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Huan Long

Shanghai Jiao Tong University

Basic set theory

Relation

Function

Spring 2018

Georg Cantor(1845-1918)oGerman mathematiciano Founder of set theory

Bertrand Russell(1872-1970)oBritish philosopher, logician, mathematician, historian, and social critic.

Ernst Zermelo(1871-1953)oGerman mathematician, foundations ofmathematics and hence on philosophy

David Hilbert (1862-1943)o German mathematicia, one of the most influential and universal mathematicians of the 19th and early 20th centuries.

Kurt Gödel(1906-1978) oAustrian American logician, mathematician, and philosopher. ZFC not ⊢ ¬CH .

Paul Cohen(1934-2007)oAmerican mathematician, 1963: ZFC not ⊢ CH,AC .

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By Georg Cantor in 1870s:

A set is an unordered collection of objects.◦ The objects are called the elements, or members, of the set. A set is

said to contain its elements.

Notation: 𝑎𝑎 ∈ 𝐴𝐴◦ Meaning that: 𝑎𝑎 is an element of the set A, or,

Set A contains 𝑎𝑎 .

Important: ◦ Duplicates do not matter. ◦ Order does not matter.

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a∈A a is an element of the set A. a∉A a is NOT an element of the set A. Set of sets {{a,b},{1, 5.2}, k} ∅ the empty set, or the null set, is set that has no elements. A⊆B subset relation. Each element of A is also an element of B. A=B equal relation. A⊆B and B⊆A. A≠B A⊂B strict subset relation. If A⊆B and A≠B |A| cardinality of a set, or the number of distinct elements. Venn Diagram

Spring 2018UV UBA

𝑎𝑎 ∈ {𝑎𝑎, 𝑒𝑒 𝑖𝑖, 𝑜𝑜,𝑢𝑢} a ∉{{a}} ∅ ∉∅ ∅ ∈ ∅ ∈ {{∅}} {3,4,5}={5,4,3,4} ∅⊆S ∅ ⊂{∅ } S ⊆S |{3, 3, 4, {2, 3},{1,2,{f}} }|=4

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Union IntersectionDifferenceComplementSymmetric differencePower set

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Definition Let A and B be sets. The union of the sets A and B, denoted by A∪B, is the set that contains those elements that are either in A or in B, or both.

A U B={x | x∈A or x∈B} Example: {1,3,5} U {1,2,3}={1,2,3,5} Venn Diagram representation

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A UUB

Definition Let A and B be sets. The intersection of the sets A and B, denoted by A ∩ B, is the set that containing those elements in both A and B.

A ∩ B={x | x∈A and x∈B} Example: {1,3,5} ∩ {1,2,3}={1,3} Venn Diagram Representation

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A B

Definition Let A and B be sets. The difference of the sets A and B, denoted by A - B, is the set that containing those elements in A but not in B.

𝐴𝐴 − 𝐵𝐵 = 𝑥𝑥 𝑥𝑥 ∈ 𝐴𝐴 𝑏𝑏𝑢𝑢𝑏𝑏 𝑥𝑥∉𝐵𝐵} = 𝐴𝐴 ∩ �𝐵𝐵 Example: {1,3,5}-{1,2,3}={5} Venn Diagram Representation

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A UUB

Definition Let U be the universal set. The complement of the sets A, denoted by �̅�𝐴 or −𝐴𝐴, is the complement of with respect to U.

�̅�𝐴 = 𝑥𝑥 𝑥𝑥∉𝐴𝐴} = 𝑈𝑈 − 𝐴𝐴 Example: -E = O Venn Diagram Representation

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UA

Definition Let A and B be sets. The symmetric difference of A and B, denoted by A ⊕ B, is the set containing those elements in either A or B, but not in their intersection.

A ⊕ B={x| (x∈A ∨ x∈B) ∧ x∉ A∩B }=(A-B)∪(B-A)

Venn Diagram: A ⊕ B A ⊕ B ⊕ c

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A B ?

Many problems involves testing all combinations of elements of a set to see if they satisfy some property. To consider all such combinations of elements of a set S, we build a new set that has its members all the subsets of S.

Definition: Given a set S, the power set of S is the set of all subsets of the set S. The power set of S is denoted by P(S) or ℘S .

Example: o P({0,1,2})={ɸ, {0},{1},{2}, {0,1},{0,2},{1,2},{0,1,2} }o P(∅)={∅}o P({∅})={∅,{∅}}

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1. Identity laws

2. Domination laws

3. Idempotent laws

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4. Complementation law

5. Commutative laws

6. Associative laws

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7. Distributive laws

8. De Morgan’s laws

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Basic set theory

Relation

Function

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In set theory {1,2}={2,1}What if we need the object <1,2> that will

encode more information:o 1 is the first componento 2 is the second component

Generally, we say<x, y> =<u, v> iff x=u ∧ y=v

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A×B={<x,y> | x∈A ∧ y ∈B } is the Cartesian product of set A and set B.

ExampleA={1,2} B={a,b,c}A×B={<1,a>,<1,b>,<1,c>,

<2,a>,<2,b>,<2,c>}

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Definition A relation is a set of ordered pairs. Examples

o <={<x,y>∈R×R| x is less than y}o M={<x,y> ∈People× People| x is married to y}

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R

dom R

ran

R

A relation as a subset of the plane

Let 𝑅𝑅 denote any binary relation on a set 𝑥𝑥, we say: 𝑅𝑅 is reflexive, if (∀𝑎𝑎 ∈ 𝑥𝑥)(𝑎𝑎𝑅𝑅𝑎𝑎); 𝑅𝑅 is symmetric, if (∀𝑎𝑎, 𝑏𝑏 ∈ 𝑥𝑥)(𝑎𝑎𝑅𝑅𝑏𝑏 → 𝑏𝑏𝑅𝑅𝑎𝑎); 𝑅𝑅 is transitive , if ∀𝑎𝑎, 𝑏𝑏, 𝑐𝑐 ∈ 𝑥𝑥 [ 𝑎𝑎𝑅𝑅𝑏𝑏 ∧ 𝑏𝑏𝑅𝑅𝑐𝑐 → (𝑎𝑎𝑅𝑅𝑐𝑐)];

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Definition 𝑅𝑅 is an equivalence relation on 𝐴𝐴 iff𝑅𝑅 is a binary relation on 𝐴𝐴 that iso Reflexive o Symmetrico Transitive

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Definition A partition π of a set A is a set of nonempty subsets of A that is disjoint and exhaustive. i.e.(a) no two different sets in π have any

common elements, and(b) each element of A is in some set in π.

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If R is an equivalence relation on A, then the quotient set (equivalence class) A/R is defined as

A/R={ [x]R | ∈A }Where A/R is read as “A modulo R”

The natural map (or canonical map)α:A→A/R defined by

α(x)= [x]R

Theorem Assume that R is an equivalence relation on A. Then the set {[x]R |x ∈A} of all equivalence classes is a partition of A.

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X R Y

X’ R Y’

Let 𝜔𝜔 = {0,1,2, … }; and 𝑚𝑚 ∼ 𝑛𝑛 ⇔𝑚𝑚− 𝑛𝑛 is divisible by 6.Then ∼ is an equivalence relation on 𝜔𝜔. The quotient set ⁄𝜔𝜔 ∼ has six members:

0 = 0,6,12, … ,1 = 1,7,13, … ,……5 = 5,11,17, …

Clique (with self-circles on each node) : a graph in which every edge is presented. Take the existence of edge as a relation. Then the equivalence class decided by such relation over the graph would be clique.

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Linear order/total ordero transitiveo trichotomy

Partial ordero reflexiveo anti-symmetrico transitive

Well order o total ordero every non-empty subset of S has a least element in this ordering.

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Basic set theory

Relation

Function

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Definition A function is a relation F such that for each x in dom F there is only one y such that x F y. And y is called the value of F at x.

Notation F(x)=y Example f(x) = x2 f : R → R, f(2) = 4, f(3) = 9, etc. Composition (f∘g)(x)=f(g(x)) Inverse The inverse of F is the set

𝐹𝐹−1={<u,v> | v F u} 𝐹𝐹−1 is not necessarily a function (why?)

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We say that F is a function from A into B or that F maps A into B (written F: A→B) iff F is a function, dom F=A and ran F⊆B.o If, in addition, ran F=B, then F is a function from A onto

B. F is also named a surjective function.o If, in addition, for any x∈dom F, y∈dom F, with x≠y,

F(x)≠F(y), then F is an injective function. or one-to-one (or single-rooted).

o F is bijective function : f is surjective and injective.

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Main Referenceso Herbert B. Enderton, Elements of Set Theory, ACADEMIC

PRESS, 1977 o Yiannis Moschovakis, Notes on Set Theory (Second

Edition), Springer, 2005o Keith Devlin, The Joy of Sets: Fundamentals of

Contemporary Set Theory, Springer-Verlag, 1993o Kenneth H. Rosen, Discrete Mathematics and Its

Applications (Sixth Edition), 2007o 沈恩绍,集论与逻辑,科学出版社,(集合论部分),

2001

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Huan LongShanghai Jiao Tong University

•Paradox and ZFC Paradox

•EquinumerosityEquinumerosity

•Ordering Cardinal Numbers

•Countable setsInfinite Cardinals

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Naive set theory

Paradox

Axiomatic set theory

Modern set theory

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Russell`s paradox(1902) Bertrand Russell(1872-1970) British philosopher, logician, mathematician,

historian, and social critic. In 1950 Russell was awarded the Nobel Prize in

Literature, "in recognition of his varied and significant writings in which he champions humanitarian ideals and freedom of thought."

What I have lived for? Three passions, simple but overwhelmingly strong, have governed my life: the longing for love, the search for knowledge, and unbearable pity for the suffering of mankind.…

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Barber Paradox

Suppose there is a town with just one male barber. The barber shaves all and only those men in town who do not shave themselves.

Question: Does the barber shave himself? If the barber does NOT shave himself, then he MUST abide by the

rule and shave himself. If he DOES shave himself, according to the rule he will NOT shave

himself.

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Formal Proof

Theorem There is no set to which every set belongs.[Russell, 1902]

Proof: Let A be a set; we will construct a set not belonging to A. Let

B={x∈A | x∉x}We claim that B∉A. we have, by the construction of B.

B∈B iff B∈A and B∉BIf B∈A, then this reduces to

B∈B iff B∉B, Which is impossible, since one side must be true and the other false. Hence B∉A

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Natural Numbers in Set Theory

∙ Constructing the natural numbers in terms of sets is part of the process of

“Embedding mathematics in set theory”

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John von Neumann

∙ December 28, 1903 – February 8, 1957. Hungarian American mathematician who made major contributions to a vast range of fields:

∙ Logic and set theory∙ Quantum mechanics∙ Economics and game theory∙ Mathematical statistics and econometrics∙ Nuclear weapons∙ Computer science

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Natural numbers

∙ By von Neumann: Each natural number is the set of all smaller natural numbers.

0= ∅1={0}={∅}2={0,1}={∅, {∅}}3={0,1,2}={∅, {∅}, {∅, {∅}}}……

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Some properties from the first four natural numbers

0= ∅1={0}={∅}2={0,1}={∅, {∅}}3={0,1,2}={∅, {∅}, {∅, {∅}}}

0∈ 1 ∈ 2 ∈ 3 ∈⋯0⊆1 ⊆ 2 ⊆ 3 ⊆⋯

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•Paradox and ZFC Paradox

•EquinumerosityEquinumerosity

•Ordering Cardinal Numbers

•Countable setsInfinite Cardinals

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Motivation

To discuss the size of sets. Given two sets A and B, we want to consider such questions as: Do A and B have the same size? Does A have more elements than B?

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Example

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Equinumerosity

Definition A set A is equinumerous to a set B (written A≈B) iff there is a one-to-one function from A onto B.

A one-to-one function from A onto B is called a one-to-one correspondence between A and B.

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Example: ω× ω ≈ ω

The set ω × ω is equinumerous to ω. There is a function J mapping ω × ω one-to-one onto ω.

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J(m,n)=((m+n)2+3m+n)/2

Example: ω≈Q

f: ω→ Q

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Example: (0,1) ≈ R

(0,1)={x ∈ R | 0<x<1}, then (0,1) ≈ R

f(x)= tan(π(2x-1)/2)

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Examples

(0,1) ≈ (n,m) Proof: f(x) = (n-m)x+m

(0,1) ≈ {x| x∈ω ∧ x>0} =(0,+∞) Proof: f(x)=1/x -1

[0,1] ≈ [0,1) Proof: f(x)=x if 0≤x<1 and x≠1/(2n), n∈ω

f(x)=1/(2n+1) if x=1/(2n), n∈ω [0,1) ≈ (0,1) Proof: f(x)=x if 0<x<1 and x≠1/(2n), n∈ω

f(0)=1/2 x=0f(x)=1/(2n+1) if x=1/(2n), n∈ω

[0,1] ≈ (0,1)

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Example: ℘(A) ≈ A2

For any set 𝐴𝐴, we have 𝑃𝑃 𝐴𝐴 ≈ 𝐴𝐴2.

Proof: Define a function 𝐻𝐻 from 𝑃𝑃(𝐴𝐴) onto 𝐴𝐴2 as:For any subset 𝐵𝐵 of 𝐴𝐴, 𝐻𝐻(𝐵𝐵) is the characteristic function of 𝐵𝐵:

1 if 𝑥𝑥 ∈ 𝐵𝐵𝑓𝑓𝐵𝐵 𝑥𝑥 =

0 if 𝑥𝑥 ∈ 𝐴𝐴 − 𝐵𝐵𝐻𝐻 is one-to-one and onto.

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Theorem

For any sets A, B and C:A ≈ AIf A ≈ B then B ≈ AIf A ≈ B and B ≈ C then A ≈ C.Proof:

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Theorem(Cantor 1873)

The set ω is not equinumerous to the set R of real numbers.

No set is equinumerous to its power set.

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The set ω is not equinumerous to the set

R of real numbers.

Proof: show that for any functon f: ω→ R, there is a real number z not belonging to ran f

f(0) =32.4345…,f(1) =-43.334…,f(2) = 0.12418…,

……z: the integer part is 0, and the (n+1)st decimal place of z is 7 unless the (n+1)st decimal place of f(n) is 7, in which case the (n+1)st decimal place of z is 6.Then z is a real number not in ran f.

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No set is equinumerous to its power set.

Proof: Let g: A→℘(A); we will construct a subset B of A that is not in ran g. Specifically, let

B={x∈ A | x∉ g(x)}Then B⊆A, but for each x∈ A

x∈ B iff x∉ g(x)Hence B≠g(x).

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•Paradox and ZFC Paradox

•EquinumerosityEquinumerosity

•OrderingCardinal Numbers

•Countable setsInfinite Cardinals

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Ordering Cardinal Numbers

Definition A set A is dominated by a set B(written A≼B) iff there is a one-to-onefunction from A into B.

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Examples

Any set dominates itself. If A⊆B, then A is dominated by B.A≼B iff A is equinumerous to some subset of B.

BF

A B

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Schröder-Bernstein Theorem

If A≼B and B≼A, then A≈B.

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Proof:

𝑓𝑓:𝐴𝐴 → 𝐵𝐵, 𝑔𝑔:𝐵𝐵 → 𝐴𝐴. Define 𝐶𝐶𝑛𝑛by recursion:𝐶𝐶0 = 𝐴𝐴 − 𝑟𝑟𝑟𝑟𝑟𝑟 𝑔𝑔 and 𝐶𝐶𝑛𝑛+ = 𝑔𝑔 𝑓𝑓 𝐶𝐶𝑛𝑛ℎ 𝑥𝑥 = 𝑓𝑓(𝑥𝑥) if 𝑥𝑥 ∈ 𝐶𝐶𝑛𝑛 for some 𝑟𝑟,

g−1 x otherwise

ℎ(𝑥𝑥) is one-to-one and onto.Spring 2018

Application of the Schröder-Bernstein Theorem

ExampleIf A⊆B⊆C and A≈C, then all three sets are

equinumerous.The set R of real numbers is equinumerous

to the closed unit interval [0,1].

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ℵ0 is the least infinite cardinal. i.e. ω≼A for

any infinite A.ℵ0 ∙2

ℵ0 =?2ℵ0≤ ℵ0 ∙2

ℵ0≤ 2ℵ0 ∙2 ℵ0 =2ℵ0

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•Paradox and ZFC Paradox

•EquinumerosityEquinumerosity

•Ordering Cardinal Numbers

•Countable setsInfinite Cardinals

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Countable Sets

Definition A set A is countable iff A≼ω,

Intuitively speaking, the elements in a countable set can be counted by means of the natural numbers.

An equivalent definition: A set A is countable iffeither A is finite or A ≈ ω .

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Example

ω is countable, as is Z and QR is uncountableA, B are countable sets ∀ C⊆A, C is countable A∪B is countable A × B is countable

For any infinite set A, ℘(A) is uncountable.

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Continuum Hypothesis

Are there any sets with cardinality between ℵ0 and 2ℵ0 ? Continuum hypothesis (Cantor): No.

i.e., there is no λ with ℵ0 < λ < 2ℵ0 . Or, equivalently, it says: Every uncountable set of real numbers is

equinumerous to the set of all real numbers.

GENERAL VERSION: for any infinite cardinal κ, there is no cardinal number between κ and 2κ .

HISTORY Georg Cantor: 1878, proposed the conjecture David Hilbert: 1900, the first of Hilbert’s 23 problems. Kurt Gödel: 1939, ZFC ⊢ ¬CH . Paul Cohen: 1963, ZFC ⊢ CH .

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