Perfect numbers: Difference between revisions
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or, restricting the search space to improve performance:
<lang haskell>isPerfect :: Int -> Bool
isPerfect n =
let lows = filter ((0 ==) . rem n) [1 .. floor (sqrt (fromIntegral n))]
in 1 < n &&
(lows ++
[ y
[ y | x <- lows, let y = n `quot` x, x /= y ])▼
, x /= y ]) `quot`
2 ==
n
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