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Propositional Logic

Propositional logic, also known as propositional calculus or sentential logic, forms a foundational sub-field of mathematical logic along with other sub-fields such as first-order logic, higher-order logic, modal logic, intuitionistic logic, temporal logic, set theory, model theory, proof theory, and recursion theory. Propositional logic provides a formal system for representing and analyzing statements that are either true or false.

It includes:

  • Syntax — the formal structure, including propositions (typically represented by variables like p, q, r) and logical connectives (AND, OR, NOT, implies, and others).
  • Semantics — how truth values are determined using truth tables, and how tautologies (always true) and contradictions (always false) are identified.
  • Equivalences — laws like De Morgan’s laws, distributive laws, and other equivalences that allow for simplification. Not all logical equivalences are laws: all laws are logical equivalences, but some equivalences are derived, conditional, or context-dependent rather than fundamental enough to be named as laws. Learn more about the basic laws of propositional logic and other equivalences.
  • Inference rules — formal rules such as modus ponens, modus tollens, and others that allow for step-by-step proofs.
  • Normal forms — standard ways to represent logical formulas, like conjunctive normal form (CNF) and disjunctive normal form (DNF).
  • Proof techniques — methods like proof by contradiction, direct proof, and truth tables to establish the validity of arguments.
  • Truth tables — used in sentential logic to determine the truth values of logical expressions based on their components.
  • Applications — propositional logic is widely used in computer science (circuit design, program verification), artificial intelligence (knowledge representation, automated reasoning), philosophy (formal analysis of arguments), and many other fields.

The study of propositional logic establishes the foundation for more complex logical systems while providing essential tools for formal reasoning across numerous disciplines. The dedicated propositional logic section develops each of these topics in depth.

Introduction to Mathematical Logic

Mathematical logic studies the principles of reasoning and provides a rigorous framework to analyze the structure of statements and arguments. It begins with the basics of propositional logic, where statements are combined using logical operators like and, or, and not, to form compound statements and evaluate their truth.

A deeper layer is predicate logic, which extends propositional logic by introducing quantifiers like for all and there exists, allowing reasoning about objects and their properties. Central to this is the concept of logical validity, which examines whether conclusions follow from premises regardless of specific interpretations.

Mathematical logic also explores formal systems, which consist of axioms, rules of inference, and symbols for constructing proofs. Key topics include set theory, the foundation of mathematics, and model theory, which studies the relationship between formal languages and their interpretations.

Another significant area is computability theory, which asks fundamental questions about what problems can be solved by algorithms, and proof theory, which investigates the nature and structure of mathematical proofs.

Applications of mathematical logic are vast, influencing fields like computer science, where it underpins algorithms and programming languages, and philosophy, where it sharpens reasoning. It develops skills in abstraction, critical thinking, and formal reasoning, making it a cornerstone of mathematical rigor.

Mathematical Logic Formulas Reference

32 itemsSee All Mathematical Logic Formulas Reference
This reference collects 32 logical equivalences and inference patterns organized into 15 categories - the standard toolkit for simplifying propositional formulas, converting between normal forms, and constructing rigorous proofs.

Idempotent Laws, Commutative Laws, and Associative Laws cover the basic structural rules for AND and OR. Distributive Laws drive normal-form conversion to DNF and CNF. Identity Laws, Domination Laws, and Negation Laws describe interactions with the truth and falsity constants. Double Negation, De Morgan Laws, Absorption Laws, and Redundancy Laws handle simplification through negation and structural elimination. Monotonicity Laws provide the tautological inference patterns of disjunction introduction and conjunction elimination. Conditional Equivalences and Biconditional Equivalences rewrite implication and if-and-only-if statements into more useful forms. Tautology and Contradiction Duality closes the set with the relationship between the truth and falsity constants under negation.

Each entry shows the formula in LaTeX, an explanation of what the law captures, and where applicable a derivation, truth-table verification, common variants, and links to closely related formulas and definitions.
View All Mathematical Logic Formulas Reference

Logic Terms and Definitions

28 itemsSee All Logic Terms and Definitions
The Logic Terms and Definitions page presents key concepts and terminology organized in multiple categories including Logic Basics, Reasoning, Formal Logic, Proof Methods, Logical Principles, and Structures. It covers fundamental concepts like propositions and predicates, reasoning methods, formal systems, and proof techniques. Each term is clearly defined to help understand the building blocks of mathematical logic and logical reasoning.
Syntax11
PropositionA declarative statement that carries exactly one truth value: true or false, but not bothRead more →Elementary PropositionAn atomic, indivisible statement PP that cannot be broken into simpler logical componentsRead more →Compound PropositionA proposition built by combining simpler propositions using logical connectives: φ:=PQ\varphi := P \circ Q where…Read more →Well-Formed Formula (WFF)An expression constructed according to the formation rules of propositional logic, ensuring unambiguous syntactic structureRead more →LiteralAn atomic proposition or its negation: PP (positive literal) or ¬P\neg P (negative literal)Read more →Logical ConnectiveAn operator that combines or modifies propositions to form compound propositions: ¬\neg, \land, \lor, \to, \leftrightarrowRead more →NegationA unary connective ¬P\neg P that reverses the truth value of PPRead more →ConjunctionA binary connective PQP \land Q that is true only when both PP and QQ are trueRead more →DisjunctionA binary connective PQP \lor Q that is true when at least one of PP or QQ is trueRead more →Disjunctive Normal Form (DNF)A formula expressed as a disjunction of conjunctions of literals: (L1L2)(L3L4)(L_1 \land L_2) \lor (L_3 \land L_4) \lor \ldotsRead more →Conjunctive Normal Form (CNF)A formula expressed as a conjunction of disjunctions of literals: (L1L2)(L3L4)(L_1 \lor L_2) \land (L_3 \lor L_4) \land \ldotsRead more →
Conditional (Implication)PQP \to Q: false only when PP is true and QQ is false; true in all other casesRead more →BiconditionalPQP \leftrightarrow Q: true when PP and QQ share the same truth value, equivalent to (PQ)(QP)(P \to Q) \land (Q \to P)Read more →AntecedentIn a conditional PQP \to Q, the antecedent is PP — the hypothesis or "if" partRead more →ConsequentIn a conditional PQP \to Q, the consequent is QQ — the conclusion or "then" partRead more →ConverseThe converse of PQP \to Q is QPQ \to P — the conditional with antecedent and consequent swappedRead more →ContrapositiveThe contrapositive of PQP \to Q is ¬Q¬P\neg Q \to \neg P — always logically equivalent to the originalRead more →Inverse (of Conditional)The inverse of PQP \to Q is ¬P¬Q\neg P \to \neg Q — not logically equivalent to the originalRead more →Logical EquivalenceTwo formulas φ\varphi and ψ\psi are logically equivalent (φψ\varphi \equiv \psi) if they have identical truth values under every possible assignmentRead more →TautologyA formula φ\varphi that evaluates to true under every possible truth assignment. Notation: φ\models \varphi or φ\varphi \equiv \topRead more →ContradictionA formula φ\varphi that evaluates to false under every possible truth assignment. Notation: φ\varphi \equiv \botRead more →ContingencyA formula that is neither a tautology nor a contradiction — it is true under some assignments and false under othersRead more →SatisfiabilityA formula φ\varphi is satisfiable if there exists at least one truth assignment under which φ\varphi evaluates to trueRead more →Truth TableA tabular listing of all possible truth value combinations for a formula's variables and the resulting truth value of the formulaRead more →Assignment (Valuation)A function vv that maps each propositional variable to a truth value: v:{P,Q,R,}{T,F}v: \{P, Q, R, \ldots\} \to \{T, F\}Read more →AbsorptionP(PQ)PP \land (P \lor Q) \equiv P and P(PQ)PP \lor (P \land Q) \equiv PRead more →Law of Excluded MiddleP¬PP \lor \neg P is always true — every proposition is either true or false, with no third optionRead more →Non-contradiction¬(P¬P)\neg(P \land \neg P) is always true — no proposition can be simultaneously true and falseRead more →
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Basic laws of Propositional Logic

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Explore Basic laws of Propositional Logic

Dynamic Truth Table Generator

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Generate dynamic truth tables for logical expressions with our interactive Logic Calculator. Learn about logical operators, Boolean algebra, and propositional logic.
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Logic Symbols Reference

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Our Mathematical Logic Symbols page provides a comprehensive collection of symbols essential for working with formal logic systems. This reference includes detailed categorization of symbols across multiple domains of mathematical logic.

Explore symbols organized by functional categories including logical operations (¬, ∧, ∨), quantifiers (∀, ∃), set operations (∈, ⊆, ∩), relation symbols (=, ≠, ≤), and specialized notation systems. The page features both basic symbols like implication (→) and biconditional (↔), as well as advanced concepts from temporal modal logic (□, ◇), type theory and lambda calculus (λ, ≡).

Each symbol is presented with its corresponding LaTeX code and a brief explanation of its meaning and usage, making this an invaluable resource for students, educators, and professionals working with mathematical proofs, set theory, or formal logic systems.

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