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Exercise 1858Logic, sets and proofs › Quantifiers and negationL1Added 3 Nov 2026

Scope of quantifiers

#scope#quantifiers#ambiguity

Statement

Explain the difference between:
- P1P_1: ∀x, P(x)⇒Q\forall x,\ P(x)\Rightarrow Q;
- P2P_2: ∀x, (P(x)⇒Q)\forall x,\ (P(x)\Rightarrow Q).

Hint

Consider whether QQ depends on xx.

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Solution

P1P_1. ∀x, P(x)⇒Q\forall x,\ P(x)\Rightarrow Q: if all xx satisfy PP, then QQ. Here QQ does not depend on xx; the quantifier applies only to P(x)P(x).

P2P_2. ∀x, (P(x)⇒Q)\forall x,\ (P(x)\Rightarrow Q): for every xx, if P(x)P(x) then QQ. Each implication is quantified separately.

Example. P(x)P(x): "xx is a counterexample"; QQ: "the conjecture is false". P1P_1: "if all are counterexamples, the conjecture is false". P2P_2: "every counterexample makes the conjecture false".

Conclusion. Scope matters: ∀x, P(x)⇒Q\forall x,\ P(x)\Rightarrow Q and ∀x, (P(x)⇒Q)\forall x,\ (P(x)\Rightarrow Q) differ in meaning.