-- Let d(n) be defined as the sum of proper divisors of n (numbers less than n which divide evenly into n). -- If d(a) = b and d(b) = a, where a ≠ b, then a and b are an amicable pair and each of a and b are called amicable numbers. -- -- For example, the proper divisors of 220 are 1, 2, 4, 5, 10, 11, 20, 22, 44, 55 and 110; therefore d(220) = 284. -- The proper divisors of 284 are 1, 2, 4, 71 and 142; so d(284) = 220. -- -- Evaluate the sum of all the amicable numbers under 10000. import ProjectEuler (sumProperDivisors) properDivisors :: (Integral a) => a -> [a] properDivisors n = [ x | x <- [1..n-1], n `mod` x == 0] amicable :: (Integral a) => a -> a -> Bool amicable x y = x /= y && sumProperDivisors x == y && sumProperDivisors y == x sumAmicable :: (Integral a) => a -> a sumAmicable n = sum [ x | x <- [1..n-1], amicable x $ sumProperDivisors x ] main = do let result = sumAmicable 10000 putStrLn $ "Project Euler, Problem 21\n" ++ "Answer: " ++ show result