Colorless Sight Wiki / ProjectEular/Problem21

ProjectEular

-- Problem 21

module Main (main) where

import qualified List   as L (nub)
import qualified Common as C (proper_divisors)

main :: IO ()
main = print $ problem21

problem21 :: Int
    problem21 = sum $ map sum_pair $ amicable_pairs 9999
    where sum_pair (x,y) = x + y

amicable_pairs :: Int -> [(Int,Int)]
amicable_pairs max_n =
    [ (n, d n) | n <- [2..max_n], n < d n, n == d (d n) ]

d :: Int -> Int
d n = sum $ C.proper_divisors n

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