Logical Equivalences to Know for Formal Logic I

Logical equivalences are key principles in formal logic that help simplify and analyze statements. They connect various logical operations, making it easier to understand relationships between propositions and their truth values, which is essential in formal reasoning and mathematical logic.

1. Double Negation

   • A statement is equivalent to the negation of its negation: ¬(¬P) ≡ P.
   • This principle simplifies expressions by eliminating double negatives.
   • It is foundational in proving other logical equivalences.

2. Commutativity

   • The order of operands does not affect the outcome: P ∧ Q ≡ Q ∧ P and P ∨ Q ≡ Q ∨ P.
   • This property allows for flexibility in rearranging logical statements.
   • It applies to both conjunction (AND) and disjunction (OR).

3. Associativity

   • The grouping of operands does not change the result: (P ∧ Q) ∧ R ≡ P ∧ (Q ∧ R) and (P ∨ Q) ∨ R ≡ P ∨ (Q ∨ R).
   • This property helps in simplifying complex logical expressions.
   • It is applicable to both conjunction and disjunction.

4. Distributivity

   • Distributes one operation over another: P ∧ (Q ∨ R) ≡ (P ∧ Q) ∨ (P ∧ R) and P ∨ (Q ∧ R) ≡ (P ∨ Q) ∧ (P ∨ R).
   • This principle is crucial for transforming expressions into simpler forms.
   • It combines the operations of AND and OR effectively.

5. De Morgan's Laws

   • Relates conjunctions and disjunctions through negation: ¬(P ∧ Q) ≡ ¬P ∨ ¬Q and ¬(P ∨ Q) ≡ ¬P ∧ ¬Q.
   • These laws are essential for understanding how negation interacts with logical operations.
   • They are frequently used in proofs and logical reasoning.

6. Implication

   • Expresses a conditional relationship: P → Q is equivalent to ¬P ∨ Q.
   • Understanding implication is key to analyzing logical arguments.
   • It highlights the relationship between antecedents and consequents.

7. Contraposition

   • States that P → Q is equivalent to ¬Q → ¬P.
   • This principle is useful for proving the validity of implications.
   • It emphasizes the importance of the relationship between a statement and its contrapositive.

8. Biconditional

   • Indicates equivalence: P ↔ Q is true if both P and Q are either true or false.
   • It can be expressed as (P → Q) ∧ (Q → P).
   • Understanding biconditional statements is crucial for logical equivalences.

9. Idempotence

   • States that repeating an operation does not change the outcome: P ∧ P ≡ P and P ∨ P ≡ P.
   • This property simplifies expressions by removing redundancies.
   • It applies to both conjunction and disjunction.

10. Identity Laws

    • Establishes that combining a statement with a neutral element yields the original statement: P ∧ True ≡ P and P ∨ False ≡ P.
    • These laws help in identifying the core components of logical expressions.
    • They reinforce the concept of identity in logical operations.

11. Domination Laws

    • States that certain values dominate the outcome: P ∧ False ≡ False and P ∨ True ≡ True.
    • These laws simplify expressions by eliminating unnecessary components.
    • They highlight the influence of certain truth values in logical operations.

12. Negation Laws

    • Establishes that a statement and its negation cover all possibilities: P ∨ ¬P ≡ True and P ∧ ¬P ≡ False.
    • These laws are fundamental in understanding the nature of truth in logic.
    • They emphasize the completeness of logical statements.

13. Absorption Laws

    • Shows how certain combinations can be simplified: P ∧ (P ∨ Q) ≡ P and P ∨ (P ∧ Q) ≡ P.
    • These laws help in reducing complex expressions to their simplest form.
    • They illustrate the redundancy in certain logical combinations.

14. Exportation

    • States that (P ∧ Q) → R is equivalent to P → (Q → R).
    • This principle is useful for transforming implications into a more manageable form.
    • It highlights the relationship between conjunctions and implications.

15. Tautology and Contradiction

    • A tautology is a statement that is always true (e.g., P ∨ ¬P), while a contradiction is always false (e.g., P ∧ ¬P).
    • Understanding these concepts is essential for evaluating the validity of logical expressions.
    • They serve as benchmarks for determining the truth values of more complex statements.