 ?
--
isDeficient :: Integral a => a -> Bool
isDeficient n = n + n > sumOfDivisors n

--
-- 友愛数のもう片方を探す
--
-- ex : findAmicableNumber 220 => Jus 284
-- ex : findAmicableNumber 221 => Nothing
sumOfDivisors :: Integral a => a -> a
sumOfDivisors n = product [f p i | (p, i) <- factorize n]
    where f p i = div (p ^ (i + 1) - 1) (p - 1)

--
-- 完全数か ?
--
sumOfDivisors n = product [f p i | (p, i) <- factorize n]
    where f p i = div (p ^ (i + 1) - 1) (p - 1)

--
-- 完全数か ?
--