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Turnbull China Bikeride
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Text File
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1989-04-01
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2KB
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47 lines
{------------------------------------------------------------------------
Factorizing Gaussian integers using Gofer
------------------------------------------------------------------------}
prime_divisors :: Int -> [Int]
prime_divisors x = f (abs x) where
f (2*n) = 2:f n -- deal with even case
f (3*n) = 3:f n
f (5*n) = 5:f n
f (7*n) = 7:f n
f n | n<2 = []
| otherwise = d:f (n/d) where
d = maybe 11
maybe k | n `mod` k == 0 = k
| k*k > n = n -- n has to be prime
| otherwise = maybe (k+2) -- next odd candidate
sum_of_squares :: Int -> [Gauss Int]
sum_of_squares n = [ (a:+!b) | a<-[1..m], b<-[1..(n-a*a)], n == a*a+b*b]
where m:_ = [ k-1 | k <- [1..], k*k > n ]
irreducible :: Int -> [Gauss Int]
irreducible p | p == 2 = [unit + i, unit - i]
| p `rem` 4 == 3 = let x = p:+!0 in [x,x]
| otherwise = sum_of_squares p
divides :: (Gauss Int) -> (Gauss Int) -> Bool
z `divides` z' = a `rem` d == 0 && b `rem` d == 0 where
a :+! b = z' * (conjugate z)
d = norm z
factors :: (Gauss Int) -> [Gauss Int]
factors z | z == zero = []
| norm z == 1 = [z]
| otherwise = w:factors (z/w) where
w | u `divides` z = u
| otherwise = u'
where
u:u':_ = irreducible p
p:_ = prime_divisors (norm z)
factorise :: Gauss Int -> String
factorise = show . factors
----- End Complex