GateAI
Propositional and First-Order Logic

GATE CSE 2023 · Q26

MSQ1 markMedium

Time

Geetha has a conjecture about integers, which is of the form ∀x(P(x)  ⟹  ∃yQ(x,y)),\forall x\Big(P(x) \implies \exists y Q(x,y)\Big), where PP is a statement about integers, and QQ is a statement about pairs of integers. Which of the following (one or more) option(s) would imply Geetha's conjecture?

Select all that applyOne or more correct

Pick every correct option

Solution

Step-by-step verified solution

Official answer: B) ∀ x∀ y Q(x,y); C) ∃ y∀ x (P(x) Q(x,y) )

Propositional and First-Order Logic

Geetha's conjecture is given by:

G≡∀x(P(x)  ⟹  ∃yQ(x,y))G \equiv \forall x \big(P(x) \implies \exists y Q(x, y)\big)

We need to determine which statement SS logically implies GG (i.e., S  ⟹  GS \implies G is valid):

  • Option B (∀x∀yQ(x,y)\forall x \forall y Q(x, y)): If for all xx and all yy, Q(x,y)Q(x, y) is true, then for every xx, there definitely exists at least one yy such that Q(x,y)Q(x, y) is true (since the domain of integers is non-empty). Thus, the consequent ∃yQ(x,y)\exists y Q(x, y) is always true for every xx, making the conditional P(x)  ⟹  ∃yQ(x,y)P(x) \implies \exists y Q(x, y) true for all xx. Therefore, ∀x∀yQ(x,y)  ⟹  G\forall x \forall y Q(x, y) \implies G.

  • Option C (∃y∀x(P(x)  ⟹  Q(x,y))\exists y \forall x (P(x) \implies Q(x, y))): Suppose there exists an integer y0y_0 such that for all xx, P(x)  ⟹  Q(x,y0)P(x) \implies Q(x, y_0). Then for any arbitrary xx, if P(x)P(x) holds, Q(x,y0)Q(x, y_0) must hold. Since y0y_0 is a witness, ∃yQ(x,y)\exists y Q(x, y) holds. Thus, for every xx, P(x)  ⟹  ∃yQ(x,y)P(x) \implies \exists y Q(x, y). Therefore, ∃y∀x(P(x)  ⟹  Q(x,y))  ⟹  G\exists y \forall x (P(x) \implies Q(x, y)) \implies G.

  • Options A and D (∃x(… )\exists x (\dots)): Both state existential properties for only *some* integer xx. An existential statement about a single element cannot guarantee that a condition holds universally for *all* integers xx. Hence, neither implies GG.

Thus, the correct options are B and C.

Try the question first, then check your work.

Think the official key or this write-up is wrong?

Report error or challenge

Examiner trap

The misconception this question tests

This Propositional and First-Order Logic previous year question is Q26 of the GATE CSE 2023 paper. Find more GATE CSE PYQs on Propositional and First-Order Logic in Engineering Mathematics, or browse all GATE CSE PYQs.

Free diagnostic

Where do you stand for GATE?

Take the 20-minute diagnostic: the 3 topics that cost you the most marks, and a rank range. Or sit 5 related Propositional and First-Order Logic PYQs now with no account.

One GATE CSE PYQ every morning

An official question by email at 7 AM IST, with a link to its verified solution. Unsubscribe in one click.

Propositional and First-Order Logic in GATE CSE

Propositional and First-Order Logic has 18 GATE CSE PYQs across 16 papers from 2011 to 2026. 2 of them are 1-mark MSQs like this one, and it is the only one in GATE CSE 2023.

How GATE tests Propositional and First-Order Logic

More from GATE CSE 2023

Share this solution

A plain-text card for WhatsApp or Telegram study groups.

GateAI solution cardGATE CSE 2023 · Q26 · MSQ

Geetha has a conjecture about integers, which is of the form ∀ x (P(x) ∃ y Q(x,y) ), where P is a statement about integers, and Q is a statement about pairs of integers. Which of the following (one or more) option(s) would imply Geetha's conjecture?

Topic-wise GATE CS PYQs with verified steps

Independent practice explanation verified by the GateAI team. GATE is conducted by the IITs; question text follows the official paper.