Definition of mapping: If each element in set A has a unique element corresponding to it in set B under the action of correspondence law F, then sets A, B and correspondence law F are called mappings from sets A to B.

To verify whether it is a mapping, we only need to pay attention to several important words in the mapping definition, 1,' every element in a', 2,' only one element in set b', and it is easy to verify whether it is a mapping by paying attention to these two words. Simply put, you need to find every element in A from A, but you can only find one in B.

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