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In mathematics, the Lucas–Lehmer test (LLT) is a primality test for Mersenne numbers. The test was originally developed by Édouard Lucas in 1856[1] and subsequently improved by Lucas in 1878 and Derrick Henry Lehmer in the 1930s.

Contents

The testEdit

The Lucas–Lehmer test works as follows. Let Mp = 2p − 1 be the Mersenne number to test with p an odd prime. The primality of p can be efficiently checked with a simple algorithm like trial division since p is exponentially smaller than Mp. Define a sequence   for all i ≥ 0 by

 

The first few terms of this sequence are 4, 14, 194, 37634, ... (sequence A003010 in the OEIS). Then Mp is prime if and only if

 

The number sp − 2 mod Mp is called the Lucas–Lehmer residue of p. (Some authors equivalently set s1 = 4 and test sp−1 mod Mp). In pseudocode, the test might be written as

// Determine if Mp = 2p − 1 is prime for p > 2
Lucas–Lehmer(p)
    var s = 4
    var M = 2p − 1
    repeat p − 2 times:
        s = ((s × s) − 2) mod M
    if s == 0 return PRIME else return COMPOSITE

Performing the mod M at each iteration ensures that all intermediate results are at most p bits (otherwise the number of bits would double each iteration). The same strategy is used in modular exponentiation.

Alternate starting valuesEdit

Starting values s0 other than 4 are possible, for instance 10, 52, and others (sequence A018844 in the OEIS). The Lucas-Lehmer residue calculated with these alternative starting values will still be zero if Mp is a Mersenne prime. However, the terms of the sequence will be different and a non-zero Lucas-Lehmer residue for non-prime Mp will have a different numerical value from the non-zero value calculated when s0 = 4.

It is also possible to use the starting value (2 mod Mp)(3 mod Mp)−1, usually denoted by 2/3 for short.[2] This starting value equals (2p + 1) /3, the Wagstaff number with exponent p.

Starting values like 4, 10, and 2/3 are universal, that is, they are valid for all (or nearly all) p. There are infinitely many additional universal starting values.[2] However, some other starting values are only valid for a subset of all possible p, for example s0 = 3 can be used if p = 3 (mod 4).[3] This starting value was often used where suitable in the era of hand computation, including by Lucas in proving M127 prime.[4] The first few terms of the sequence are 3, 7, 47, ... (sequence A001566 in the OEIS).

Sign of penultimate termEdit

If sp−2 = 0 mod Mp then the penultimate term is sp−3 = ± 2(p+1)/2 mod Mp. The sign of this penultimate term is called the Lehmer symbol ϵ(s0p).

In 2000 S.Y. Gebre-Egziabher proved that for the starting value 2/3 and for p ≠ 5 the sign is:

 

That is, ϵ(2/3, p) = +1 iff p = 1 (mod 4) and p ≠ 5.[2]

The same author also proved that the Lehmer symbols for starting values 4 and 10 when p is not 2 or 5 are related by:

 

That is, ϵ(4, p) × ϵ(10, p) = 1 iff p = 5 or 7 (mod 8) and p ≠ 2, 5.[2]

OEIS sequence A123271 shows ϵ(4, p) for each Mersenne prime Mp.

Time complexityEdit

In the algorithm as written above, there are two expensive operations during each iteration: the multiplication s × s, and the mod M operation. The mod M operation can be made particularly efficient on standard binary computers by observing that

 

This says that the least significant n bits of k plus the remaining bits of k are equivalent to k modulo 2n−1. This equivalence can be used repeatedly until at most n bits remain. In this way, the remainder after dividing k by the Mersenne number 2n−1 is computed without using division. For example,

916 mod 25−1 = 11100101002 mod 25−1
= ((916 mod 25) + int(916 ÷ 25)) mod 25−1
= (101002 + 111002) mod 25−1
= 1100002 mod 25−1
= (100002 + 12) mod 25−1
= 100012 mod 25−1
= 100012
= 17.

Moreover, since s × s will never exceed M2 < 22p, this simple technique converges in at most 1 p-bit addition (and possibly a carry from the pth bit to the 1st bit), which can be done in linear time. This algorithm has a small exceptional case. It will produce 2n−1 for a multiple of the modulus rather than the correct value of 0. However, this case is easy to detect and correct.

With the modulus out of the way, the asymptotic complexity of the algorithm only depends on the multiplication algorithm used to square s at each step. The simple "grade-school" algorithm for multiplication requires O(p2) bit-level or word-level operations to square a p-bit number. Since this happens O(p) times, the total time complexity is O(p3). A more efficient multiplication algorithm is the Schönhage–Strassen algorithm, which is based on the Fast Fourier transform. It only requires O(p log p log log p) time to square a p-bit number. This reduces the complexity to O(p2 log p log log p) or Õ(p2).[5] Currently the most efficient known multiplication algorithm, Fürer's algorithm, only needs   time to multiply two p-bit numbers.

By comparison, the most efficient randomized primality test for general integers, the Miller–Rabin primality test, requires O(k n2 log n log log n) bit operations using FFT multiplication for an n-digit number, where k is the number of iterations and is related to the error rate. For constant k, this is in the same complexity class as the Lucas-Lehmer test. In practice however, the cost of doing many iterations and other differences leads to worse performance for Miller–Rabin. The most efficient deterministic primality test for any n-digit number, the AKS primality test, requires Õ(n6) bit operations in its best known variant and is extremely slow even for relatively small values.

ExamplesEdit

The Mersenne number M3 = 23−1 = 7 is prime. The Lucas–Lehmer test verifies this as follows. Initially s is set to 4 and then is updated 3−2 = 1 time:

  • s ← ((4 × 4) − 2) mod 7 = 0.

Since the final value of s is 0, the conclusion is that M3 is prime.

On the other hand, M11 = 2047 = 23 × 89 is not prime. Again, s is set to 4 but is now updated 11−2 = 9 times:

  • s ← ((4 × 4) − 2) mod 2047 = 14
  • s ← ((14 × 14) − 2) mod 2047 = 194
  • s ← ((194 × 194) − 2) mod 2047 = 788
  • s ← ((788 × 788) − 2) mod 2047 = 701
  • s ← ((701 × 701) − 2) mod 2047 = 119
  • s ← ((119 × 119) − 2) mod 2047 = 1877
  • s ← ((1877 × 1877) − 2) mod 2047 = 240
  • s ← ((240 × 240) − 2) mod 2047 = 282
  • s ← ((282 × 282) − 2) mod 2047 = 1736

Since the final value of s is not 0, the conclusion is that M11 = 2047 is not prime. Although M11 = 2047 has nontrivial factors, the Lucas–Lehmer test gives no indication about what they might be.

Proof of correctnessEdit

The proof of correctness for this test presented here is simpler than the original proof given by Lehmer. Recall the definition

 

The goal is to show that Mp is prime iff  

The sequence   is a recurrence relation with a closed-form solution. Let   and  . It then follows by induction that   for all i:

 

and

 

The last step uses   This closed form is used in both the proof of sufficiency and the proof of necessity.

SufficiencyEdit

The goal is to show that   implies that   is prime. What follows is a straightforward proof exploiting elementary group theory given by J. W. Bruce[6] as related by Jason Wojciechowski.[7]

Suppose   Then

 

for some integer k, so

 

Multiplying by   gives

 

Thus,

 

For a contradiction, suppose Mp is composite, and let q be the smallest prime factor of Mp. Mersenne numbers are odd, so q > 2. Informally,[note 1] let   be the integers modulo q, and let   Multiplication in   is defined as

 

Clearly, this multiplication is closed, i.e. the product of numbers from X is itself in X. The size of X is denoted by  

Since q > 2, it follows that   and   are in X.[note 2] The subset of elements in X with inverses forms a group, which is denoted by X* and has size   One element in X that does not have an inverse is 0, so  

Now   and  , so

 

in X. Then by equation (1),

 

in X, and squaring both sides gives

 

Thus   lies in X* and has inverse   Furthermore, the order of   divides   However  , so the order does not divide   Thus, the order is exactly  

The order of an element is at most the order (size) of the group, so

 

But q is the smallest prime factor of the composite  , so

 

This yields the contradiction  . Therefore,   is prime.

NecessityEdit

In the other direction, the goal is to show that the primality of   implies  . The following simplified proof is by Öystein J. Rödseth.[8]

Since   for odd  , it follows from properties of the Legendre symbol that   This means that 3 is a quadratic nonresidue modulo   By Euler's criterion, this is equivalent to

 

In contrast, 2 is a quadratic residue modulo   since   and so   This time, Euler's criterion gives

 

Combining these two equivalence relations yields

 

Let  , and define X as before as the ring   Then in the ring X, it follows that

 

where the first equality uses the Binomial Theorem in a finite field, which is

 ,

and the second equality uses Fermat's little theorem, which is

 

for any integer a. The value of   was chosen so that   Consequently, this can be used to compute   in the ring X as

 

All that remains is to multiply both sides of this equation by   and use  , which gives

 

Since   is 0 in X, it is also 0 modulo  

ApplicationsEdit

The Lucas–Lehmer test is the primality test used by the Great Internet Mersenne Prime Search (GIMPS) to locate large primes. This search has been successful in locating many of the largest primes known to date.[9] The test is considered valuable because it can probably test a large set of very large numbers for primality within an affordable amount of time. In contrast, the equivalently fast Pépin's test for any Fermat number can only be used on a much smaller set of very large numbers before reaching computational limits.

See alsoEdit

NotesEdit

  1. ^ Formally, let   and  .
  2. ^ Formally,   and   are in X. By abuse of language   and   are identified with their images in X under the natural ring homomorphism from   to X which sends   to T.

ReferencesEdit

  1. ^ The Largest Known Prime by Year: A Brief History
  2. ^ a b c d Jansen, B.J.H. (2012). Mersenne primes and class field theory (Doctoral thesis). Leiden University. p. iii. Retrieved 2018-12-17.
  3. ^ Robinson, Raphael M. (1954). "Mersenne and Fermat numbers". Proc. Amer. Math. Soc. 5: 842–846. doi:10.1090/S0002-9939-1954-0064787-4.
  4. ^ Haworth, Guy (1990). Mersenne numbers (PDF) (Technical report). p. 20. Retrieved 2018-12-17.
  5. ^ Colquitt, W. N.; Welsh, L., Jr. (1991), "A New Mersenne Prime", Mathematics of Computation, 56 (194): 867–870, doi:10.2307/2008415, The use of the FFT speeds up the asymptotic time for the Lucas–Lehmer test for Mp from O(p3) to O(p2 log p log log p) bit operations.
  6. ^ J. W. Bruce (1993). "A Really Trivial Proof of the Lucas–Lehmer Test". The American Mathematical Monthly. 100 (4): 370–371. doi:10.2307/2324959.
  7. ^ Jason Wojciechowski. Mersenne Primes, An Introduction and Overview. 2003.
  8. ^ Öystein J. Rödseth. A note on primality tests for N = h · 2n − 1. Department of Mathematics, University of Bergen.
  9. ^ The "Top Ten" Record Primes, The Prime Pages

External linksEdit