Factors of a Mersenne number
A Mersenne number is a number in the form of 2P-1
.
If P
is prime, the Mersenne number may be a Mersenne prime. (If P
is not
prime, the Mersenne number is also not prime.)
In the search for Mersenne prime numbers it is advantageous to eliminate exponents by finding a small factor before starting a, potentially lengthy, Lucas-Lehmer test.
There are very efficient algorithms for determining if a number divides 2P-1
(or equivalently, if 2P mod (the number) = 1
).
Some languages already have built-in implementations of this exponent-and-mod operation (called modPow or similar).
The following is how to implement this modPow yourself:
For example, let's compute 223 mod 47
.
Convert the exponent 23 to binary, you get 10111. Starting with square = 1
,
repeatedly square it.
Remove the top bit of the exponent, and if it's 1 multiply square
by the base
of the exponentiation (2), then compute square modulo 47
.
Use the result of the modulo from the last step as the initial value of square
in the next step:
Remove Optional square top bit multiply by 2 mod 47
1*1 = 1 1 0111 1*2 = 2 2 2*2 = 4 0 111 no 4 4*4 = 16 1 11 16*2 = 32 32 32*32 = 1024 1 1 1024*2 = 2048 27 27*27 = 729 1 729*2 = 1458 1
Since 223 mod 47 = 1
, 47 is a factor of 2P-1
.
(To see this, subtract 1 from both sides: 223-1 = 0 mod 47
.)
Since we've shown that 47 is a factor, 223-1
is not prime.
Further properties of Mersenne numbers allow us to refine the process even more.
Any factor q
of 2P-1
must be of the form 2kP+1
, k
being a positive
integer or zero. Furthermore, q
must be 1
or 7 mod 8
.
Finally any potential factor q
must be
prime.
As in other trial division algorithms, the algorithm stops when
2kP+1 > sqrt(N)
.These primarily tests only work on Mersenne numbers where P
is prime. For example, M4=15
yields no factors using these techniques, but
factors into 3 and 5, neither of which fit 2kP+1
.
Test
{{test}}Console output