h11

import Data.List

strings :: Int -> [String]
strings 0 = [""]
strings n = concat (map (\x -> map (\tail -> x:tail) tails) ['a'..'z'])
    where tails = strings (n-1)

--a
palindromes :: [String]
palindromes = filter (\x -> x ==(reverse x)) (palinHelper 0)
        where
                palinHelper n = strings n ++ palinHelper (n+1)

--b
divisors :: Int -> [Int]
divisors x = filter (\y -> mod x y == 0) [1..div x 2]

perfects :: [Int]
perfects = filter (\x -> x==sum(divisors x)) [1..]

--c
semiperfects :: [Int]
semiperfects = filter(\x ->(any(\y -> (sum y)==x))(subsequences (divisors x))) [2..]