That is the definition of a closed set.
An empty set is considered a finite set because it contains zero (0) elements and zero is a finite number.
there are 5 diffeerent sets Natural Numbers whole numbers integers rational numbers irrational numbers.
Empty set or null set
The collection of all sets minus the empty set is not a set (it is too big to be a set) but instead a proper class. See Russell's paradox for why it would be problematic to consider this a set....
The set contained in another set is termed as a sub-set.
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