 # Discrete Math Problems

## Question

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## Transcribed Text

1. For the following functions determine if they are (i) injective, (ii) surjective and (iii) bijective. Prove your answers. (a) f : R → R defined by f(x) = 17x − 4. (b) f : Z → Z defined by f(x) = 17x − 4. (c) f : R → Z defined by f(x) = bxc. (Recall that bxc is the floor of x, this is the largest integer that is less than or equal to x.) (d) f : Zm → Zm with f([x]) = [x] 2 . (Be careful with this one, it depends on m) (e) f : Z10 → Z10 f([a]) = [a] −1 if [a] is invertible; [a] otherwise. (You can use question 3 for this.) (f) f : Z × Z → Z defined by f(x, y) = 3x + 2y. 2. or each function in Question 1 that is a bijection give the inverse of the function. 3. Let Pn be the set of integers {1, 2, . . . , n}. Let f : Pn→ Pn. (a) Prove that if f is injective then it is also surjective. (b) Prove that if f is surjective then it is also injective. (So this means that if f is a function from a finite set X to a finite set Y where the size of X and Y are them same, then f is injective if and only if it is surjective. 4. Let P be the set of all positive integers. Find a map f : P → P that is injective but not surjective. Find a map g : P → P that is surjective but not injective. (So the result in Question 3 only holds for finite sets!) 5. Prove that the set of even positive integers (E +) has the same cardinality as the set of all positive integers (P). Also prove that the set of odd positive integers (O+) also has the same cardinality. 6. Let A4 = {1, 2, 3, 4} and let S4 = {(x1, x2, x3, x4) | xi = 1 or xi = 0} (set of all binary sequences of length 4). i) Show that if P(A4) is the set of all subsets of A4, then we can find a bijection between A4 and S4. ii) If An = {1, 2, . . . , n} where n ∈ P, define a function from P(A) to Sn that is a bijection.(You do not need to prove it is a bijection!) iii) What is the cardinality of P(An)?

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