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math-debrief
  • Math Debrief
  • Math: TIMELINE
  • 100-fundamentals
    • debrief-name: math section-code: 000 section-name: general section-desc: Elementary topics pervasive
      • About Mathematics
      • abstraction-in-math
      • About Math
      • Axiom schema
      • Basic concepts in math
      • Collections
      • Elementary concepts in objects
      • Elements of mathematics
      • math-as-a-language
      • Mathematical structures
      • List of mathematics-based methods
      • Mathematics and Reality
      • Mathematics: General
      • Controversial mathematics
      • the-elements-of-math
      • What is mathematics
    • The foundation of mathematics
      • Mathematical foundations
      • Foundations of Mathematics
      • Axiomatization of mathematics
      • Foundational crisis of mathematics
      • Foundations
      • Hilbert's problems
      • impl-of-math-in-set-theory
      • Gödel's Incompleteness Theorem
      • Theorems in the foundations of mathematics
      • The list of FOM candidates
      • Logicism
    • Philosophy of mathematics
      • Constructive mathematics
      • Constructive mathematics
      • Metamathematics
      • Philosophy of mathematics
      • Schools of mathematics
    • terms
      • Terms
      • Arithmetic
      • Axiom
      • The Axiomatic Method
      • discrete-math
      • 201 Discrete mathematics
      • Euclidean space
      • Formal system
      • Function
      • Generalization
      • Geometry
      • Higher-order
      • Impredicativity
      • Level of measurement
      • Mathematical definition
      • FAQ
      • Mathematical function
      • Mathematical induction
      • Mathematical object
      • Mathematical object
      • Equivalent definitions of mathematical structures
      • Mathematics
      • Mathematical model
      • mathematical-notation
      • Mathematical pages
      • Mathematical terminology
      • Mathematical adjective
      • Numbers
      • plane
      • Primer: Set Theory
      • Mathematical primitive
      • Set
      • Space
      • theory
      • Variable
  • 200 Set and Set theory
    • Sets: Hierarchy
    • set.TERMS
    • SETS › TOPICS
    • 201 Set concepts
      • Mathematical collections
      • The notion of sets
      • Specification of sets
    • Set cardinality
      • Cardinality of the continuum
      • Cardinality
      • Set Cardinality
      • cardinality2
      • Set cardinality
    • Set operations
      • Disjoint sets
      • Overlapping sets
      • Product
      • set-interactions
      • Set qualities
      • Set relations
    • Set properties
      • Basic set properties
      • Set properties
    • Set relations
      • Basic set relations
      • Disjoint sets
      • Inclusion relation
      • Membership Relation
      • Set membership
    • Summary
      • Set FAQ
      • Sets: Summary
    • Set theories
      • Axiomatic set theory
      • Set Theories
      • Naive Set Theory
      • Morse-Kelley set theory
      • von Neumann-Bernays-Gödel Set Theory
      • Quine's New Foundations (NF)
      • Cantor's set theory
      • Zermelo-Fraenkel set theory
    • Axioms of set theory
      • axiom-of-choice
      • The Axiom of Extensionality
      • Axiom of infinity
      • axiom-of-pairing
      • Axiom of powerset
      • Axiom of Regularity
      • Axiom of replacement
      • Axiom of union
      • Axiom of well-ordering
      • axiom-schema-of-comprehension
      • Axiom Schema of Specification
      • Axioms of set theories
      • List of axioms in set theory
      • ZFC Axioms
    • Sets: Terms
      • Bell Number
      • Cardinal number
      • Class (set theory)
      • Closure
      • empty-set
      • Extended set operations
      • Extensions by definitions
      • Family of sets
      • Fundamental sets
      • fundamental-sets2
      • Georg Cantor
      • History of set theory
      • Implementation of mathematics in set theory
      • Indexed family of sets
      • Extensional and intensional definitions
      • Involution
      • list-of-axioms-of-set-theory
      • Implementation of mathematics in set theory
      • Set membership
      • Naive Set Theory
      • Number of relations
      • empty-relation
      • Set Partitioning
      • Powerset
      • Russell's paradox
      • Set-builder notation
      • Set equivalence
      • Set Notation in latex
      • Set notation
      • Set partition
      • Intensional and extensional set specification
      • Set notation
      • Basic concepts in set theory
      • set-theory
      • Set Types
      • set
      • subset
      • Transfinite number
      • Tuples
      • ur-elements
  • Relations
    • basic-concepts
      • algebraic-axioms
      • Elements of a relation
      • Types of Relations
      • Named Relations
      • Relation theory
      • Relations
      • Types of relations
    • Relations
      • Definitions
      • Reflexivity
      • Symmetry
      • Transitivity
    • relation-properties
      • Uniqueness properties of relations
    • Types of relations
      • Transitivity
      • Binary Relation
      • Congruence relation
      • Connex relation
      • axioms-sets-zfc
      • Endorelation
      • Equivalence relation
      • Euclidean
      • Finitary relation
      • Heterogeneous relation
      • Homogeneous relation
      • Transitivity
      • Partial equivalence relation
      • Transitivity
      • Transitivity
      • Reflexive relation
      • Reflexivity
      • Index of relations
      • Serial relation
      • Symmetry
      • Transitivity
      • Ternary relation
      • Trichotomy
      • Universal relation
      • Well-foundedness
    • terms
      • Relations
      • Binary relation
      • Relations
      • _finitary-rel
      • Relations: Overview
      • Relations
      • Index of relations
      • Binary relations
      • Composition of relations
      • Equivalence class
      • Notation
      • Relation
      • Relations
      • Sets: Summary
      • Aggregation: Sets, Relations, Functions
  • Order theory
    • Order theory
    • List of order structures in mathematics
    • List of order theory topics
    • Order theory
      • Hasse diagram
      • Order theory
      • ordered-set
      • Partial order
      • Partially ordered set
      • Total order
  • Function Theory
    • Function Theory: GLOSSARY
    • Function Theory: HIERARCHY
    • Function Theory: LINKS
    • Function Theory: TERMS
    • Function Theory: TOPIC
    • Function Theory: WIKI
    • _articles
      • about-functions
      • Function
      • Formal definition
      • Definition
      • constant
      • Introduction
      • Types of functions
      • Functions: Summary of Notations
      • Functions: Overview
      • Properties of functions
      • Function properties
      • Functions: Summary
      • Function
    • Abjections
      • Bijective function
      • Function (abjections)
      • Injective function
      • Surjective function
    • topics
      • Function: TERMS
      • Codomain
      • Composition of functions
      • Currying
      • Division of functions
      • Domain
      • Function fixed points
      • Function cardinality
      • Function definition
      • Elements of a function
      • Function in mathematics
      • Function notion
      • Function operations
      • Function properties
      • Functional statements
      • Functions in programing languages
      • Image and Preimage
      • Image
      • Inverse function
      • Notion of functions
      • Number and types of functions between two sets
      • Operation
      • Range
      • Successor function
      • Time complexity classes
  • debrief-name: math section-code: 280 section-name: domain-theory section-desc:
    • Domain theory: LINKS
    • Domain theory
  • Logic
    • Logic: CHRONO TERMS
    • Logic: CLUSTERS
    • lo.GLOSSARY
    • Logic: Wiki links
    • 305-basic-concepts
      • Introduction to Logic
      • Argumentation
      • Logic: Basic terminology
      • Logic: Terminology
      • Truth function
      • Truth function
    • README
      • Mathematical Logic
      • Types of Logic
      • BHK interpretation
      • FOL
      • Index of Logic Forms
      • History of logic
      • Logic Indices
      • Interpretation of symbols in logic and math
      • logic-systems
      • Mathematical Logic: People and Events
      • Index of logical fallacies
      • Logical symbols
      • Mathematical conjecture
      • Mathematical induction
      • Mathematical lemma
      • Mathematical Logic
      • Mathematical proof
      • Mathematical theorem
      • Mathematical theory
      • Monotonicity of entailment
      • Satisfiability Modulo Theories
      • Sequent Calculus
      • Sequent
      • Tableaux
      • Truth tables
    • 360-propositional-logic
      • Propositional Logic
      • Propositional Logic
    • 370-predicate-logic
      • Predicate Logic
      • First-order logic
      • Predicate calculus
      • Examples of predicate formulae
    • 380-proof-theory
      • Argument-deduction-proof distinctions
      • Direct proof
      • Mathematical induction
      • Mathematical induction
      • Mathematical proof
      • Natural deduction
      • Natural deduction
      • Proof by induction
      • Proof by induction
      • proof-calculus
      • Proof Theory
      • Structural induction
      • System L
      • Proof theory
    • Logic: Indices
      • Gödel's Incompleteness Theorem
      • The History of Mathematical Logic
      • forallx
      • Logic for CS
      • Lectures in Logic and Set Theory
      • _logicomix
    • Logic
      • Logical connectives
      • Logical equivalence
    • Rules of Inference
      • WIKI
      • Conjunction elimination
      • Conjunction introduction
      • Cut rule
      • Disjunction elimination
      • Disjunction introduction
      • Disjunctive syllogism
      • Exportation
      • implication-elimination
      • implication-introduction
      • Rules of Inference: Index
      • Rules of inference
      • Rules of Inference for Natural Deduction
      • Logical Inference
      • Reiteration
      • Rule of inference
      • Structural rules
      • substitution
    • Logic
      • The principle of bivalence
      • The principle of explosion
      • The Law of Identity (ID)
      • Laws of thought
      • Properties of logic systems
      • List of laws in logic
      • The law of non-contradiction
    • Logic
      • Logic systems: LINKS
      • Logic system
      • logic-systems
      • logic-typ
      • logics-by-purpose
      • _logics
      • Affine logic
      • Algebraic logic
      • Bunched logic
      • Classical logic
      • Traditional first-order logic
      • Hoare logic
      • Linear logic
      • Modal logic
      • Non-monotonic logic
      • Syntax
      • Predicate logic
      • Propositional Logic
      • Relevance logic
      • Separation logic
      • Substructural logics
      • Syllogistic logic
    • Logic: Sections: Elementary
    • Logic: Topics
      • Pages in Logic
      • Logic ❱ Terms ❱ List
      • Logic ❱ Terms ❱ Definitions
      • Absoluteness
      • Assumption
      • Automated theorem proving
      • Canonical normal form
      • Categorical proposition
      • Classical linear logic
      • Consequence
      • Decidability
      • Deduction systems
      • deduction-theorem
      • Deductive reasoning
      • Diagonal lemma
      • Fallacy
      • Fitch notation
      • Formal language
      • formal-system
      • Formalism
      • Formula
      • functionally complete
      • Hilbert system
      • Hoare logic
      • horn-clause
      • Mathematical induction
      • Induction
      • Inductive Reasoning
      • Intuitionistic logic
      • Intuitionistic logic
      • Intuitionistic logic
      • Judgement
      • Judgments
      • Linear logic
      • Logic in computer science
      • Logic
      • Logical connective
      • Logical consequence
      • Logical constant
      • Logical form
      • axioms-sets
      • Logical reasoning
      • Ludics
      • Non-logical symbol
      • Predicate
      • Premise
      • Quantification
      • Realizability
      • Boolean satisfiability problem
      • DPLL algorithm
      • Satisfiability
      • Semantics of logic
      • Skolemization
      • SAT and SMT
      • Syntax
      • Tautology
      • Term
      • Unification
      • Validity
  • 510 Lambda Calculi
    • Lambda Calculus: GLOSSARY
    • Lambda calculi: LINKS
    • Lambda Calculus: OUTLINE
    • Lambda Calculus: Basic concepts
      • Introduction
      • Lambda expressions
      • Free variables
    • Lambda Calculi
      • Lambda calculus: LINKS
      • Lambda calculus combinators in Haskell
      • Lambda calculus: Combinators
      • Combinators
      • combos-all.js
      • combos-bird.js
      • combos-birds-list.js
      • combos-birds.js
      • Fixed-point combinator
      • Fixpoint operator
      • Lambda calculus: Fixpoint
    • combinatory-logic
      • algebraic-structures
      • Combinatory logic
      • Combinatory logic
      • relation-classification
      • 04-definition
    • Lambda calculus encoding schemes
      • bohm-berarducci-encoding
      • Index of Church encodings
      • Church encodings
      • Church Numerals
      • Encoding data structures
      • Encoding schemes in lambda calculi
      • Lambda encoding
      • Mogensen-Scott encoding
      • Parigot encoding
      • encodings
        • Encoding data structures
        • Encoding of Data Types in the λ-calculus
        • church-booleans
        • Church data structures
        • Church encoding
        • Church Numerals: Church encoding of natural numbers
        • Lambda Calculus: Church encoding
        • Lambda Calculus: Church encoding
        • church-numerals
        • Lambda Calculus: Church encoding: Numerals
        • Church pair
        • Pair
        • Lambda Calculus: Church encoding
        • Alternative encodings
        • Encoding schemes
        • Encoding schemes
        • Encodings in Untyped Lambda Calculus
        • Lambda calculus
        • Scott encoding
        • Lambda calculus: Scott encoding
    • lambda-calculus-evaluation
      • Call-by-name
      • Call-by-need
      • Call-by-value
    • lambda-calculus-forms
      • Beta normal form
      • Lambda terms
      • Fixity of lambda-terms
    • lambda-calculus-reductions
      • Alpha conversion
      • Beta reduction
      • Delta reduction
      • Eta conversion
      • Eta conversion
      • Lambda calculus: η-conversion
    • lambda-calculus
      • Alonzo Church
      • Inference rules for lambda calculus
      • Lambda Calculus: Introduction
      • Lambda abstraction
      • Lambda application
      • Lambda Calculus: Definition
      • About λ-calculus
      • Type inference
      • Lambda Calculus
      • Lambda Calculus: Introduction
      • Introduction to λ-calculus
      • Lambda calculus
      • Definition of Lambda Calculus
      • Functions in lambda calculus
      • History of Lambda Calculus
      • Using the Lambda Calculus
      • Name capturing
      • Variable occurrences
      • Variables
    • Lambda Calculus
      • Church-Rosser theorem
      • Curry's paradox
      • De Bruijn index
      • de Bruijn notation
      • Deductive lambda calculus
      • Kleene-Rosser paradox
      • Aspects of the lambda calculus
      • Function Refactoring
      • Lambda lifting
      • Let expression
      • Reduction strategy
      • Substitution
    • typed-lambda-calculi
      • Lambda Cube
      • Simply typed lambda calculus
      • System F
      • Typed lambda calculi
  • Type theory
    • Type Theory: GLOSSARY
    • Type theorists
    • Type Theory: SUMMARY
    • TERMS: Type Theory
      • Types
      • History of type theory
      • History of Type Theory
    • curry-howard-correspondence
      • The Curry-Howard Correspondence in Haskell
      • Curry-Howard correspondence
      • Curry-Howard correspondence
      • Curry-Howard correspondence
      • Curry-Howard-Lambek correspondence - HaskellWiki
    • dependent-types
      • Dependent type
      • Dependent type
    • Hindley-Milner type-system
      • Hindley-Milner type system
      • Monomorphism vs polymorphism
      • Let-polymorphism
      • The Hindley-Milner type system
      • Algorithm W in Haskell
      • Hindley-Milner Type Inference: W Algorithm
      • hindley–milner-type-system
      • Hindley-Milner type system
      • HM inference examples
      • HM in ML
      • Type Inference
    • Homotopy type theory
      • Homotopy type theory
      • Univalent Type theory as the foundations of mathematics
    • Intuitionistic type theory
      • Inductive definition
      • Inductive type
      • Intuitionistic type theory
    • Type Theory
      • TTTools
      • Coinduction
      • Impredicativity
      • Lean
      • Subsumption
    • Type Theory : Topics
      • Type Theory : Terms
      • Recursion types
      • Recursive data type
      • Subtyping
      • Type Class
      • Type Equivalence
      • Type Inference
      • Type rule
      • Type system
      • Variance
    • type-theories
      • Calculus of Constructions
      • Constructive type theory
      • ramified-type-theory
      • simple-type-theory
      • Substructural type systems
    • type-theory-general
      • Linear types
      • History of Type Theory
      • Type Theory
      • Overview
      • Type Theory
  • Abstract Algebra
    • 410-group-theory
      • Abelian group
    • algebras
      • Associative Algebra
      • Field
      • Group-like algebraic structures
      • group
      • Lattice
      • Magma
      • monoid
      • Overview of Algebras
      • Quasigroup
      • Rack and quandle
      • Ring
      • Semigroup
      • Algebra of sets
      • Setoid
    • boolean-algebra
      • Boolean algebra
      • Axioms in Boolean Algebra
      • Boolean algebra
      • Boolean Algebra Laws
      • Boolean Algebra Laws
      • Two-element Boolean algebra
      • Boolean algebra
      • Boolean domain
    • terms
      • Algebra
      • Axioms of abstract algebra
      • Algebraic notation for algebraic data types
      • Algebraic structure
      • Algebraic structure
      • Field of sets
      • Homomorphism
      • Isomorphism
      • Algebraic structures
      • Mathematical structure
      • Polynomials
      • Relation algebra
  • Category Theory
    • CT GLOSSARY
    • Category Theory: OUTLINE
    • CT SUMMARY
    • A First Introduction to Categories (2009)
      • Sets, maps, composition
      • 02-history
      • axioms-logic
      • Bijection of functions
      • Commutative diagram
      • Directed graph
      • CT prerequisites
      • String diagram
      • Transitive closure
    • Category Theory Fundamentals
      • Introduction
      • Interpretation
      • Fundamental concepts
      • Category theory
      • Category
      • Category Theory: Definitions
    • Key concepts
      • Duality
      • Functor
      • Homeset
      • Initial Object
      • Morphism
      • Natural transformation
      • Object
      • Terminal Object
    • Categorical constructions
      • Categorical constructions
      • Coproduct
      • Diagram
      • Product
      • Universal construction
    • Types of categories
      • Concrete category
      • Discrete category
      • Functor category
      • Groupoid
      • Hask
      • Kleisli category
      • Locally small category
      • Monoid
      • monoidal-categories.md
      • Index of named categories
      • Opposite category
      • Ordered category
      • Set category
      • Small category
      • Subcategory
    • Types of Functors
      • Adjoint functor
      • relation-arity
      • Endofunctor
      • Faithful functor
      • Forgetful functor
      • Hom functor
      • Identity functor
      • Inverse functor
      • Monad
      • Powerset functor
    • Types of Morphisms
      • Anamorphism
      • Automorphism
      • Catamorphism
      • Endomorphism
      • Epimorphism
      • Homomorphism
      • Hylomorphism
      • Idempotent morphism
      • Identity morphism
      • Inverse morphism
      • Isomorphism
      • Metamorphism
      • monomorphism
      • Natural isomorphism
      • Split morphism
    • 20-advanced-concepts
      • Coalgebra
      • (Co)Inductive types
      • Recursion Schemes
    • Category Theory
      • Category Theory: TERMS
      • Algebraic Data Types
      • Category Theory
      • Category
      • Coproduct
      • Function type
      • Functoriality
      • Initial Object
      • Limits and Colimits
      • Natural Transformation
      • 5. Products
      • Terminal Object
    • Category Theory :: Contents
      • CT :: Links
      • Category Theory :: Terms
      • Category :: Definition
      • F-Algebra
      • Functor
      • Initial object
      • Monoid
      • Natural Transformation
      • Number of morphisms
      • Terminal object
      • Transitive closure
      • Types of morphisms
      • Categories by cardinality
      • Types of functors
  • Number Theory
    • Invariance and Monovariance Principle
    • 615-arithmetic
      • Addition
      • Aliquot sum
      • Arithmetic function
      • Laws
      • Arithmetic operations
      • Index of arithmetic operations
      • Arithmetic operations
      • Arithmetic
      • Divisibility rules
      • Divisibility
      • division
      • Divisor Function
      • Divisor Summatory Function
      • Divisor
      • Euclidean division
      • Hyperoperations
      • hyperops
      • Modular arithmetic
      • Multiplication
      • Number Theory: primer in numbers
      • Percentage
      • Rules of Divisibility
      • Subtraction
    • The fundamental sets of numbers
      • Algebraic numbers
      • Complex numbers
      • Fractions
      • Fundamental number sets
      • Imaginary numbers
      • Integers
      • Irrational numbers
      • Natural number
      • Rational numbers
      • Real numbers
      • Transcendental numbers
      • Ulam's spiral
      • The whole numbers
    • COUNTING THEORY
      • Counting Theory
      • counting
      • Fundamental Counting Rules
    • 630-combinatorics
      • Combinatorics
      • Combinations
      • Combinatorics
      • Counting theory
      • Counting theory
      • Enumerative combinatorics
      • Partition
      • Pascals triangle
      • Permutations
      • Twelvefold way
    • Probability theory
      • Statistics › Probability theory: Glossary
      • Statistics › Probability theory › Topics
      • Statistics › Probability theory › Wiki Links
      • Conditional Probability
      • Distribution
      • Probability theory
      • Probability
    • Number theory
      • euclids-lemma
      • gcd-lcm
      • Induction
      • Infinity
      • Numbers and numerals with interesting properties
      • Lagrange's four-square theorem
      • Matrix
      • Matrix
      • List of Number Systems
      • Number Theory
      • Number Theory with Glenn Olsen
      • Number
      • Arithmetic
      • Numbers
      • numeral-prefixes
      • Numeral system
      • Numeral
      • Ordinal numbers
      • Parity
      • Peano axioms
      • Polynomial
      • Polynomial
      • Positional notation
      • Probability
      • Symbol
      • Well Ordering Principle
    • topics
      • Coprimality
      • Facorization of composite numbers
      • Fundamental Theorem of Arithmetic
      • Prime factorisation
      • Prime number
      • Prime numbers
  • Theory of computation
    • Theory of computation: Abbreviations
    • Theory of computation: CHRONOLOGICAL TOPICS
    • Theory of computation: GLOSSARY
    • Theory of Computation: HIERARCHY
    • Theory of computation: LINKS
    • Theory of computation: TERMS
    • Theory of computation: TOPICS
    • Theory of computation: WIKI
    • Theory of Computation
      • _toc-more
      • Theory of Computation
    • 610-automata-theory
      • Abstract machine
      • Automata Theory
      • Automaton
      • Edit distance
      • Finite-state Machine
      • Automata Theory: WIKI
    • Formal systems
      • Abstract interpretation
      • Alphabet
      • Binary combinatory logic
      • Chomsky hierarchy
      • Epsilon calculus
      • Formal language
      • Iota and Jot
      • Regular expression
      • Regular Language
      • SKI combinator calculus
    • 621-grammar
      • Backus-Naur Form (BNF)
      • Context-free grammar
      • Context-sensitive grammar
      • Extended Backus–Naur Form (EBNF)
      • Regular Language
      • Terminal and nonterminal symbols
    • 622-syntax
      • Syntax
    • 624-semantics
      • Axiomatic semantics
      • Denotational Semantics: Summary
      • Denotational Semantics
      • Denotational Semantics
      • Denotational semantics
      • Formal semantics
      • Operational semantics
      • Semantics in CS
      • Semantics
    • 630-computability-theory
      • Computability (recursion) theory: TERMS
      • Computability (recursion) theory: TOPICS
      • Effective Computability
      • Church Thesis
      • Church-Turing Thesis
      • Computability theory
      • Computability
      • Computable function
      • Entscheidungsproblem
      • Halting problem
      • Machine that always halts
      • McCarthy Formalism
      • Super-recursive algorithm
      • Recursion theory
    • 632-recursive-function-theory
      • Recursion Theory
      • Ackermann function
      • General recursive function
      • Minimization operator
      • Partial functions
      • Recursion Function Theory
      • Sudan function
    • 634-primitive-recursive-functions
      • Primitive Recursive Function
      • Initial functions
      • The list of primitive recursive functions
      • Primitive combination
      • Primitive composition
      • Primitive recursion
      • Successor function
    • 640-models-of-computation
      • Models of computation: Summaries
      • Model of computation
    • 680-complexity-theory
      • Algorithmic Complexity
      • Complexity Theory
  • debrief-name: math section-code: 900 section-name: aggregations section-desc: Aggregations, indices,
    • Index of closures
    • List of mathematical entities
    • List of mathematical objects
    • Enumeration of mathematical structures
    • Math : Axioms as Formulae
    • 950-math-areas
      • Areas of mathematics
      • Areas of mathematics
    • 970-links
      • check
      • Math: Links
      • Math Debrief: Links
      • Math Primer: LINKS
      • Links
      • Math: LINKS: ncatlab
      • Math: LINKS
      • WIKI
      • WIKI
      • WIKI_ALL
      • Math: Wiki lists
      • Glossary of areas of mathematics
      • WIKI_collections
      • Mathematics for Computer Science
      • Mathematics Classification
      • math
      • Resources
      • Math on YouTubel Video Playlists
      • wiki resources
    • 980-hierarchy
      • HIERAR
      • Math: Hierarchy
      • Math HIERARCHY
      • classification
        • Mathematics
        • https://ncatlab.org/nlab/all_pages https://ncatlab.org/nlab/all_pages/reference https://ncatlab.org/
        • Math Classification and Topical Pages
        • Areas of mathematics
        • Areas of mathematics
        • Math Classification: CCS
        • Math hierarchy
        • Computational mathematics
        • Taxonomy: Mathematics
        • Areas of mathematics
        • Mathematics Subject Classification
        • Math fields
        • math-topics
        • Mathematics Subject Classification – MSC
        • MSC Classification Codes
        • mss-top-levels-filenames
        • MSC classification: Top Levels
        • Math classification
    • 990-appendix
      • Math glossary at ENCYCLOPÆDIA BRITANNICA
      • Bibliography
      • Math: Abbreviations
      • math.GLOSSARY
    • Math : Canon
      • Main branches of mathematics
      • Enumeration: Math paradigms
      • enum-math-symbols
      • List of mathematical theories
      • enum-algebras
        • Group-like algebraic structures
        • Group
        • Groupoid
        • magma
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  1. Relations
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Relations

Cartesian product

If XXX and YYY are sets, then their Cartesian product X×YX \times YX×Y is the set of all ordered pairs (x,y)(x,y)(x,y) with x∈Xx\in Xx∈X and y∈Yy\in Yy∈Y, denoted by:

X×Y={∀x,∀y∣(x,y)→x∈X∧y∈Y}X \times Y = \{\forall x, \forall y \mid (x,y) \to x\in X \land y\in Y\}X×Y={∀x,∀y∣(x,y)→x∈X∧y∈Y}

It is possible that X=YX = YX=Y, in which case there is only a single set, XXX, and the dot product is between itself:

X×X=X2={(x,x)}X \times X = X^2 = \{(x, x)\}X×X=X2={(x,x)} so the elements of the dot product have the template (x,x)(x, x)(x,x).

Relations

The dot product is also a relation, called the total relation. Each and every element of the domain set is combined with each and every element of the codomain set. However, the elements of the dot product (thereby, of any relation also) are not the same as the elements of either domain or codomain, but a new structure, called an ordered pair is formed and denoted as (x,y)(x,y)(x,y).

In an ordered pair, (x,y)(x,y)(x,y), the xxx is an element of the domain - the elements of the domain always make the first component of an ordered pair. The yyy comes from the codomain - the elements of the codomain always make the second component of an ordered pair.

An ordered pair, (x,y)(x, y)(x,y), consists of two components, xxx and yyy, with xxx, always a member of the domain, in place of the first component, and yxyxyx, always a member of the codomain, in place of the second component.

For example, let a relation RRR between two sets A={1,2,3}A = \{1,2,3\}A={1,2,3} and B={a,b}B = \{a,b\}B={a,b} be the total relation i.e. their dot product; it is denoted by:

R={(1,a),(1,b)(2,a),(2,b)(3,a),(3,b)}R = \{ \\ \quad (1,a), (1,b) \\ \quad (2,a), (2,b) \\ \quad (3,a), (3,b) \\ \}R={(1,a),(1,b)(2,a),(2,b)(3,a),(3,b)}

These two sets have a large number of possible relations - every relation between the empty relation and their total relation is a possible relation.

There is only one empty relation, in general, regardless of the sets involved.

And, between any two sets, there is only one total relation; that is, there's only one provided the two sets don't exchange their roles of domain and codomain (if they are allowed to do so, then it could be said there are two total relations between any two sets).

In between these two extremes are all other possible relations. Each one is made up of an arbitrary number of elements (i.e ordered pairs). Two relations, just like sets, are equal if all their members are equal, that is, iff one is the subset of the other and vice versa.

The relations are classified according to several criteria; the most fundamental kinds of relations are determined by the presence/absence of the certain elements.

A relation is a subset of Cartesian product. In fact, the Cartesian product is the total relation, and any other relation between two sets is a subset of their dot product.

Considering a relation on a set, e.g. <<< on the set N\mathbb{N}N, and considering the set of all pairs of numbers (n,m)(n,m)(n,m) where n<mn < mn<m, i.e.

R={(n,m):n,m∈N,n<m}R=\{(n,m):n,m \in \mathbb{N}, n \lt m\}R={(n,m):n,m∈N,n<m},

then there is a close connection between the number nnn being less than a number mmm and the corresponding pair (n,m)(n,m)(n,m) being a member of RRR, namely, n<mn < mn<m iff (n,m)∈R(n,m) \in R(n,m)∈R. In a sense, we can consider the set RRR to be the <<< relation on the set N\mathbb{N}N.

A binary relation on a set XXX is a subset of X2X^2X2. If R⊆X2R \subseteq X^2R⊆X2 is a binary relation on XXX and x,y∈Xx, y \in Xx,y∈X, we write xRyxRyxRy (or RxyRxyRxy) for (x,y)∈R(x,y) \in R(x,y)∈R.

The set N2=N×N\mathbb{N^2} = \mathbb{N} \times \mathbb{N}N2=N×N of ordered pairs of natural numbers (starting and ending curly-braces elided):

(0,0),(0,1),(0,2),(0,3),…(1,0),(1,1),(1,2),(1,3),…(2,0),(2,1),(2,2),(2,3),…(3,0),(3,1),(3,2),(3,3),…⋮⋮⋮⋮⋱fig.1\begin{matrix} (0,0), & (0,1), & (0,2), & (0,3), & \dots \\ (1,0), & (1,1), & (1,2), & (1,3), & \dots \\ (2,0), & (2,1), & (2,2), & (2,3), & \dots \\ (3,0), & (3,1), & (3,2), & (3,3), & \dots \\ \vdots & \vdots & \vdots & \vdots & \ddots \end{matrix}\\ \text{fig.1}(0,0),(1,0),(2,0),(3,0),⋮​(0,1),(1,1),(2,1),(3,1),⋮​(0,2),(1,2),(2,2),(3,2),⋮​(0,3),(1,3),(2,3),(3,3),⋮​…………⋱​fig.1

The subset of all pairs resting on the diagonal (fig.1), (0,0),(1,1),(2,2),…(0,0), (1,1), (2, 2),\dots(0,0),(1,1),(2,2),…, is the identity relation on N\mathbb{N}N, Id={(x,x):x∈X}I_{d}=\{(x,x):x \in X\}Id​={(x,x):x∈X} for any set XXX.

The subset of all pairs above the diagonal, L={(0,1),(0,2),(0,3),…,(1,2),(1,3),…,(2,3),… }L=\{(0,1), (0,2), (0,3),\dots , (1,2), (1,3), \dots , (2,3), \dots\}L={(0,1),(0,2),(0,3),…,(1,2),(1,3),…,(2,3),…}, is the less than relation, nLm  ⟺  n<mnLm \iff n \lt mnLm⟺n<m (the symbol   ⟺  \iff⟺ means "if and only if").

The subset of all pairs below the diagonal is the greater than relation, nGm  ⟺  n>mnGm \iff n \gt mnGm⟺n>m.

The union Le=L∪IdL_{e} = L \cup I_{d}Le​=L∪Id​ is the less than or equal to relation, nLem  ⟺  n≤mnL_{e}m \iff n \le mnLe​m⟺n≤m.

The union Ge=G∪IdG_{e} = G \cup I_{d}Ge​=G∪Id​ is the greater than or equal to relation, nGem  ⟺  n≥mnG_{e}m \iff n \ge mnGe​m⟺n≥m.

L,G,Le,GeL, G, L_{e}, G_{e}L,G,Le​,Ge​ are special kinds of relations called orders.

LLL and GGG have the property that no number bears LLL or GGG to itself i.e. for all nnn, neither nLnnLnnLn nor nGnnGnnGn (e.g. neither 3 < 3 nor 3 > 3). Relations with this property are called irreflexive, and, if they also happen to be orders, they are called strict orders.

Any subset of X2X^2X2 is a relation on XXX. Even the empty set is a relation on any set i.e. the empty relation, which consists of no pairs.

Also X2X^2X2 itself is a relation on XXX called universal (full) relation, consisting of all the pairs of the Cartasian product X×XX \times XX×X.

Special Properties of Relations

Some relations are so common they've been given special names. For instance, ≤\le≤ and ⊆\subseteq⊆ both relate their respective domains (e.g. N\mathbb{N}N in the case of ≤\le≤, and P(X)\mathcal{P}(X)P(X) in the case of ⊆\subseteq⊆) in similar ways.

To get at exactly how these relations are similar, and how they differ, we categorize them according to some special properties that relations can have. It turns out that (combinations of) some of these special properties are especially important, like orders and equivalence relations.

Reflexivity

A relation is reflexive if every element of the set is related to itself: a relation R⊆X2R \subseteq X^2R⊆X2 is reflexive iff ∀x∈X:(x,x)∈R\forall x \in X:(x,x) \in R∀x∈X:(x,x)∈R. To be reflexive, e.g. a relation RRR to the set N2\mathbb{N^2}N2 (see fig.1), must contain all the identity (diagonal) pairs; it may also contain other, non-identity pairs.

Examples

  • the empty relation is not reflexive

  • the universal relation is reflexive

A relation is irreflexive if it doesn't conatin no identity (diagonal) pairs (it may contain other pairs as long as they're not id pairs). Irreflexive relation, R⊆X2R \subseteq X^2R⊆X2, is denoted by ∀x∈X:(x,x)∉R\forall x \in X : (x,x)\not \in R∀x∈X:(x,x)∈R.

Examples

  • the empty relation is irreflexive

  • the universal relation is not irreflexive

Transitivity

A relation is transitive iff, whenever xRyxRyxRy and yRzyRzyRz, then also xRzxRzxRz. To be transitive a relation must satisfy this condition: if it contains the pair (x,y) AND the pair (y,z), ONLY THEN it must also contain the pair (x,z). This means that empty relation is transitive. A relation that contains the pairs (x,y) and (x,z) is transitive as well.

Only when it contains (x,y) and (y,z) i.e. only when there's a connecting element: an element that's the second entry in one pair and the first entry in another, then it must also contain the pair (x,z) in order to be a transitive relation.

Examples

  • the empty relation is transitive

  • the universal relation is transitive

Symmetry

A relation R⊆X2R \subseteq X^2R⊆X2 is symmetric iff, whenever xRyxRyxRy, then also yRxyRxyRx. If a relation contains a pair (x,y) then it must also contain a pair (y,x) to be symmetric.

Examples

  • the empty relation is symmetric

  • the universal relation is symmetric

  • a relation conting only a pair (1,1) is symmetric

A relation R⊆X2R \subseteq X^2R⊆X2 is anti-symmetric iff, whenever both xRyxRyxRy and yRxyRxyRx then x=yx = yx=y; i.e. if x≠yx \neq yx=y then either ¬xRy\lnot xRy¬xRy or ¬yRx\lnot yRx¬yRx. If a relation contains a pair (x,y) then it must not also contain a pair (y,x) in order to be anti-symmetric, unless that pair was (x,x) to begin with.

Examples

  • the empty relation is anti-symmetric

  • a relation conting only a pair (1,2) is anti-symmetric

  • a relation conting only a pair (1,1) is anti-symmetric, even though it acually contains (x,y) and (y,x), but since x=y, it is allowed.

  • In a symmetric relation, xRy and yRx always hold together, or neither holds.

  • In an anti-symmetric relation, the only way for xRy and yRx to hold is if x = y. This doesn't require that xRy and yRx hold when x = y, only that it isn't ruled out.

  • an anti-symmetric relation can be reflexive

  • not every anti-symmetric relation is reflexive

  • being anti-symmetric and not being symmetric are different conditions

  • a relation can be both symmetric and anti-symmetric at the same time, e.g. the identity relation.

Connectivity: A relation R⊆X2R \subseteq X^2R⊆X2 is connected if ∀x,y∈X:x≠y\forall x,y \in X:x \neq y∀x,y∈X:x=y, then either xRyxRyxRy or yRxyRxyRx.

Partial order: A relation R⊆X2R \subseteq X^2R⊆X2 that is reflexive, transitive, and anti-symmetric.

Linear order: A partial order that is also connected.

Equivalence relation: A relation R⊆X2R \subseteq X^2R⊆X2 that is reflexive, symmetric and transitive.

Orders

Very often we are interested in comparisons between objects, where one object may be less or equal or greater than another in a certain respect. Size is the most obvious example of such a comparative relation, or order. But not all such relations are alike in all their properties. For instance, some comparative relations require any two objects to be comparable, others don’t. (If they do, we call them linear or total). Some include identity (like ≤) and some exclude it (like <).

  • Preorder: a relation which is both reflexive and transitive; Re+Tr

  • Partial order: a preorder which is also anti-symmetric; Re+vS+Tr

  • Linear (total) order: a partial order which is also connected. Re+vS+Tr+Co

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