Discrete Structures

Distribution of Quantifiers
over Conjunction and Disjunction

Last Update: 3 February 2009

Note: NEW or UPDATED material is highlighted


Note: In what follows,


Theorem:

  1. ∀x[P(x) ∧ Q(x)] ≡ (∀xP(x) ∧ ∀xQ(x))

  2. ∀x[P(x) ∨ Q(x)] is not ≡ (∀xP(x) ∨ ∀xQ(x))

  3. ∃x[P(x) ∧ Q(x)] is not ≡ (∃xP(x) ∧ ∃xQ(x))

  4. ∃x[P(x) ∨ Q(x)] ≡ (∃xP(x) ∨ ∃xQ(x))

I.e., the universal quantifier distributes over conjunction, but not disjunction, and the existential quantifier distributes over disjunction, but not conjunction.


proof:

  1. See Rosen, p. 39

  2. Counterexample:

    Then: tval(LHS) = T, but tval(RHS) = F

  3. Counterexample:

    Then: tval(LHS) = F, but tval(RHS) = T

  4. To prove this, we need a rule of inference that, from p, we can infer (p ∨ q)
    (i.e., (p → (p ∨ q)) is a tautology)
    (this is the rule that we will soon be calling "addition" or "∨-introduction").

    Show: LHS & RHS always have same tval:

    Case 1: Suppose tval(LHS) = T, & show tval(RHS) = T:

    Case 2: Suppose tval(RHS) = T & show tval(LHS) = T:

QED.


Copyright © 2009 by William J. Rapaport (rapaport@cse.buffalo.edu)
http://www.cse.buffalo.edu/~rapaport/191/S09/distqfroverandor.html-20090203