Check whether the relation R on R defined by R = {(a, b) : a ≤ b3} is reflexive, symmetric or transitive.
We have the set of real numbers, R.
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.
We have
R = {(a, b): a ≤ b3}
Check for Reflexivity:
For a ∈ R
If (a, a) ∈ R,
⇒ a ≤ a3, which is not true.
Say, if a = – 2.
a ≤ a3
⇒ – 2 ≤ – 8
⇒ –2 ≤ –8, which is not true as – 2 > – 8.
Hence, (a, a) ∉ R
So, ∀ a ∈ R, then (a, a) ∉ R.
⇒ R is not reflexive.
∴ R is not reflexive.
Check for Symmetry:
∀ a, b ∈ R
If (a, b) ∈ R
⇒ a ≤ b3
Replace a by b and b by a, we get
⇒ b ≤ a3
[Take a = –2 and b = 3.
a ≤ b3
⇒ –2 ≤ 33
⇒ –2 ≤ 27, which is a true statement.
Now, b ≤ a3
⇒ 3 ≤ (–2)3
⇒ 3 ≤ –8, which is not a true statement as 3 > –8]
⇒ (b, a) ∉ R
So, if (a, b) ∈ R, then (b, a) ∉ R
∀ a, b ∈ R
⇒ R is not symmetric.
∴ R is not symmetric.
Check for Transitivity:
∀ a, b, c ∈ R
If (a, b) ∈ R and (b, c) ∈ R
⇒ a ≤ b3 and b ≤ c3
⇒ a ≤ c3 or not.
Let us check.
Take a = 3, and
.
a ≤ b3
⇒ 3 ≤ 3.37, which is true.
b ≤ c3
⇒ 1.5 ≤ 1.728
a ≤ c3
⇒ 3 ≤ 1.728, which is not true as 3 > 1.728.
Hence, (a, c) ∉ R.
So, if (a, b) ∈ R and (b, c) ∈ R, then (a, c) ∉ R.
∀ a, b, c ∈ ℝ
⇒ R is not transitive.
∴ R is not transitive.
Hence, R is neither reflexive, nor symmetric, nor transitive.