MATH7861 Discrete Mathematics Semester One Examinations, 2022
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Semester One Examinations, 2022
MATH7861 Discrete Mathematics
1. The logical binary operation NAND, denoted |, is defined by
p | q = ~ (p ^ q).
(a) (2 marks) Write down the truth table for ~ (p | q). Show all working.
(b) (3 marks) Prove, using truth tables or otherwise, that (p | p) = (~ p). Show all working.
(c) (3 marks) Prove, using truth tables or otherwise, that ((p | p) | (q | q)) = p ^ q . Show all working.
2. (a) Consider the statement
There exists a real number x such that for all real numbers y , xy = 1.
(i) (2 marks) Write the negation of this statement.
(ii) (2 marks) State which of the original statement or its negation is true. Justify your answer, showing all working.
(b) (4 marks) Consider the statement
For all integers a and b, if (ab)2 is odd then a is odd and b is odd.
Use either a direct proof, a proof by contraposition or a proof by contradiction to prove this statement is true. Show all working.
3. (a) (2 marks) Let A = {0, 1, 2, 3, 4}, B = {3, 4, 5, 6} and C = {1, 2, 4, 5, 7} and let
R = ((A - B) ∩ (A - C)) n (B ∩ C ∩ A).
List the elements of the set R.
(b) (2 marks) Let X = p(0) and Y = p(p(0)). List the elements of each of the following sets.
(i) X
(ii) Y
(iii) X ∩ Y
(iv) X n Y
4. (5 marks) Prove that |Z+ x Z+ | = |Z+ |.
5. (a) (4 marks) Let a be an integer such that the remainder is 1 when a is divided by 4. Prove that when a4 is divided by 16 the remainder is also 1.
(b) (4 marks) Use Euclid’s Algorithm to verify that gcd(2000, 1713) = 1. Clearly show all steps in the execution of the algorithm.
6. (8 marks) Use mathematical induction to prove that the following statement is true.
For all integers n > 0, 3n < (n + 3)!
7. Consider the function f : Q ∩ [1, o) - Q ∩ [2, o) given by
f (x) = (x - 1)2 + 2 for all x e Q ∩ [1, o).
(a) (4 marks) Prove that the function f is injective.
(b) (3 marks) Prove that the function f is not surjective.
8. Consider the relation ρ on Z given by
aρb if and only if (a - b)(a + b) = 4k for some k e Z.
(a) (6 marks) Prove that ρ is an equivalence relation.
(b) (2 marks) Determine the number of equivalence classes of ρ . Justify your answer.
9. (a) (1 marks) Determine whether the groups (Z, +) and (R - {0}, .) are isomorphic. Justify your answer.
(b) (3 marks) Determine whether the groups (Z4 , +), (Z5 - {[0]}, .) are isomorphic. Justify your answer.
(c) (4 marks) Find a subset S of Z10 such that (S, .) is a group of order 4 (where . denotes multiplication modulo 10). Justify your answer.
10. For this question, you may leave your answers as expressions involving factorials.
(a) (2 marks) How many distinct arrangements can be made of the letters of the word WOOLLOONGABBA?
(b) (3 marks) If the letters of the word WOOLLOONGABBA are permuted at random, what is the probability that we get the same word back?
(c) (3 marks) How many distinct arrangements are there of the letters of the word WOOLLOONGABBA such that exactly one of the following is true?
● The two As are not next to each other.
● The two Bs are not next to each other.
● The two Ls are not next to each other.
11. (a) (4 marks) Let T be a tree on p vertices where p > 11, T has precisely six vertices of degree
1 and precisely four vertices of degree 3. Determine the degree of each of the remaining p - 10 vertices of T.
(b) (4 marks) Let d be a positive integer and let T be a tree such that T has a vertex of degree d. Prove that T has at least d vertices of degree 1.
2022-11-06