-- What is the smallest odd composite that cannot be written as the sum of a
-- prime and twice a square?

import Euler

composites = [1..] \\\ primes
isTwiceSquare n = 2 * ((floor $ sqrt $ (/ 2) $ fromIntegral n) ^ 2) == n

main = print $ head $ do
   n <- tail $ filter odd composites
   n <$ (guard $ not $ any isTwiceSquare $ map (n-) $ takeWhile (< n) primes)