Hi, In exercise 2 of http://www.walenz.org/Dijkstra/page0145.html we need to write a function that holds (1) The value 1 is in the sequence (2) If x and y are in the sequence, so is f(x,y), where f has the properties a. f(x,y) > x b. (y1 > y2) => (f(x,y1)>f(x,y2)) This is a solution for this problem, but an inefficient one hammingff :: [Integer] hammingff = 1 : merge [ h x y | x <- hammingff, y <- hammingff ] [ h x y | y <- hammingff, x <- hammingff ] h x y = 2*x+3*y merge :: (Ord a) => [a] -> [a] -> [a] merge [] xs = xs merge ys [] = ys merge (x:xs) (y:ys) = case compare x y of LT -> x : merge xs (y:ys) GT -> y : merge (x:xs) ys EQ -> x : merge xs ys anybody has a better solution?. Thanks Jose