p∧q true iff both true
p and q is true exactly when both p and q are true.
Conjunction of two propositions.
- p, q
- propositions
Use when — Combining two statements with 'and'.
Common trap — And/or confusion.
p∨q false iff both false
p or q is false exactly when both p and q are false.
Inclusive disjunction of two propositions.
- p, q
- propositions
Use when — Standard inclusive OR, unless stated otherwise.
Common trap — Exclusive OR assumption.
p→q ≡ ¬p∨q
p implies q is equivalent to not p or q.
Implication restated as a disjunction.
- p, q
- propositions
Use when — Propositional logic transformations.
Common trap — Replacing implication by its converse.
p→q ≡ ¬q→¬p
p implies q is equivalent to not q implies not p.
An implication and its contrapositive are logically equivalent.
- p, q
- propositions
Use when — Proving an implication via its contrapositive.
Common trap — Using the inverse instead of the contrapositive.
q→p ≡ ¬p→¬q
q implies p is equivalent to not p implies not q.
The converse and the inverse of an implication are equivalent to each other.
- p, q
- propositions
Use when — Comparing converse and inverse.
Common trap — Assuming either equals the original implication.
p↔q ≡ (p→q)∧(q→p)
p if and only if q is equivalent to p implies q and q implies p.
Biconditional as the conjunction of both implication directions.
- p, q
- propositions
Use when — Proving 'if and only if' statements.
Common trap — Proving one direction only.
¬(p∧q) ≡ ¬p∨¬q
Not (p and q) is equivalent to not p or not q.
De Morgan's law for conjunction.
- p, q
- propositions
Use when — Negating a conjunction.
Common trap — Wrong connective after negation.
¬(p∨q) ≡ ¬p∧¬q
Not (p or q) is equivalent to not p and not q.
De Morgan's law for disjunction.
- p, q
- propositions
Use when — Negating a disjunction.
Common trap — Wrong connective after negation.
¬[∀x P(x)] ≡ ∃x ¬P(x)
Not (for all x, P of x) is equivalent to there exists x such that not P of x.
Negation of a universally quantified statement.
- P(x)
- predicate over a declared domain
Use when — Negating a 'for all' statement.
Common trap — Missing or dropping the declared domain.
¬[∃x P(x)] ≡ ∀x ¬P(x)
Not (there exists x such that P of x) is equivalent to for all x, not P of x.
Negation of an existentially quantified statement.
- P(x)
- predicate over a declared domain
Use when — Negating a 'there exists' statement.
Common trap — Reversing the quantifier without negating the predicate.