Problem 234

Project Euler Haskell

234 Semidivisible numbers
Problem 234 - Project Euler
prime sqaure root という，素数の世界でのルートについての問題．
lps, ups が決まると範囲が決まるので，それを利用．

import Number (primes)

p234 :: Integer -> Integer
p234 n = sum.filter ( <= n ).concat.zipWith semiIn (takeWhile (<= u) primes) $ tail primes
    where u = floor.sqrt.fromIntegral $ n

semiIn :: (Num a, Enum a) => a -> a -> [a]
semiIn l u = filter ( /= l * u )  $ semiL ++ semiU
    where semiL = [l * (l+1), l * (l+2)..u * u - 1]
          semiU = reverse [u * (u-1), u * (u-2)..l * l + 1]

main :: IO ()
main = print.p234 $ 999966663333