Giuga numbers: Difference between revisions
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The first 5 Giuga numbers are:
[30 858 1722 66198 2214408306]
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=={{header|Haskell}}==
<syntaxhighlight>
--for obvious theoretical reasons the smallest divisor of a number bare 1
--must be prime
primeFactors :: Int -> [Int]
primeFactors n = snd $ until ( (== 1) . fst ) step (n , [] )
where
step :: (Int , [Int] ) -> (Int , [Int] )
step (n , li) = ( div n h , li ++ [h] )
where
h :: Int
h = head $ tail $ divisors n --leave out 1
divisors :: Int -> [Int]
divisors n = [d | d <- [1 .. n] , mod n d == 0]
isGiuga :: Int -> Bool
isGiuga n = (divisors n /= [1,n]) && all (\i -> mod ( div n i - 1 ) i == 0 )
(primeFactors n)
solution :: [Int]
solution = take 4 $ filter isGiuga [2..]</syntaxhighlight>
{{out}}
<pre>
[30,858,1722,66198]
</pre>
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