foo (x:xs) | x `mod` 3 == 0 = x + foo xs foo (_:xs) = foo xs foo [] = 0(2)
bar n = [ (x, y) | x <- [1..n], y <- [(x * x) .. (2 * x * x)], odd (x + y)]Ⅱ
(1) [1,3,9,27,81]
(2) [0,1,4,11,26]
(3) [5,6,10,11,15,16,20,21]
(4) [(2,3),(3,6),(4,5),(4,9),(4,13)]