square :: Int -> Int
square x = x * x

sumUpList :: (Num a) => [a] -> a  
sumUpList [] = 0  
sumUpList (x:xs) = x + sumUpList xs

isPrime :: Integer -> Bool
isPrime k = null [ x | x <- [2..k `div` 2], k `mod` x  == 0]

isOdd :: Int -> Int
isOdd x = x `mod` 2

powerOf :: Int -> Int -> Int
powerOf x y  = x^y

interesting :: (Ord a) => [a] -> [a]  
interesting [] = []
interesting (x:xs) = (interesting l) ++ [x] ++ (interesting g)
    where
        l = filter (< x) xs
        g = filter (>= x) xs