Example:
Is the set {1, 1/2, 1/3, ...} perfect ? How about the set
{1, 1/2, 1/3, ...} 
{0} ?
- The first set is not closed. Hence it is not perfect.
- The second set is closed, and {0} is an accumulation point. However, every point
different from 0 is isolated, and can therefore not be an accumulation point. Therefore, this
set is not perfect either.
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