Primality of numbers of the form 2n^2-1 (Euler, 216)

-- ### Project Euler, problem 216 ### --
isPrime n = all (/= 0) . map (n `mod`) $ [2..squareRoot n]
  where
    squareRoot = truncate . sqrt . fromIntegral

list = map (subtract 1 . (*2) . (^2)) [2..]  -- 2n^2-1

main = do
  putStrLn . show $ solveit 10000     -- control-case
  putStrLn . show $ solveit 50000000  -- problem
    where
      solveit n = length . filter isPrime $ take (n-1) list