MAT1348C Test 4 Discrete Mathematics for Computing
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Discrete Mathematics for Computing
MAT1348C Test 4
Tuesday, March 14, 2017
Q1. In each part, give an example of a non-empty binary relation R on the set A = {1, 2, 3} that has the prescribed properties: (you do not need to justify your answers )
1a) Properties: reflexive, symmetric,
{ |
and antisymmetric: |
} |
1b) Properties: transitive and symmetric,
R = { |
but neither antisymmetric nor reflexive: |
} |
[Q1 out of 4 points]
Q2. Let A = {f1 , f2 , f3 , f4 , f5 , f6 , f7 , f8 , f9 },
where f1 , . . . , f9 are the following functions from R to R:
f1 (x) = x2
f2 (x) = (x _ 1)2
f3 (x) = x2 _ 1
f4 (x) = 2x
f5 (x) = 0
f6 (x) = 1
f7 (x) = 2x _ 1
x + 1
2
2
Let R be the relation on A defined by the following rule:
For all fi , fj e A, /fi , fj、e R if and only if fi (1) = fj (1)
Note. R is an equivalence relation on A but you do not need to prove that in order to answer these questions.
2a) List all elements of A which are related to f1 by the relation R: |
2b) Determine the elements of the equivalence class of f2 with respect to the relation R: |
|
[f2 ┐R = { |
} |
2c) Determine the partition p of A into equivalence classes with respect to R:
{ |
} |
[Q2 out of 6 points]
Q3. Let f and g be functions defined as follows:
f : /Q × Q、→ /Q × Q、
f (q, r) = /q + r, 2q、
g : Q → /Q × Q、
g(x) = /3x, 5 _ x、
For the following questions, no justification is needed; only your final answers will count.
Q3a) Does f · g exist? Circle the correct answer: YES NO If you circled YES, give the expression for f · g and give its domain and codomain: |
Q3b) Does g · f exist? Circle the correct answer: YES NO If you circled YES, give the expression for g · f and give its domain and codomain: |
Note. The function f is invertible; you do not need to prove that for this question. Q3c) Give the expression (formula) for f_1 and give its domain and codomain. |
[Q3 out of 6 points]
MuLrIPLE.CHoIcE QuEsrIoNs.
Indicate the correct responses in the appropriate boxes. No justification is needed.
Q4. Let A and B be finite sets, and let f : A → B and g : B → A be two functions. Which three of the following statements are true in general?
(A) If f_1 exists, then f_1 。f is the identity function on B .
(B) If f_1 exists, then f_1 。f is the identity function on A.
(C) If f_1 exists, then f is a bijection.
(D) If f and g are both bijections, then f 。g = g 。f .
(E) If f and g are both bijections, then f 。g is a bijection.
(F) If f and g are both bijections, then f 。g is the identity function.
True statements of Q4:
and
and
[Q4 out of 3 points]
Q5. Let A and B be finite sets. Which two of the following statements are true in general? (A) The set of all binary relations from A to B is p /A × B、.
(B) The set of all binary relations on the set A is p /A、.
(C) The number of binary relations from A to B is IAI . IBI.
(D) The number of binary relations from A to B is 2|A|. |B| .
(E) If A = Ø, then there are no binary relations from A to B .
True statements of Q5:
and
[Q5 out of 2 points]
Q6. Let X = {1, 2, 3}. Recall that p(X) =,S : S S X}.
Define a relation R on p(X) according to the following rule:
For all A, B e p(X), /A, B、e R if and only if A n B Ø .
Which three of the following statements are true?
(A) R is reflexive.
(B) R is symmetric.
(C) R is antisymmetric.
(D) R is transitive.
(E) /{3}, X、e R.
(F) /X, X、e R.
True statements of Q6:
and
and
[Q6 out of 3 points]
Q7. Let X = {1, 2, 3, 4, 5, 6, 7}.
Which two of the following collections is not a partition of X ?
(A) 91 = {{1, 4, 5}, {2, 3, 6, 7}}
(B) 92 = {{1, 2, 3}, {3, 4}, {4, 5, 6, 7}}
(C) 93 = {{1, 2}, {4, 5, 6, 7}}
(D) 94 = {{1, 2, 3, 4, 5, 6, 7}}
(E) 95 = {1}, {2}, {3}, {4}, {5}, {6}, {7}
The non-partitions of Q7:
and
[Q7 out of 2 points]
LoNc.ANswER QuEsrIoNs.
Detailed solutions are required.
Q8. Let A, B, and C be sets, and let f : A → B and g : B → C be two functions.
For the following statement, determine whether it is true or false.
. If you claim that the statement is true, give a proof.
. If you claim that the statement is false, give a concrete counterexample (i. e. specify the elements of the sets A, B, and C and the functions f : A → B and g : B → C
which demonstrate that the statement can be false).
Statement: If f and g are both surjective, then g · f is surjective.
[Q8 out of 5 points]
Q9. Let R be a binary relation on the set ❩ defined by the following rule:
For all m, n e ❩ , /m, n、e R if and only if m = qn for some q e Q+ Note. Q+ denotes the set of positive rational numbers.
Prove that R is an equivalence relation on ❩ .
[Q9 out of 5 points]
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2023-03-13