existed.
The 3PRP is NOT faster than a LL. The time for the subtraction at each step is completely negligible. And we'll need the LL test anyway so we can actually prove primality of Mersenne numbers. Why should we add another algorithm that is exactly as fast as LL but inferior in that it is not sufficient for a primality proof? Alex 
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