1. functions: f : S → T, is a binary relation f ⊆ S ×T such that for all s ∈ S there is exactly one t ∈ T such that (s,t) ∈ f
unique element related to s时写作f(s)
partial function: f : S ！→ T is a binary relation f ⊆ S ×T such that for all s ∈ S there is at most one t ∈ T such that (s,t) ∈ f . That is, it is a function f : S‘→ T for S’ ⊆ S
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3. Functions on finite sets：For a finite set S and f : S → S the properties surjective and injective are equivalent
4. Inverse function
5. Properties of the inverse
6. basic matrix operations
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第一个的第一行与第二个的第一列依次相乘并求出加和，A*B不等于B*A
8. Big O除了9024课上的定义还有一种为
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11. 练习
