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Relations R1, R2, R3 and R4 are defined on a set A = {a, b, c} as follows :R1 = {(a, a) (a, b) (a, c) (b, b) (b, c), (c, a) (c, b) (c, c)}R2 = {(a, a)}R3 = {(b, a)}R4 = {(a, b) (b, c) (c, a)}Find whether or not each of the relations R1, R2, R3, R4 on A is (i) reflexive (iii) symmetric (iii) transitive. |
Answer» We have set, A = {a, b, c} Here, R1, R2, R3, and R4 are the binary relations on set A. So, recall that for any binary relation R on set A. We have, R is reflexive if for all x ∈ A, xRx. R is symmetric if for all x, y ∈ A, if xRy, then yRx. R is transitive if for all x, y, z ∈ A, if xRy and yRz, then xRz. So, using these results let us start determining given relations. We have R1 = {(a, a) (a, b) (a, c) (b, b) (b, c) (c, a) (c, b) (c, c)} (i). Reflexive: For all a, b, c ∈ A. [∵ A = {a, b, c}] Then, (a, a) ∈ R1 (b, b) ∈ A (c, c) ∈ A [∵ R1 = {(a, a) (a, b) (a, c) (b, b) (b, c) (c, a) (c, b) (c, c)}] So, ∀ a, b, c ∈ A, then (a, a), (b, b), (c, c) ∈ R. ∴ R1 is reflexive. (ii). Symmetric: If (a, a), (b, b), (c, c), (a, c), (b, c) ∈ R1 Then, clearly (a, a), (b, b), (c, c), (c, a), (c, b) ∈ R1 ∀ a, b, c ∈ A [∵ R1 = {(a, a) (a, b) (a, c) (b, b) (b, c) (c, a) (c, b) (c, c)}] But, we need to try to show a contradiction to be able to determine the symmetry. So, we know (a, b) ∈ R1 But, (b, a) ∉ R1 So, if (a, b) ∈ R1, then (b, a) ∉ R1. ∀ a, b ∈ A ∴ R1 is not symmetric. (iii). Transitive: If (b, c) ∈ R1 and (c, a) ∈ R1 But, (b, a) ∉ R1 [Check the Relation R1 that does not contain (b, a)] ∀ a, b ∈ A [∵ R1 = {(a, a) (a, b) (a, c) (b, b) (b, c) (c, a) (c, b) (c, c)}] So, if (b, c) ∈ R1 and (c, a) ∈ R1, then (b, a) ∉ R1. ∀ a, b, c ∈ A ∴ R1 is not transitive. Now, we have R2 = {(a, a)} (i). Reflexive: Here, only (a, a) ∈ R2 for a ∈ A. [∵ A = {a, b, c}] [∵ R2 = {(a, a)}] So, for a ∈ A, then (a, a) ∈ R2. ∴ R2 is reflexive. (ii). Symmetric: For symmetry, If (x, y) ∈ R, then (y, x) ∈ R ∀ x, y ∈ A. Notice, in R2 we have R2 = {(a, a)} So, if (a, a) ∈ R2, then (a, a) ∈ R2. Where a ∈ A. ∴ R2 is symmetric. (iii). Transitive: Here, (a, a) ∈ R2 and (a, a) ∈ R2 Then, obviously (a, a) ∈ R2 Where a ∈ A. [∵ R2 = {(a, a)}] So, if (a, a) ∈ R2 and (a, a) ∈ R2, then (a, a) ∈ R2, where a ∈ A. ∴ R2 is transitive. Now, we have R3 = {(b, a)} (i). Reflexive: ∀ a, b ∈ A [∵ A = {a, b, c}] But, (a, a) ∉ R3 Also, (b, b) ∉ R3 [∵ R3 = {(b, a)}] So, ∀ a, b ∈ A, then (a, a), (b, b) ∉ R3 ∴ R3 is not reflexive. (ii). Symmetric: If (b, a) ∈ R3 Then, (a, b) should belong to R3. ∀ a, b ∈ A. [∵ A = {a, b, c}] But, (a, b) ∉ R3 [∵ R3 = {(b, a)}] So, if (a, b) ∈ R3, then (b, a) ∉ R3 ∀ a, b ∈ A ∴ R3 is not symmetric. (iii). Transitive: We have (b, a) ∈ R3 but do not contain any other element in R3. Transitivity can’t be proved in R3. [∵ R3 = {(b, a)}] So, if (b, a) ∈ R3 but since there is no other element. ∴ R3 is not transitive. Now, we have R4 = {(a, b) (b, c) (c, a)} (i). Reflexive: ∀ a, b, c ∈ A [∵ A = {a, b, c}] But, (a, a) ∉ R4 Also, (b, b) ∉ R4 and (c, c) ∉ R4 [∵ R4 = {(a, b) (b, c) (c, a)}] So, ∀ a, b, c ∈ A, then (a, a), (b, b), (c, c) ∉ R4 ∴ R4 is not reflexive. (ii). Symmetric: If (a, b) ∈ R4, then (b, a) ∈ R4 But (b, a) ∉ R4 [∵ R4 = {(a, b) (b, c) (c, a)}] So, ∀ a, b ∈ A, if (a, b) ∈ R4, then (b, a) ∉ R4. ⇒ R4 is not symmetric. It is sufficient to show only one case of ordered pairs violating the definition. ∴ R4 is not symmetric. (iii). Transitivity: We have, (a, b) ∈ R4 and (b, c) ∈ R4 ⇒ (a, c) ∈ R4 But, is it so? No, (a, c) ∉ R4 So, it is enough to determine that R4 is not transitive. ∀ a, b, c ∈ A, if (a, b) ∈ R4 and (b, c) ∈ R4, then (a, c) ∉ R4. ∴ R4 is not transitive. |
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