Let A, B be sets, let P, Q ⊆ A be subsets of A, and let f: A → B be an injective function. Prove that f(P) − f(Q) = f(P − Q).

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Problem 1 : Let A, B be sets, let P, Q ⊆ A be subsets of A, and let f: A → B be an injective function.

Prove that f(P) − f(Q) = f(P − Q).

Using the definition of injectivity : a function that maps distinct elements of it's domain to distinct elements that f(x1) = f(x2) implies x1 = x2.

Problem 12 Let f: A → B be a function.

• Prove that f is injective if and only if E = f −1 (f(E)) for all subsets E ⊆ A.

• Prove that f is surjective if and only if F = f(f −1 (F)) for all subsets F ⊆ B.

 

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