Math.NumberTheory.Primes.Factorisation

Various functions related to prime factorisation. Many of these functions use the prime factorisation of an Integer. If several of them are used on the same Integer, it would be inefficient to recalculate the factorisation, hence there are also functions working on the canonical factorisation, these require that the number be positive and in the case of the Carmichael function that the list of prime factors with their multiplicities is ascending.

Factorisation functions

Factorisation of Integers by the elliptic curve algorithm after Montgomery. The algorithm is explained at http://programmingpraxis.com/2010/04/23/modern-elliptic-curve-factorization-part-1/ and http://programmingpraxis.com/2010/04/27/modern-elliptic-curve-factorization-part-2/

The implementation is not very optimised, so it is not suitable for factorising numbers with several huge prime divisors. However, factors of 20-25 digits are normally found in acceptable time. The time taken depends, however, strongly on how lucky the curve-picking is. With luck, even large factors can be found in seconds; on the other hand, finding small factors (about 12-15 digits) can take minutes when the curve-picking is bad.

Given enough time, the algorithm should be able to factor numbers of 100-120 digits, but it is best suited for numbers of up to 50-60 digits.

factorise :: Integer -> [(Integer, Int)]

factorise n produces the prime factorisation of n. factorise 0 is an error and the factorisation of 1 is empty. Uses a StdGen produced in an arbitrary manner from the bit-pattern of n.

defaultStdGenFactorisation :: StdGen -> Integer -> [(Integer, Int)]

defaultStdGenFactorisation first strips off all small prime factors and then, if the factorisation is not complete, proceeds to curve factorisation. For negative numbers, a factor of -1 is included, the factorisation of 1 is empty. Since 0 has no prime factorisation, a zero argument causes an error.

stepFactorisation :: Integer -> [(Integer, Int)]

stepFactorisation is like factorise', except that it doesn't use a pseudo random generator but steps through the curves in order. This strategy turns out to be surprisingly fast, on average it doesn't seem to be slower than the StdGen based variant.

factorise' :: Integer -> [(Integer, Int)]

Like factorise, but without input checking, hence n > 1 is required.

defaultStdGenFactorisation' :: StdGen -> Integer -> [(Integer, Int)]

Like defaultStdGenFactorisation, but without input checking, so n must be larger than 1.

Trial division

trialDivisionTo :: Integer -> Integer -> [(Integer, Int)]

trialDivisionTo bound n produces a factorisation of n using the primes <= bound. If n has prime divisors > bound, the last entry in the list is the product of all these. If n <= bound^2, this is a full factorisation, but very slow if n has large prime divisors.

Partial factorisation

smallFactors :: Integer -> Integer -> ([(Integer, Int)], Maybe Integer)

smallFactors bound n finds all prime divisors of n > 1 up to bound by trial division and returns the list of these together with their multiplicities, and a possible remaining factor which may be composite.

stdGenFactorisation

Arguments

:: Maybe Integer

Lower bound for composite divisors

-> StdGen

Standard PRNG

-> Maybe Int

Estimated number of digits of smallest prime factor

-> Integer

The number to factorise

-> [(Integer, Int)]

List of prime factors and exponents

A wrapper around curveFactorisation providing a few default arguments. The primality test is bailliePSW, the prng function - naturally - randomR. This function also requires small prime factors to have been stripped before.

curveFactorisation

Arguments

:: Maybe Integer

Lower bound for composite divisors

-> (Integer -> Bool)

A primality test

-> (Integer -> g -> (Integer, g))

A PRNG

-> g

Initial PRNG state

-> Maybe Int

Estimated number of digits of the smallest prime factor

-> Integer

The number to factorise

-> [(Integer, Int)]

List of prime factors and exponents

curveFactorisation is the driver for the factorisation. Its performance (and success) can be influenced by passing appropriate arguments. If you know that n has no prime divisors below b, any divisor found less than b*b must be prime, thus giving Just (b*b) as the first argument allows skipping the comparatively expensive primality test for those. If n is such that all prime divisors must have a specific easy to test for structure, a custom primality test can improve the performance (normally, it will make very little difference, since n has not many divisors, and many curves have to be tried to find one). More influence has the pseudo random generator (a function prng with 6 <= fst (prng k s) <= k-2 and an initial state for the PRNG) used to generate the curves to try. A lucky choice here can make a huge difference. So, if the default takes too long, try another one; or you can improve your chances for a quick result by running several instances in parallel.

curveFactorisation n requires that small (< 100000) prime factors of n have been stripped before. Otherwise it is likely to cycle forever. When in doubt, use defaultStdGenFactorisation.

curveFactorisation is unlikely to succeed if n has more than one (really) large prime factor.

Single curve worker

montgomeryFactorisation :: KnownNat n => Word -> Word -> Mod n -> Maybe Integer

montgomeryFactorisation n b1 b2 s tries to find a factor of n using the curve and point determined by the seed s (6 <= s < n-1), multiplying the point by the least common multiple of all numbers <= b1 and all primes between b1 and b2. The idea is that there's a good chance that the order of the point in the curve over one prime factor divides the multiplier, but the order over another factor doesn't, if b1 and b2 are appropriately chosen. If they are too small, none of the orders will probably divide the multiplier, if they are too large, all probably will, so they should be chosen to fit the expected size of the smallest factor.

It is assumed that n has no small prime factors.

The result is maybe a nontrivial divisor of n.