问题1.假设A是特定时间某个镇上所有人类的集合。确定以下每个关系是否是自反的,对称的和可传递的:
(i)R = {(x。y)x和y在同一位置工作}
(ii)R = {(x。y)x和y居住在同一地点}
(iii)R = {(x。y)x是y的妻子}
(iv)R = {(x。y)x是y的父亲}
解决方案:
(i) Given the relation R = {(x, y): x and y work at the same place}
Now we need to check whether the relation is reflexive or not.
Check whether the relation is reflexive or not. A relation ‘R’ on a set ‘A’ is said to be reflexive if (x R x) ∀ x ∈ A i.e. (x, x) ∈ R ∀ x ∈ A.
Let x be any element of R.
Then, x ∈ R
⇒ x and x work at the same place is true since they are same.
⇒ (x, x) ∈ R [condition for reflexive relation]
So, R is a reflexive relation.
Now we have to check whether the relation is Symmetric relation or not. A relation R on set A is symmetric if (a, b)∈ R and (b, a)∈ R for all (a, b) ∈ A.
Let (x, y) ∈ R
⇒ x and y work at the same place [since it is given in the question]
⇒ y and x work at the same place [same as “x and y work at the same place”]
⇒ (y, x) ∈ R
So, R is a symmetric relation also.
Now we have to check whether the given relation is Transitive relation or not. Relation R is said to be Transitive over set A if (x, y) ∈ R and (y, z) ∈ R then (x, z) ∈ R ∀ x, y, z ∈ A.
Let (x, y) ∈ R and (y, z) ∈ R.
Then, x and y work at the same place. [Given]
y and z also work at the same place. [(y, z) ∈ R]
⇒ x, y and z all work at the same place.
⇒ x and z work at the same place.
⇒ (x, z) ∈ R
Therefore, R is a transitive relation also.
So the relation R = {(x, y): x and y work at the same place} is a reflexive relation, symmetric relation and transitive relation as well.
(ii) Given the relation R = {(x, y): x and y live in the same locality}
Now we have to check whether the relation R is reflexive, symmetric and transitive or not.
Check whether the relation is reflexive or not. A relation ‘R’ on a set ‘A’ is said to be reflexive if (x R x) ∀ x ∈ A i.e. (x, x) ∈ R ∀ x ∈ A.
Let x be any element of relation R.
Then, x ∈ R
It is given that x and x live in the same locality is true since they are the same.
So, R is a reflexive relation.
Now we have to check whether the relation is Symmetric relation or not. A relation R on set A is symmetric if (a, b)∈ R and (b, a)∈ R for all (a, b) ∈ A.
Let (x, y) ∈ R
⇒ x and y live in the same locality [ it is given in the question ]
⇒ y and x live in the same locality [if x and y live in the same locality, then y and x also live in the same locality]
⇒ (y, x) ∈ R
So, R is a symmetric relation as well.
Now we have to check whether the given relation is Transitive relation or not. Relation R is said to be Transitive over set A if (x, y) ∈ R and (y, z) ∈ R then (x, z) ∈ R ∀ x, y, z ∈ A.
Let x, y and z be any elements of R and (x, y) ∈ R and (y, z) ∈ R.
Then,
x and y live in the same locality and y and z live in the same locality
⇒ x, y and z all live in the same locality
⇒ x and z live in the same locality
⇒ (x, z) ∈ R
So, R is a transitive relation also.
So the relation R = {(x, y): x and y live in the same locality} is a reflexive relation, symmetric relation and transitive relation as well.
(iii) Given R = {(x, y): x is wife of y}
Now we have to check whether the relation R is reflexive, symmetric and transitive relation or not.
Check whether the relation is reflexive or not. A relation ‘R’ on a set ‘A’ is said to be reflexive if (x R x) ∀ x ∈ A i.e. (x, x) ∈ R ∀ x ∈ A.
Let x be an element of R.
Then, x is wife of x cannot be true. [since the same person cannot be the wife of herself]
⇒ (x, x) ∉ R
So, R is not a reflexive relation.
Check whether the relation is Symmetric relation or not. A relation R on set A is symmetric if (a, b)∈ R and (b, a)∈ R for all (a, b) ∈ A.
Let (x, y) ∈ R
⇒ x is wife of y
⇒ x is female and y is male
⇒ y cannot be wife of x as y is husband of x
⇒ (y, x) ∉ R
So, R is not a symmetric relation.
Check whether the given relation is Transitive relation or not. Relation ‘R’ is said to be Transitive over set ‘A’ if (x, y) ∈ R and (y, z) ∈ R then (x, z) ∈ R ∀ x, y, z ∈ A.
Let (x, y) ∈ R, but (y, z) ∉ R
Since x is wife of y, but y cannot be the wife of z, since y is husband of x.
⇒ x is not the wife of z.
⇒(x, z)∈ R
So, R is a transitive relation.
Hence the given relation R = {(x, y): x is wife of y} is a transitive relation but not a reflexive and symmetric relation.
(iv) Given the relation R = {(x, y): x is father of y}
Now we have to check whether the relation R is reflexive, symmetric and transitive or not.
First let us check whether the relation is reflexive or not. A relation ‘R’ on a set ‘A’ is said to be reflexive if (x R x) ∀ x ∈ A i.e. (x, x) ∈ R ∀ x ∈ A.
Let x be an arbitrary element of R.
Then, x is father of x cannot be true. [since no one can be father of himself]
So, R is not a reflexive relation.
Check whether the relation is Symmetric relation or not. A relation R on set A is symmetric if (a, b)∈ R and (b, a)∈ R for all (a, b)∈ A.
Let (x, y)∈ R
⇒ x is the father of y.
⇒ y is son/daughter of x.
⇒ (y, x) ∉ R
So, R is not a symmetric relation.
Now, check whether the given relation is Transitive relation or not. Relation ‘R’ is said to be Transitive over set ‘A’ if (x, y) ∈ R and (y, z) ∈ R then (x, z) ∈ R ∀ x, y, z ∈ A.
Let (x, y)∈ R and (y, z)∈ R.
Then, x is father of y and y is father of z
⇒ x is grandfather of z
⇒ (x, z)∉ R
So, R is not a transitive relation.
Hence, the given relation R = {(x, y): x is father of y} is not a reflexive relation, not a symmetric relation and not a transitive relation as well.
问题2.在集合A = {a,b,c}上定义了三个关系R1,R2和R3,如下所示:
R1 = {((a,a),(a,b),(a,c),(b,b),(b,c),(c,a),(c,b),(c,c) }
R2 = {(a,a)}
R3 = {(b,c)}
R4 = {(a,b),(b,c),(c,a)}。
找出A上的每个关系R1,R2,R3,R4是否(i)自反(ii)对称和(iii)可传递。
解决方案:
i) Considering the relation R1, we have
R1 = {(a, a), (a, b), (a, c), (b, b), (b, c), (c, a), (c, b), (c, c)}
Now we have check R1 is reflexive, symmetric and transitive or not.
First let us check whether the relation is reflexive or not. A relation ‘R’ on a set ‘A’ is said to be reflexive if (x R x) ∀ x ∈ A i.e. (x, x) ∈ R ∀ x ∈ A.
Given (a, a), (b, b) and (c, c) ∈ R1 [since each element maps to itself]
So, R1 is reflexive.
Check whether the relation is Symmetric relation or not. A relation R on set A is symmetric if (a, b)∈ R and (b, a)∈ R for all (a, b)∈ A.
We see that for every ordered pair (x, y), there is a pair (y, x) present in the relation R1.
So, R1 is symmetric.
Transitive: A relation ‘R’ is said to be Transitive over set ‘A’ if (x, y) ∈ R and (y, z) ∈ R then (x, z) ∈ R ∀ x, y, z ∈ A.
In the relation, (a, b) ∈ R1, (b, c) ∈ R1 and also (a, c) ∈ R1
So, R1 is transitive.
Therefore, R1 is reflexive relation, symmetric relation and transitive relation as well.
(ii) Considering the relation R2, we have
R2 = {(a, a)}
First let us check whether the relation is reflexive or not. A relation ‘R’ on a set ‘A’ is said to be reflexive if (x R x) ∀ x ∈ A i.e. (x, x) ∈ R ∀ x ∈ A.
We can see that (a, a) ∈ R2. [since each element maps to itself]
So, R2 is a reflexive relation.
Check whether the relation is Symmetric relation or not. A relation R on set A is symmetric if (a, b)∈ R and (b, a)∈ R for all (a, b)∈ A.
We can see that (a, a) ∈ R
⇒ (a, a) ∈ R.
So, R2 is symmetric.
Check whether the relation is Transitive or not. A relation ‘R’ is said to be Transitive over set ‘A’ if (x, y) ∈ R and (y, z) ∈ R then (x, z) ∈ R ∀ x, y, z ∈ A.
R2 is clearly a transitive relation. [since there is only one element in it]
Therefore, R2 is reflexive relation, symmetric relation and transitive relation as well.
(iii) Considering the relation R3, we have
R3 = {(b, c)}
First let us check whether the relation is reflexive or not. A relation ‘R’ on a set ‘A’ is said to be reflexive if (x R x) ∀ x ∈ A i.e. (x, x) ∈ R ∀ x ∈ A.
In the relation, (a, a)∉ R3, (b, b)∉ R3 neither (c, c) ∉ R3.
So, R3 is not reflexive. [since all pairs of type (x, x) should be present in the relation]
Check whether the relation is Symmetric relation or not. A relation R on set A is symmetric if (a, b)∈ R and (b, a)∈ R for all (a, b)∈ A.
In the relation, (b, c) ∈ R3, but (c, b) ∉ R3
So, R3 is not symmetric.
Check whether the relation is Transitive or not. A relation ‘R’ is said to be Transitive over set ‘A’ if (x, y) ∈ R and (y, z) ∈ R then (x, z) ∈ R ∀ x, y, z ∈ A.
R3 has only two elements.
Hence, R3 is transitive.
Therefore, R2 is transitive relation but not a reflexive relation and not a symmetric relation also.
(iv) Considering the relation R4, we have
R4 = {(a, b), (b, c), (c, a)}
First let us check whether the relation is reflexive or not. A relation ‘R’ on a set ‘A’ is said to be reflexive if (x R x) ∀ x ∈ A i.e. (x, x) ∈ R ∀ x ∈ A.
In the relation, (a, a) ∉ R4, (b, b) ∉ R4 (c, c) ∉ R4
So, R4 is not a reflexive relation.
Check whether the relation is Symmetric relation or not. A relation R on set A is symmetric if (a, b)∈ R and (b, a)∈ R for all (a, b)∈ A.
Here, (a, b) ∈ R4, but (b, a) ∉ R4.
So, R4 is not symmetric
Check whether the relation is Transitive or not. A relation ‘R’ is said to be Transitive over set ‘A’ if (x, y) ∈ R and (y, z) ∈ R then (x, z) ∈ R ∀ x, y, z ∈ A.
In the relation, (a, b) ∈ R4, (b, c) ∈ R4, but (a, c) ∉ R4
So, R4 is not a transitive relation.
Therefore, R2 is not a reflexive relation, not a symmetric relation and neither a transitive relation as well.
问题3.测试以下关系R 1 ,R 2和R 3是否为(i)自反的(ii)对称的和(iii)传递的:
(i)对由(A,B)∈R 1⇔一个= 1 / B所定义Q0的R 1。
(ⅱ)R 2上通过(A,B)∈R 2定义ž⇔|一个- B | ≤5
(ⅲ)在R R 3所定义的(a,b)中∈R 3⇔A2 – 4AB + 3B2 = 0。
解决方案:
i) Given R1 on Q0 defined as (a, b) ∈ R1 ⇔ a = 1/b.
First let us check whether the relation is reflexive or not. A relation ‘R’ on a set ‘A’ is said to be reflexive if (x R x) ∀ x ∈ A i.e. (x, x) ∈ R ∀ x ∈ A.
Let a be an element of R1.
Then, a ∈ R1
⇒ a ≠1/a ∀ a ∈ Q0
So, R1 is not reflexive.
Now check whether the relation is Symmetric relation or not. A relation R on set A is symmetric if (a, b)∈ R and (b, a)∈ R for all (a, b)∈ A.
Let (a, b) ∈ R1
Then, (a, b) ∈ R1
Therefore, we can write ‘a’ as a =1/b
⇒ b = 1/a
⇒ (b, a) ∈ R1
So, R1 is symmetric.
Now check whether the relation is Transitive or not. A relation ‘R’ is said to be Transitive over set ‘A’ if (x, y) ∈ R and (y, z) ∈ R then (x, z) ∈ R ∀ x, y, z ∈ A.
Here, (a, b) ∈ R1 and (b, c) ∈ R2
⇒ a = 1/b and b = 1/c
⇒ a = 1/ (1/c) = c
⇒ a ≠ 1/c
⇒ (a, c) ∉ R1
So, R1 is not a transitive relation.
(ii) Given R2 on Z defined as (a, b) ∈ R2 ⇔ |a – b| ≤ 5
Now we have to check whether R2 is reflexive, symmetric and transitive or not.
First let us check whether the relation is reflexive or not. A relation ‘R’ on a set ‘A’ is said to be reflexive if (x R x) ∀ x ∈ A i.e. (x, x) ∈ R ∀ x ∈ A.
Let a be an element of R2.
Then, a ∈ R2
On applying the given condition we will get,
⇒ | a−a | = 0 ≤ 5
So, R1 is a reflexive relation.
Now check whether the relation is Symmetric relation or not. A relation R on set A is symmetric if (a, b)∈ R and (b, a)∈ R for all (a, b)∈ A.
Let (a, b) ∈ R2
⇒ |a−b| ≤ 5 [Since, |a−b| = |b−a|]
⇒ |b−a| ≤ 5
⇒ (b, a) ∈ R2
So, R2 is a symmetric relation.
Now check whether the relation is Transitive or not. A relation ‘R’ is said to be Transitive over set ‘A’ if (x, y) ∈ R and (y, z) ∈ R then (x, z) ∈ R ∀ x, y, z ∈ A.
Let (1, 3) ∈ R2 and (3, 7) ∈ R2
⇒|1−3|≤5 and |3−7|≤5
But |1−7| ≰ 5
⇒ (1, 7) ∉ R2
So, R2 is not a transitive relation.
(iii) Given R3 on R defined as (a, b) ∈ R3 ⇔ a2 – 4ab + 3b2 = 0.
Now we have to check whether R2 is reflexive, symmetric and transitive or not.
First let us check whether the relation is reflexive or not. A relation ‘R’ on a set ‘A’ is said to be reflexive if (x R x) ∀ x ∈ A i.e. (x, x) ∈ R ∀ x ∈ A.
Let a be an element of R3.
Then, a ∈ R3.
⇒ a2 − 4a × a+ 3a2= 0
So, R3 is reflexive
Now check whether the relation is Symmetric relation or not. A relation R on set A is symmetric if (a, b)∈ R and (b, a)∈ R for all (a, b)∈ A.
Let (a, b) ∈ R3
⇒ a2−4ab+3b2=0
But b2−4ba+3a2 ≠ 0 ∀ a, b ∈ R
So, R3 is not symmetric.
Now check whether the relation is Transitive or not. A relation ‘R’ is said to be Transitive over set ‘A’ if (x, y) ∈ R and (y, z) ∈ R then (x, z) ∈ R ∀ x, y, z ∈ A.
Let (1, 2) ∈ R3 and (2, 3) ∈ R3
⇒ 1 − 8 + 6 = 0 and 4 – 24 + 27 = 0
But 1 – 12 + 9 ≠ 0
So, R3 is not a transitive relation.
问题4.让A = {1,2,3},让R1 = {(1,1),(1,3),(3,1),(2,2),(2,1),( 3,3)},R2 = {(2,2),(3,1),(1,3)},R3 = {(1,3),(3,3)}。确定A上的每个关系R1,R2,R3是否(i)自反(ii)对称(iii)可传递。
解决方案:
Considering the relation R1, we have
R1 = {(1, 1), (1, 3), (3, 1), (2, 2), (2, 1), (3, 3)}
First let us check whether the relation is reflexive or not. A relation ‘R’ on a set ‘A’ is said to be reflexive if (x R x) ∀ x ∈ A i.e. (x, x) ∈ R ∀ x ∈ A.
Here, (1, 1) ∈ R, (2, 2) ∈ R, (3, 3) ∈ R
So, R1 is reflexive.
Check whether the relation is Symmetric relation or not. A relation R on set A is symmetric if (a, b)∈ R and (b, a)∈ R for all (a, b)∈ A.
In the given relation, (2, 1) ∈ R1 but (1, 2) ∉ R1
So, R1 is not symmetric.
Now check whether the relation is Transitive or not. A relation ‘R’ is said to be Transitive over set ‘A’ if (x, y) ∈ R and (y, z) ∈ R then (x, z) ∈ R ∀ x, y, z ∈ A.
In the relation, (2, 1) ∈ R1 and (1, 3) ∈ R1 but (2, 3) ∉ R1
So, R1 is not transitive.
Therefore, the relation R1 is reflexive but not symmetric and transitive relation.
Now considering the relation R2, we have
R2 = {(2, 2), (3, 1), (1, 3)}
First let us check whether the relation is reflexive or not. A relation ‘R’ on a set ‘A’ is said to be reflexive if (x R x) ∀ x ∈ A i.e. (x, x) ∈ R ∀ x ∈ A.
Clearly, (1, 1) and (3, 3) ∉ R2
So, R2 is not a reflexive relation.
Check whether the relation is Symmetric relation or not. A relation R on set A is symmetric if (a, b)∈ R and (b, a)∈ R for all (a, b)∈ A.
In the relation, (1, 3) ∈ R2 and (3, 1) ∈ R2
So, R2 is a symmetric relation.
Now check whether the relation is Transitive or not. A relation ‘R’ is said to be Transitive over set ‘A’ if (x, y) ∈ R and (y, z) ∈ R then (x, z) ∈ R ∀ x, y, z ∈ A.
In the relation, (1, 3) ∈ R2 and (3, 1) ∈ R2 but (3, 3) ∉ R2
So, R2 is not a transitive relation.
Therefore, the relation R2 is symmetric but not a reflexive and transitive relation.
Considering the relation R3, we have
R3 = {(1, 3), (3, 3)}
First let us check whether the relation is reflexive or not. A relation ‘R’ on a set ‘A’ is said to be reflexive if (x R x) ∀ x ∈ A i.e. (x, x) ∈ R ∀ x ∈ A.
In the relation, (1, 1) ∉ R3
So, R3 is not reflexive.
Check whether the relation is Symmetric relation or not. A relation R on set A is symmetric if (a, b)∈ R and (b, a)∈ R for all (a, b)∈ A.
In the relation, (1, 3) ∈ R3, but (3, 1) ∉ R3
So, R3 is not symmetric.
Now check whether the relation is Transitive or not. A relation ‘R’ is said to be Transitive over set ‘A’ if (x, y) ∈ R and (y, z) ∈ R then (x, z) ∈ R ∀ x, y, z ∈ A.
Here, (1, 3) ∈ R3 and (3, 3) ∈ R3
Also, (1, 3) ∈ R3
So, R3 is transitive.
Therefore, the relation R3 is transitive but not a reflexive and symmetric relation.
问题5.在实数集上定义以下关系。
(i)如果a – b> 0,则为aRb
(ii)aRb iff 1 + ab> 0
(iii)如果| a |,则为aRb ≤b。
查找关系是自反的,对称的还是可传递的。
解决方案:
(i) Consider the relation defined as aRb if a – b > 0
Now for this relation we have to check whether it is reflexive, transitive and symmetric or not.
First let us check whether the relation is reflexive or not. A relation ‘R’ on a set ‘A’ is said to be reflexive if (x R x) ∀ x ∈ A i.e. (x, x) ∈ R ∀ x ∈ A.
Let a be an element of R.
Then, a ∈ R
But a − a = 0 ≯ 0
So, this relation is not a reflexive relation.
Now check whether the relation is Symmetric relation or not. A relation R on set A is symmetric if (a, b)∈ R and (b, a)∈ R for all (a, b)∈ A.
Let (a, b) ∈ R
⇒ a − b > 0
⇒ − (b − a) > 0
⇒ b − a < 0
So, the given relation is not a symmetric relation.
Now check whether the relation is Transitive or not. A relation ‘R’ is said to be Transitive over set ‘A’ if (x, y) ∈ R and (y, z) ∈ R then (x, z) ∈ R ∀ x, y, z ∈ A.
Let (a, b) ∈ R and (b, c) ∈ R.
Then, a − b > 0 and b − c > 0
Adding the two, we will get
a – b + b − c > 0
⇒ a – c > 0
⇒ (a, c) ∈ R.
So, the given relation is a transitive relation.
(ii) Consider the relation defined as aRb iff (read as “if and only if”) 1 + a b > 0
Now for this relation we have to check whether it is reflexive, transitive and symmetric or not.
First let us check whether the relation is reflexive or not. A relation ‘R’ on a set ‘A’ is said to be reflexive if (x R x) ∀ x ∈ A i.e. (x, x) ∈ R ∀ x ∈ A.
Let a be an element of R.
Then, a ∈ R
⇒ 1 + a × a > 0
i.e. 1 + a2 > 0 [since, square of any number is positive]
So, the given relation is a reflexive relation.
Now check whether the relation is Symmetric relation or not. A relation R on set A is symmetric if (a, b)∈ R and (b, a)∈ R for all (a, b)∈ A.
Let (a, b) ∈ R
⇒ 1 + a b > 0
⇒ 1 + b a > 0
⇒ (b, a) ∈ R
So, the given relation is symmetric.
Now check whether the relation is Transitive or not. A relation ‘R’ is said to be Transitive over set ‘A’ if (x, y) ∈ R and (y, z) ∈ R then (x, z) ∈ R ∀ x, y, z ∈ A.
Let (a, b) ∈ R and (b, c) ∈ R
⇒1 + a b > 0 and 1 + b c >0
But 1+ ac ≯ 0
⇒ (a, c) ∉ R
So, the given relation is not a transitive relation.
(iii) Consider the relation defined as aRb if |a| ≤ b.
Now for this relation we have to check whether it is reflexive, transitive and symmetric or not.
First let us check whether the relation is reflexive or not. A relation ‘R’ on a set ‘A’ is said to be reflexive if (x R x) ∀ x ∈ A i.e. (x, x) ∈ R ∀ x ∈ A.
Let a be an element of relation R.
Then, a ∈ R [Since, |a|=a]
⇒ |a| ≮ a
So, R is not a reflexive relation.
Now check whether the relation is Symmetric relation or not. A relation R on set A is symmetric if (a, b)∈ R and (b, a)∈ R for all (a, b)∈ A.
Let (a, b) ∈ R
⇒ |a| ≤ b
⇒ |b| ≰ a ∀ a, b ∈ R
⇒ (b, a) ∉ R
So, R is not a symmetric relation.
Now check whether the relation is Transitive or not. A relation ‘R’ is said to be Transitive over set ‘A’ if (x, y) ∈ R and (y, z) ∈ R then (x, z) ∈ R ∀ x, y, z ∈ A.
Let (a, b) ∈ R and (b, c) ∈ R
⇒ |a| ≤ b and |b| ≤ c
Multiplying the corresponding sides, we will get
|a| × |b| ≤ b c
⇒ |a| ≤ c
⇒ (a, c) ∈ R
Thus, R is a transitive relation.
问题6.检查集合{1、2、3、4、5、6}中定义为R = {(a,b):b = a + 1}的关系R是自反的,对称的还是可传递的。
解决方案:
Given R = {(a, b): b = a + 1}
Now, for this relation we have to check whether it is reflexive, transitive and symmetric or not.
First let us check whether the relation is reflexive or not. A relation ‘R’ on a set ‘A’ is said to be reflexive if (x R x) ∀ x ∈ A i.e. (x, x) ∈ R ∀ x ∈ A.
Let a be an element of R.
Then, a = a + 1 cannot be true for all a ∈ A.
⇒ (a, a) ∉ R
So, R is not a reflexive relation over the given set.
Now check whether the relation is Symmetric relation or not. A relation R on set A is symmetric if (a, b)∈ R and (b, a)∈ R for all (a, b)∈ A.
Let (a, b) ∈ R
⇒ b = a + 1
⇒ −a = −b + 1
⇒ a = b − 1
So, (b, a) ∉ R
Thus, R is not a symmetric relation over the given set.
Now check whether the relation is Transitive or not. A relation ‘R’ is said to be Transitive over set ‘A’ if (x, y) ∈ R and (y, z) ∈ R then (x, z) ∈ R ∀ x, y, z ∈ A.
Let (1, 2) and (2, 3) ∈ R
⇒ 2 = 1 + 1 and 3
2 + 1 is true.
But 3 ≠ 1+1
⇒ (1, 3) ∉ R
So, R is not a transitive relation over the given set.
问题7.检查定义为R = {(a,b):a≤b 3 }的R上的关系R是自反的,对称的或可传递的。
解决方案:
We have given the relation R = {(a, b): a ≤ b3}
First let us check whether the given relation is reflexive or not.
It can be observed that (1/2, 1/2) in R as 1/2 > (1/2)3 = 1/8
So, R is not a reflexive relation.
Now, check for whether the relation is symmetric or not
(1, 2) ∈ R (as 1 < 23 = 8)
But,
(2, 1) ∉ R (as 2 > 13 = 1)
So, R is not a symmetric relation.
We have (3, 3/2), (3/2, 6/5) in “R as” 3 < (3/2)3 and 3/2 < (6/5)3
But (3, 6/5) ∉ R as 3 > (6/5)3
So, R is not a transitive relation.
Hence, R is neither reflexive, nor symmetric, nor transitive.
问题8:证明集合上的每个身份关系都是自反的,但相反不一定是正确的。
解决方案:
We will verify this by taking example.
Let A be a set.
Then, Identity relation IA=IA is reflexive, since (a, a) ∈ A ∀ a ∈ A.
The converse of this need not be necessarily true.
Now, consider the set A = {1, 2, 3}
Here, relation R = {(1, 1), (2, 2) , (3, 3), (2, 1), (1, 3)} is reflexive on A.
But, R is not an identity relation.
Hence proved, that every identity relation on a set is reflexive but the converse is not necessarily true.
问题9.如果A = {1,2,3,4},则定义A上具有以下性质的关系:
(i)反身,及物但不对称
(ii)对称但既不反身也不及物动词。
(iii)反身,对称和及物。
解决方案:
(i) We have given the set A = {1, 2, 3, 4}
The relation on A having properties of being reflexive, transitive, but not symmetric is
R = {(1, 1), (2, 2), (3, 3), (4, 4), (2, 1)}
Relation R satisfies reflexivity and transitivity.
⇒ (1, 1), (2, 2), (3, 3) ∈ R [satisfies the reflexivity property]
and (1, 1), (2, 1) ∈ R ⇒ (1, 1) ∈ R [satisfies the transitivity property]
However, (2, 1) ∈ R, but (1, 2) ∉ R [does not satisfies the symmetric property]
(ii) We have given the set A = {1, 2, 3, 4}
The relation on A having properties of being reflexive, transitive, but not symmetric is
R = {(1, 1), (2, 2), (3, 3), (4, 4), (2, 1)}
Relation R satisfies reflexivity and transitivity.
⇒ (1, 1), (2, 2), (3, 3) ∈ R [satisfies the reflexivity property]
And (1, 1), (2, 1) ∈ R ⇒ (1, 1) ∈ R [satisfies the transitivity property]
However, (2, 1) ∈ R, but (1, 2) ∉ R [does not satisfies the symmetric property]
(iii) We have given the set A = {1, 2, 3, 4}
The relation on A having properties of being symmetric, reflexive and transitive is
R = {(1, 1), (2, 2), (3, 3), (4, 4), (1, 2), (2, 1)}
Relation R satisfies reflexivity, symmetricity and transitivity.
⇒ (1, 1), (2, 2), (3, 3) ∈ R [satisfies the reflexivity property]
⇒ (1, 1) ∈ R and (2, 1) ∈ R [satisfies the symmetric property]
⇒ (1, 1), (2, 1) ∈ R ⇒ (1, 1) ∈ R [satisfies the transitivity property]
问题10:令R是在自然数N集合上定义的关系,因为R = {(x,y):x,y∈N,2x + y = 41}。找到R的域和范围。还要验证R是否(i)自反(ii)对称(iii)可传递。
解决方案:
We have given,
{(x, y) : x, y ∈ N, 2x + y = 41}
Now,
2x + y = 41
⇒ y = 41 – 2x
Put the value of x one by one to form the relation R.
The relation we will after putting x = 1, 2, 3, ……. ,20 is:
[we can’t put x=21 since y = 41 – 2(2) < 0, which is not a natural number]
R = {(1, 39), (2, 37), (3, 35)………….., (20, 1)}
So the domain of R is
Domain(R) = {1, 2, 3, ……… ,20}
And the range of R is
Range(R) = {39, 37, 33, ……. ,1} and can be rearranged as {1, 3, 5, ……….. ,39}
Now for this relation we have to check whether it is reflexive, transitive and symmetric or not.
First let us check whether the relation is reflexive or not. A relation ‘R’ on a set ‘A’ is said to be reflexive if (x R x) ∀ x ∈ A i.e. (x, x) ∈ R ∀ x ∈ A.
Let x be an any element of relation R.
Since, (2, 2) ∉ R
So, R is not a reflexive relation.
Now check whether the relation is Symmetric relation or not. A relation R on set A is symmetric if (a, b)∈ R and (b, a)∈ R for all (a, b)∈ A.
Since, (1, 39) ∈ R but (39, 1) ∉ R.
So, R is not symmetric.
Now check whether the relation is Transitive or not. A relation ‘R’ is said to be Transitive over set ‘A’ if (x, y) ∈ R and (y, z) ∈ R then (x, z) ∈ R ∀ x, y, z ∈ A.
Since, (15,11) ∈ R and (11,19) ∈ R but (15,19) ∉ R.
Thus, R is not transitive.