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1. Cardinality : the size of the set, denoted by |A|
2. inclusion-exclusion principle：|A∪B|=|A|+|B|−|A∩B|.
3. countable v.s uncountable
4. Any subset of a countable set is countable.
Any superset of an uncountable set is uncountable.
5. If A and B are countable, then A×B is also countable.
6. Conclusion
Any set C in the form of
,where indices i and j belong to some countable sets, is countable.
這也是為何Q也屬於countable
因為
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