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Sets and Relations question

2025 · 2 Apr · Shift 1 · Q41
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Sets and Relations question

2025 · 2 Apr · Shift 1 · Q41

JEE MainMathematicsSets and RelationsMCQ+4 / −1
Let A be the set of all functions f:Z→Zf: \mathbf{Z} \rightarrow \mathbf{Z}f:Z→Z and R be a relation on A such that R={(f,g):f(0)=g(1)\mathrm{R}=\{(\mathrm{f}, \mathrm{g}): f(0)=\mathrm{g}(1)R={(f,g):f(0)=g(1) and f(1)=g(0)}f(1)=\mathrm{g}(0)\}f(1)=g(0)}. Then R is :
  1. A
    Symmetric and transitive but not reflective
  2. B
    Symmetric but neither reflective nor transitive
  3. C
    Transitive but neither reflexive nor symmetric
  4. D
    Reflexive but neither symmetric nor transitive
View written solutionFree

Correct answer: B

  1. Given relation

Let AAA be the set of all functions f:Z→Zf: \mathbb{Z} \to \mathbb{Z}f:Z→Z.

The relation RRR on AAA is defined by

(f,g)∈R  ⟺  f(0)=g(1) and f(1)=g(0).(f,g) \in R \iff f(0)=g(1) \text{ and } f(1)=g(0).(f,g)∈R⟺f(0)=g(1) and f(1)=g(0).

We must check whether RRR is reflexive, symmetric, and transitive.


  1. Check reflexivity

A relation is reflexive if for every f∈Af \in Af∈A, we have (f,f)∈R(f,f) \in R(f,f)∈R.

Now,

(f,f)∈R  ⟺  f(0)=f(1) and f(1)=f(0).(f,f) \in R \iff f(0)=f(1) \text{ and } f(1)=f(0).(f,f)∈R⟺f(0)=f(1) and f(1)=f(0).

This is equivalent to just

f(0)=f(1).f(0)=f(1).f(0)=f(1).

But this is not true for every function f:Z→Zf: \mathbb{Z}\to\mathbb{Z}f:Z→Z.

For example, take

f(x)=x.f(x)=x.f(x)=x.

Then

f(0)=0,f(1)=1,f(0)=0, \quad f(1)=1,f(0)=0,f(1)=1,

and hence f(0)≠f(1)f(0) \ne f(1)f(0)=f(1). So (f,f)∉R(f,f) \notin R(f,f)∈/R.

Therefore, RRR is not reflexive.


  1. Check symmetry

A relation is symmetric if

(f,g)∈R  ⟹  (g,f)∈R.(f,g) \in R \implies (g,f) \in R.(f,g)∈R⟹(g,f)∈R.

Suppose (f,g)∈R(f,g) \in R(f,g)∈R. Then

f(0)=g(1),f(1)=g(0).f(0)=g(1), \qquad f(1)=g(0).f(0)=g(1),f(1)=g(0).

To have (g,f)∈R(g,f) \in R(g,f)∈R, we need

g(0)=f(1),g(1)=f(0).g(0)=f(1), \qquad g(1)=f(0).g(0)=f(1),g(1)=f(0).

But these are exactly the same equalities as above, just written in reverse order.

Hence whenever (f,g)∈R(f,g) \in R(f,g)∈R, we also get (g,f)∈R(g,f) \in R(g,f)∈R.

Therefore, RRR is symmetric.


  1. Check transitivity

A relation is transitive if

(f,g)∈R and (g,h)∈R  ⟹  (f,h)∈R.(f,g) \in R \text{ and } (g,h) \in R \implies (f,h) \in R.(f,g)∈R and (g,h)∈R⟹(f,h)∈R.

Assume

(f,g)∈Rand(g,h)∈R.(f,g) \in R \quad \text{and} \quad (g,h) \in R.(f,g)∈Rand(g,h)∈R.

Then

f(0)=g(1),f(1)=g(0),f(0)=g(1), \qquad f(1)=g(0),f(0)=g(1),f(1)=g(0),

and

g(0)=h(1),g(1)=h(0).g(0)=h(1), \qquad g(1)=h(0).g(0)=h(1),g(1)=h(0).

Combining,

f(0)=g(1)=h(0),f(0)=g(1)=h(0),f(0)=g(1)=h(0), f(1)=g(0)=h(1).f(1)=g(0)=h(1).f(1)=g(0)=h(1).

So we get

f(0)=h(0),f(1)=h(1).f(0)=h(0), \qquad f(1)=h(1).f(0)=h(0),f(1)=h(1).

But for (f,h)∈R(f,h) \in R(f,h)∈R, we need

f(0)=h(1),f(1)=h(0).f(0)=h(1), \qquad f(1)=h(0).f(0)=h(1),f(1)=h(0).

These do not follow in general.

So transitivity may fail.

Let us give a counterexample.

Take functions f,g,h∈Af,g,h \in Af,g,h∈A such that:

  • f(0)=0, f(1)=1f(0)=0, \ f(1)=1f(0)=0, f(1)=1
  • g(0)=1, g(1)=0g(0)=1, \ g(1)=0g(0)=1, g(1)=0
  • h(0)=0, h(1)=1h(0)=0, \ h(1)=1h(0)=0, h(1)=1

(Values at all other integers can be chosen arbitrarily.)

Then:

  • (f,g)∈R(f,g) \in R(f,g)∈R because f(0)=0=g(1),f(1)=1=g(0).f(0)=0=g(1), \qquad f(1)=1=g(0).f(0)=0=g(1),f(1)=1=g(0).
  • (g,h)∈R(g,h) \in R(g,h)∈R because g(0)=1=h(1),g(1)=0=h(0).g(0)=1=h(1), \qquad g(1)=0=h(0).g(0)=1=h(1),g(1)=0=h(0).

But (f,h)∉R(f,h) \notin R(f,h)∈/R because that would require

f(0)=h(1)  ⟺  0=1,f(0)=h(1) \iff 0=1,f(0)=h(1)⟺0=1,

which is false.

Therefore, RRR is not transitive.


  1. Conclusion

Thus RRR is:

  • not reflexive,
  • symmetric,
  • not transitive.

So the correct option is

B: Symmetric but neither reflexive nor transitive\boxed{\text{B: Symmetric but neither reflexive nor transitive}}B: Symmetric but neither reflexive nor transitive​
  1. Comparison with stored answer

Stored correct answer: B

Our derived answer: B

Hence, the derived answer agrees with the stored answer.

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