Well, I got stuck. I wanted to use dynamic programming to compute the value `p 100': ================================ module Main where import Data.Ratio import Data.Ix n = 100 q :: (Integer, Integer) -> Rational q (i,k) | i == 1 && k == 2 = 1 q (i,k) = (map q' (range dom)) !! (dom `index` (i,k)) where dom = ((1,1),(n,n)) q' (i,k) = (p i * ((n + 1 - k) % 1)) / ((n + 1 - i) % 1) p :: Integer -> Rational p k | k == 1 = 1 p k = map p' [1 .. n] !! ((1,n) `index` k) where p' k = sum (map f [1 .. (k - 1)]) where f i = q (i,k) / ((n + 1 - i) % 1) ====================== I used the lazy list pattern for memoizing previous values of `p' and `q' as I saw it elsewhere. Both `p k' and `q (i,k)' depend only on values of `p x' and `q (y,x)' where `x < k' and `y < k'. Nevertheless, it takes forever to compute even `p 20'. What's wrong? (I even tried using a strict container along the lines of `N !Rational' for storing the results of `p' and `q'. It didn't help.) The program seems to work correctly for small values of `k', by the way. I used Rational because I needed the _exact_ result. Thanks and Regards, -- Lajos Nagy Computer Science Ph.D. Student, Florida Institute of Technology