in Mathematical Logic retagged by
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10 votes
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Below is a drawing(graph representation) of a binary relation $\text{R}$ over a set $\text{P}$ of elements $\{ \text{A, B, C, D, E, F}\}:$

Which of the following first-order logic statements about $\mathrm{R}$, is/are true?

  1. $\forall x \in P .(x R x \rightarrow \exists y \in P . x R y)$
  2. $\exists x \in P . \forall y \in P . x R y$
  3. $\exists x \in P .(x R x \rightarrow \forall y \in P . x R y)$
  4. $\forall x \in P . \exists y \in P . x R y$
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4 Comments

loved this one only for option c.
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$ \large{\colorbox{yellow}{Detailed video solution of this question with direct time stamp}}$
All India Mock Test 3 - Solutions Part 1

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This is a great question, although I made it wrong
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1 Answer

4 votes
4 votes

Let's understand Option wise

In the solution, I am assuming x and y both belong to P so I am not writing repeatedly

Option A).

$∀_{x}$ ( xRx → $∃_{y}$ . xRy) which is equivalent to  $∀_{x}$ x   $∃_{y}$ .( xRx →  xRy)

it translates to “ for every value of x there is some y such that if x is reflexive or xRx then xRy”.

this is true because we don’t have any counter-example for x, for example for element “f” fRf is true so xRx → xRy is also true as there is no mention of x != y.

Option B). $∃_{x}$ $∀_{y}$ xRy

it translates to “There is some x for all y in a domain such that xRy”.

as we need some witness in this case and we have no witness in the domain P.

Option C).$∃_{x}$ ( xRx → $∀_{y}$ . xRy) 

(I also made a mistake in this option and didn’t mark it but now I realize my silly mistake.)

This is true as it translates to “  There is some x for all y such that if xRx then xRy”.

now here implication is given and it is also true when the antecedent part is false.

so we just need to find such x which is not related to itself we have such x in domain P which is “element C”.

so C is our witness here, C doesn’t relate to itself and C doesn’t relate to all elements in the domain too. 

so F→ F = T.

Option D).

$∀_{x}$   $∃_{y}$  xRy 

it translates to “ for every x there is some y such that xRy

This is also true as we don’t have any counter-example to make it false, for element “f”  fRf as there is no mention of x != y.

{Most trickiest option is Option C I must say.}

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