CSC165H1: Problem Set 1

CSC165H1 Problem Set 1
CSC165H1: Problem Set 1
General instructions
Please read the following instructions carefully before starting the problem set. They contain important
information about general problem set expectations, problem set submission instructions, and reminders
of course policies.
• Your problem sets are graded on both correctness and clarity of communication. Solutions that are
technically correct but poorly written will not receive full marks. Please read over your solutions
carefully before submitting them.
• Each problem set may be completed in groups of up to three. If you are working in a group for this
problem set, please consult https://github.com/MarkUsProject/Markus/wiki/Student_Groups
for a brief explanation of how to create a group on MarkUs.
Exception: Problem Set 0 must be completed individually.
• Solutions must be typeset electronically, and submitted as a PDF with the correct filename. Handwritten submissions will receive a grade of ZERO.
The required filename for this problem set is problem set1.pdf.
• Problem sets must be submitted online through MarkUs. If you haven’t used MarkUs before, give
yourself plenty of time to figure it out, and ask for help if you need it! If you are working with a
partner, you must form a group on MarkUs, and make one submission per group. “I didn’t know
how to use MarkUs” is not a valid excuse for submitting late work.
• Your submitted file should not be larger than 9MB. This may happen if you are using a word
processing software like Microsoft Word; if it does, you should look into PDF compression tools to
make your PDF smaller, although please make sure that your PDF is still legible before submitting!
• Submissions must be made before the due date on MarkUs. You may use grace tokens to extend
the deadline; please see the Problem Set page for details on using grace tokens.
• The work you submit must be that of your group; you may not refer to or copy from the work of
other groups, or external sources like websites or textbooks. You may, however, refer to any text
from the Course Notes (or posted lecture notes), except when explicitly asked not to.
Additional instructions
• Final expressions must have negation symbols (¬) applied only to predicates or propositional variables, e.g. ¬p or ¬P rime(x). To express “a is not equal to b,” you can write a 6= b.
• When rewriting logical formulas into equivalent forms (e.g., simplifying a negated formula or removing implication operators), you must show all of the simplification steps involved, not
just the final result. We are looking for correct use of the various simplification rules here.
• You may not define your own predicates or sets for this problem set; please work with the definitions
provided in the questions.
Page 1/5
CSC165H1, Winter 2018 Problem Set 1
1. [6 marks] Propositional formulas. For each of the following propositional formulas, find the following
two items:
(i) The truth table for the formula. (You don’t need to show your work for calculating the rows of the
table.)
(ii) A logically equivalent formula that only uses the ¬, ∧, and ∨ operators; no ⇒ or ⇔. (You should
show your work in arriving at your final result. Make sure you’re reviewed the “extra instructions”
for this problem set carefully.)
(a) [3 marks] (p ⇒ q) ⇒ ¬q.
(b) [3 marks] (p ⇒ ¬r) ∧ (¬p ⇒ q).
Page 2/5
CSC165H1, Winter 2018 Problem Set 1
2. [8 marks] Fixed Points.
Let f be a function from N to N. A fixed point of f is an element x ∈ N such that f(x) = x. A least
fixed point of f is the smallest number x ∈ N such that f(x) = x. A greatest fixed point of f is the largest
number x ∈ N such that f(x) = x.
(a) [1 mark] Express using the language of predicate logic the English statement:
“f has a fixed point.”
You may use an expression like “f(x) = [something]” in your solution.
(b) [2 marks] Express using the language of predicate logic the English statement:
“f has a least fixed point.”
You may use the predefined function f as well as the predefined predicates = and <. You may not
use any other predefined predicates.
(c) [2 marks] Express using the language of predicate logic the English statement:
“f has a greatest fixed point.”
You may use the predefined function f as well as the predefined predicates = and <. You may not
use any other predefined predicates.
(d) [3 marks] Consider the function f from N to N defined as f(x) = x mod 7.1
Answer the following questions by filling in the blanks.
The fixed points of f are:
The least fixed point of f is:
The greatest fixed point of f is:
1 Here we are using the modulus operator. Given a natural number a and a positive integer b, a mod b is the natural
number less than b that is the remainder when a is divided by b. In Python, the expression a % b may be used to compute
the value of a mod b.
Page 3/5
CSC165H1, Winter 2018 Problem Set 1
3. [6 marks] Partial Orders. A binary predicate R on a set D is called a partial order if the following
three properties hold:
(1) (reflexive) ∀d ∈ D, R(d, d)
(2) (transitive) ∀d, d0
, d00 ∈ D, (R(d, d0
) ∧ R(d
0
, d00)) ⇒ R(d, d00)
(3) (anti-symmetric) ∀d, d0 ∈ D, (R(d, d0
) ∧ R(d
0
, d)) ⇒ d = d
0
A binary predicate R on a set D is called a total order if it is a partial order and in addition the following
property holds: ∀d, d0 ∈ D, R(d, d0
) ∨ R(d
0
, d).
For example, here is a binary predicate R on the set {a, b, c, d} that is a total order:
R(a, b) = R(a, c) = R(a, d) = R(b, c) = R(b, d) = R(c, d) = R(a, a) = R(b, b) = R(c, c) = R(d, d) = True
and all other values are False.
(a) [2 marks] Give an example of a binary predicate R on the set N that is a partial order but that is
not a total order.
(b) [2 marks] Let R be a partial order predicate on a set D. R specifies an ordering between elements
in D. Whenever R(d, d0
) is True, we will say that d is less than or equal to d
0
, or that d
0
is greater
than or equal to d. The following formula in predicate logic expresses that there exists a greatest
element in D; that is, an element in D that is greater than or equal to every other element in D:
∃d ∈ D, ∀d
0 ∈ D, R(d
0
, d)
An element in D is said to be maximal if no other element in D is larger than this element. The
following formula in predicate logic expresses that there exists a maximal element in D:
∃d ∈ D, ∀d
0 ∈ D, d = d
0 ∨ ¬R(d, d0
)
Give an example of a partial order order R over {a, b, c, d} such that every element is maximal.
(c) [2 marks] Give an example of a partial order R over {a, b, c, d} such that a ∈ D is maximal but a
is not a greatest element. Justify your answer briefly.
Page 4/5
CSC165H1, Winter 2018 Problem Set 1
4. [13 marks] One-to-one functions. So far, most of our predicates have had sets of numbers as their
domains. But this is not always the case: we can define properties of any kind of object we want to study,
including functions themselves!
Let S and T be sets. We say that a function f : S → T is one-to-one if no two distinct inputs
are mapped to the same output by f. For example, if S = T = Z, the function f1(x) = x + 1 is
one-to-one, since every input x gets mapped to a distinct output. However, the function f2(x) = x
2
is not one-to-one, since f2(1) = f2(−1) = 1. Formally we express “f : S → T is one-to-one” as:
∀x1 ∈ S, ∀x2 ∈ S, f(x1) = f(x2) ⇒ x1 = x2.
We say that f : S → T is onto if every element in T gets mapped to by at least one element in S. The
above function f(x) = x + 1 is onto over Z but is not onto over N. Formally we express “f : S → T is
onto” as:
∀y ∈ T, ∃x ∈ S, f(x) = y
Let t ∈ T. We say that f outputs t if there exists s ∈ S such that f(s) = t.
(a) [1 mark] How many functions are there from {1, 2, 3} to {a, b, c, d}?
(b) [1 mark] How many one-to-one functions are there from {1, 2, 3} to {a, b, c, d}?
(c) [1 mark] How many onto functions are there from {1, 2, 3, 4} to {a, b, c}?
(d) [2 marks] Now let R be a binary predicate with domain N×N. We say that R represents a function
if, for every x ∈ N, there exists a unique y ∈ N, such that R(x, y) (is True). In this case, we write
expressions like y = f(x).
Define a predicate F unction(R), where R is a binary predicate with domain N × N, that expresses
the English statement:
“R represents a function.”
You may use the predicates <, ≤, =, R, but may not use any other predicate or function symbols.
In parts (e)-(h) below, you may use the predicate F unction(R) (in addition to the predicates <, ≤
, =, R) in your solution, but may not use any other predicate or function symbols.
(e) [2 marks] Define a predicate that expresses the following English statement.
“R represents an onto function.”
(f) [2 marks] Define a predicate that expresses the following English statement.
“R represents a one-to-one function.”
(g) [2 marks] Define a predicate that expresses the following English statement.
“R represents a function that outputs infinitely many elements of N.”
(h) [2 marks] Now define a predicate that expresses the following English statement.
“R represents a function that outputs all but finitely many elements of N.”
Page 5/5