Colorless Sight Wiki / ProjectEular/Problem28

ProjectEular

-- Problem 28

module Main (main) where

main :: IO ()
main = print $ problem28

problem28 :: Int
problem28 = calc_iter 1001

calc_iter :: (Integral a) => a -> a
calc_iter n
    | n < 1     = error "must be a positive odd."
    | even n    = error "must be a positive odd."
    | n == 1    = 1
    | otherwise = 4*n^2 - 6*(n-1) + calc_iter (n-2)

{-
     n^2-(n-1)  ...     n^2
         :               :
         :       1       :
         :               :
    n^2-2*(n-1) ... n^2-3*(n-1)
-}

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