Discrete Math - Problem Set 1 - Summer I 2022
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Discrete Math - Problem Set 1 - Summer I 2022
Exercise I
For the following statements A and B, say if A =⇒ B , B =⇒ A, A ⇐⇒ B, or neither. Explain quickly why. No need to be very formal, and you can use anything that you know.
1. A: x ≥ 0, B: x is the square of a real number (here, x is a real number).
2. A: p is prime, B: p is divisible by 2 (here p is an integer).
3. A: p is not prime, B: p is divisible by 2 (here p is an integer).
4. A: a2 + b2 = 10000, B: a and b have the same parity (here, a and b are integers).
5. A: a and b are both prime, B: a + b is prime (here, a and b are integers).
Exercise II
1. Define the terms “perfect square” and “consecutive perfect squares” . Then, show that the difference between consecutive perfect squares is odd.
2. Consider the following definition of the “◁” symbol.
Let x and y be integers. Write x ◁ y if 3x + 5y = 7k for some integer k.
(a) Show that 1 ◁ 5, 3 ◁ 1, and 0 ◁ 7.
(b) Prove that if a ◁ b and c ◁ d, then (a + c) ◁ (b + d).
Exercise III
1. Show that an integer n is odd if and only if 2n + 2 is divisible by 4.
2. Consider a,b,d ∈ Z. Show that a and b are divisible by d if and only if au + bv is divisible by d for any u,v ∈ Z.
Exercise IV
For each statement below, explain why it is false. If you give a counterexample, make sure your explanation is complete.
1. For any n ∈ N, 22n + 1 is prime.
2. For all n ∈ N, n2 + n + 41 is prime.
3. The expression ((¬p ∧ q) ∨ ((¬r ∧ p) ∧ ¬p)) is a contradiction.
4. Let a,b ∈ Z. If a|b and b|a, then a = b.
5. Let n ∈ N. If n is not prime, then it has a divisor d with 1 < d < n.
Exercise V
1. Simplify the following expressions using computational rules.
(a) (¬x ∨ ¬y) → (x ∧ y)
(b) (((x ∧ y) ∨ z) ∧ (¬x)) ∨ (¬z)
2. Here is another Boolean operation called exclusive or; it is denoted by the symbol ∨ . It is defined in the following table.
x |
y |
x |
∨y |
T T F F |
T F T F |
F T T F |
(a) Prove that x ∨y is logically equivalent to (x ∧ ¬y) ∨ ((¬x) ∧ y).
(b) Prove that x ∨y is logically equivalent to (x ∨ y) ∧ (¬(x ∧ y)).
(c) Explain why the operation ∨ is called exclusive or.
2022-06-05