[Haskell-cafe] Zumkeller numbers

[Henning Thielemann]

On Tue, 8 Dec 2009, Richard O'Keefe wrote:

> is_Zumkeller :: Int -> Bool
> is_Zumkeller n =
>   let facs = factors n
>       fsum = sum facs
>   in  mod fsum 2 == 0 &&

I see this test is essential. I didn't do it and thus my program did not 
find that 1800 is not a Zumkeller number within an hour. With this check 
my program becomes dramatically faster.

>       can_part facs (div fsum 2)
>
> main = print (filter is_Zumkeller [1..5000])