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MATH7861 ASSIGNMENT
Marking. Each question is worth 10 marks. Total 80 marks.
Submission. Please submit online via blackboard. You must submit ONE PDF file with your
NAME, STUDENT NUMBER and TUTORIAL GROUP NUMBER clearly at the top of
the first page. Make sure that your pdf file is legible and that the file size is not excessive. If you take
photos with your phone, the file size will be very large, and you must compress the size.1
Help. You can get help for doing the questions but solutions must be written in your own words.
Copying from any source (including AI) constitutes cheating and will be detrimental to your studies
at UQ.
Problems.
(1) (10 marks) Define f : {1, 2} × Z→ Z by f(a, b) := 2b+ a.
Determine the following (make sure to justify your answers):
(a) (2 marks) Domain of f .
(b) (2 marks) Range of f .
(c) (2 marks) Is f one to one?
(d) (2 marks) Is f onto?
(e) (2 marks) Is f bijective?
(2) (10 marks) Prove that for every positive integer n, and sets A and B1, B2, . . . , Bn
A−
n⋂
i=1
Bi =
n⋃
i=1
(A−Bi).
(3) (10 marks) Let X be a set and (Y,≤) a partially ordered set. Let F denote the set of functions
from X to Y . Define a relation ≤ on F by setting f ≤ g if f(x) ≤ g(x) for all x ∈ X. Show
that ≤ is a partial ordering on F .
(4) (10 marks) Suppose X, Y are two disjoint countable sets. Prove that X ∪ Y is also countable.
1See, for instance, https://pdfcompressor.com for compressing tools.
1
(5) (10 marks) Let f be a function from the set A to the set B. Let S and T be subsets of A.
(a) (2 marks) Prove that f(S ∪ T ) = f(S) ∪ f(T ).
(b) (2 marks) Prove that f(S ∩ T ) ⊆ f(S) ∩ f(T ).
(c) (3 marks) Give an example of sets A, B, S, T and a function f for which f(S ∩ T ) ⊂
f(S) ∩ f(T ).
(d) (3 marks) Prove that if f is one-one, then f(S ∩ T ) = f(S) ∩ f(T ).
(6) (10 marks) Let S be the set of all binary strings of length 6. Consider the relation ρ on the
set S in which for all a, b ∈ S, (a, b) ∈ ρ if and only if the length of a longest substring of
consecutive ones in a is the same as the length of a longest substring of consecutive ones in b.
(a) (2 marks) Is 011010 related to 000011? Explain why or why not.
(b) (6 marks) Prove that ρ is an equivalence relation.
(c) (2 marks) List the elements of the equivalence class [111100].
(7) (10 marks) Find an explicit bijection (i.e. give a formula that explains what f(x) is given any
x ∈ [0, 1]) between [0, 1] and [0, 1).
(8) (10 marks) Define a relation R on Z by xRy if x2 − y2 is divisible by 4. Is R an equivalence
relation? Justify your answer. If it is an equivalence relation, determine the equivalence classes.