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])
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