MATH 220

# Final Review

Updated 2017-12-17

## Chapter 1

### Set

• Denoted by $\{\}$
• Element in the set: $1\in \{1, 2\}$
• Subset: $\{1\}\subset \{1,2\}$

### Power Set

Suppose $A=\{1, 2\}$ then its power set is a set of all subsets of $A$: $\mathcal P(A)=\{\emptyset, \{1\}, \{2\}, \{1, 2\}\}$

### Set Operations

Suppose sets $A=\{1,2,3\}, B=\{3, 4, 5\}$

#### Union

If $x\in A$ or $x\in B$ then $x\in A \cup B$

#### Intersection

If $x\in A$ and $x\in B$ then $x \in A\cap B$

#### Difference

If $x\in A$ and $x\notin B$ then $x\in A-B$

#### Compliment

If $x\in A$ then $x\notin \bar A$

### Direct Product

Direct product of two sets is $A\times B$ that outputs an ordered set $(a, b)$ such that $a\in A, b\in B$.

### Indexed Sets Operations

#### Union

$\bigcup_{i=1}^{n}A_i=A_1\cup A_2\cup A_3\cup \dots$

#### Intersection

$\bigcap_{i=1}^{n}A_i=A_1\cap A_2\cap A_3\cap \dots$

## Chapter 2

### Statement

A statement is a claim. A statement can be true or false.

### Logical Operations

#### Conjunction

Binary operator also known as and. Denoted by $\wedge$. Suppose $A$ and $B$ are statements $A\wedge B$ is true if and only if $A$ and $B$ are both true.

#### Disjunction

Binary operator also known as or. Denoted by $\vee$. $A\vee B$ is true if $A$ is true, $B$ is true, or both is true.

#### Negation

Denoted by $\neg$. If $A=\neg B$, then $A$ is true if and only if $B$ is false.

### Open Sentence

A statement that can’t be determined to be true or false unless a quantifier is given. An example would be:

• $x$ is an even number.

### Logical Equivalences

#### DeMorgan

$\neg (A\cap B)=\neg A\cup \neg B$

#### Distributive

$A\cap(B\cup C)=(A\cap B)\cup(A\cap C)$

### Implications

Implication: $P\implies Q$

Converse: $Q\implies P$

Contrapositive: $\neg Q \implies \neg P$

Negation: $\neg (P\implies Q)\equiv \neg P \wedge Q$

### Biconditional Implications

Also referred to as “if and only if”

$P\iff Q\equiv (P\implies Q)\wedge(Q\implies P)$

### Quantifier

An example of quantifiers to the open sentence example (above) would be:

• $\forall x\in \mathbb N$ (this would make the statement false)
• $\exists x\in \mathbb N$ (this would make the statement true)

#### Negation

$\neg \forall \equiv \exists,\quad \neg \exists \equiv \forall$

## Chapter 4-10

### Direct Proof

To prove $P\implies Q$, the structure is:

\begin{align} &\text{Assume P is true}\\ &\vdots\\ &\text{Therefore Q is true} \end{align}

### Proof By Cases

Usually use when there there is a disjunction in the proof.

### Proof By Contrapositive

To prove $P\implies Q$, the structure is:

\begin{align} &\text{Assume Q is false}\\ &\vdots\\ &\text{Therefore P is false} \end{align}

To prove $P\implies Q$, the structure is:

$\text{Assume the hypothesis is negated}: \neg Q \wedge P,\\ \vdots\\ \text{finds contradiction}\\ \text{Therefore the negated statement is false (original hypothesis is true)}$

### Existence Proof

To prove $\exists x, R(x)$, where $R(x)$ is the statement. Just provide an example.

#### Uniqueness Proof

To prove $\exists x, R(x)$, $x$ is unique, provide an example $x=d$ for which $R(d)$ is true (existence proof) and show that $d$ is the only unique example.

#### Constructive Proof

Gives an explicit example that proves and existence proof.

#### Non-Constructive Proof

Proves an example exists without actually providing it.

### Mathematical Induction

#### Proof By Induction

To prove the statements $S_1, S_2, S_2\dots$ are all true, the structure is:

\begin{align} &\textbf{(basis step)}\text{ Prove }S_1\text{ is true}\\ &\textbf{(induction step)}\text{ Given any integer } k\geq1, \text{Prove } S_k\implies S_{k+1}\\ &\vdots\\ &\text{By induction hypothesis...}\\ &\vdots\\ &\text{Therefore }S_n\text{ is true} \end{align}

#### Strong Mathematical Induction

Strong induction works the same as regular induction, except:

• instead of assuming $S_k$ is true, we assume $S_1, S_2,\dots, S_k$ all to be true.

#### Proof By Smallest Counterexample

This is a hybrid of proving by induction and proving by contradiction. To prove $S_1, S_2, \dots$ are all true, the structure is:

\begin{align} \textbf{(basis)}&\text{ Prove } S_1 \text{ is true}\\ \textbf{(contradict)}& \text{ For the sake of contradiction, assume not every }S_n\text{ is true}\\ &\text{Let smallest integer }k>1 \text{ such that } S_k \text{ is false}\\ &\text{Then }S_{k-1} \text{ is true and } S_k \text{ is false; get contradiction} \end{align}

## Chapter 11

### Relation

Definition: a relation $R$, on a set $A$ is a subset $R\subset A\times A$.

#### Relation Properties

• Relation $R$ is reflexive iff $\forall x \in A, xRx$
• Relation $R$ is symmetric iff $\forall x, y\in A, xRy\implies yRx$
• Relation $R$ is transitive iff $\forall x, y, z\in A, ((xRy)\wedge(yRz))\implies xRz$

### Equivalence Relation

A relation $R$ is an equivalence relation iff $R$ is reflexive, symmetric, and transitive.

### Equivalence Class

Equivalence relations on $A$ divides $A$ into subsets called equivalence class. The equivalence class containing $a$ is the subset $\{x\in A:xRa\}$ of $A$ consists of all elements of $A$ that relates to $a$.

• Denoted as $[a]$
• Where $[a]=\{x\in A: xRa\}$
• If there exists $[a]=[b]$, there is still only one equivalence class.

Theorem 11.1: Suppose $R$ is an equivalence relation on set $A$ and $a,b\in A$. Then $[a]=[b]$ iff $aRb$

#### Partition

A partition of set $A$ is a set of non-empty subsets of $A$ such that the union of all subsets equals $A$ and intersection of any two subsets equals $\emptyset$.

Theorem 11.2: Suppose $R$ is an equivalence relation on set $A$. Then the set $\{[a]:a\in A\}$ forms a partition of $A$.

### Integer Modulo n

The equivalence classes of the equivalence relation $\equiv (\text{mod } n)$ are $[0], [1], \dots, [n-1]$, for some $n\in \mathbb N$. Then the integer modulo n is the set $\mathbb Z_n=\{[0], [1], \dots, [n-1]\}$.

• Property 1: $[a]+[b]=[a+b]$
• Property 2: $[a]\cdot[b]=[a\cdot b]$

## Chapter 12

### Function

• A function is denoted as $f:A\rightarrow B$, where $A$ is the domain and $B$ is the codomain
• The range of function $f$ is the set $\{f(a):a\in A\}=b:(a,b)\in f$
• Two functions $f$ and $g$ are the same iff $\forall x\in A, f(x)=g(x)$

### Injective

A function is injective iff $\forall x,y\in A, x\neq y\implies f(x)\neq f(y)$.

To prove a function is injective, the proof structure is (contrapositive):

\begin{align} &\text{Suppose }x,y\in A \text{ and } f(x)=f(y)\\ &\vdots\\ &\text{Therefore x=y} \end{align}

### Surjective

A function is surjective iff $\forall b\in B, \exists a\in A$ such that $f(a)=b$

To prove a function is surjective, the proof structure is (direct):

\begin{align} &\text{Suppose } b\in B\\ &\vdots\\ &\text{*Existential proof for } \exists a\in A \text{ for which } f(a)=b* \end{align}

### Bijective

A function bijective iff the function is both injective and surjective

### Pigeonhole Principle

Suppose $A$ and $B$ are finite sets and $f:A\rightarrow B$ is any function, then:

• If $\lvert A \rvert >\lvert B\rvert$ then $f$ is not injective
• If $\lvert A \rvert <\lvert B\rvert$ then $f$ is not surjective

Suppose $A$ and $B$ are finite sets and $A\neq \emptyset, B\neq \emptyset$, then:

• If $\lvert A \rvert \geq \lvert B \rvert$ then $\exists$ a surjective $f:A\rightarrow B$
• If $\lvert A \rvert \leq\lvert B \rvert$ then $\exists$ an injective $g:A\rightarrow B$
• If $\lvert A \rvert =\lvert B \rvert$ then $\exists$ a bijective $h: A\rightarrow B$

### Composition

Suppose $f:A\rightarrow B, g:B\rightarrow C$, then the composition is $g\circ f:A\rightarrow C$

Theorem 12.1: If $f:A\rightarrow B$, $g:B\rightarrow C$, $h:C\rightarrow D$, then $(h\circ g)\circ f=h\circ (g\circ f)$

Theorem 12.2: If $f:A\rightarrow B$ and $g: B\rightarrow C$, if both $f$ and $g$ are injective, then $g\circ f$ is injective

### Inverse Function

For a bijective $f:A\rightarrow B$, its inverse is the function $f^{-1}:B\rightarrow A$.

Composition with inverse yields identity function:

• $f^{-1}\circ f = i_A$
• $f\circ f^{-1}=i_B$

#### Identity Function

$\forall x\in A, i_A(x)=x$

Theorem 12.3: Function $f$ is bijective iff the reverse relation $f^{-1}$ is a function $B\rightarrow A$

### Image & Preimage

Suppose $f:A\rightarrow B$:

• If $X\subset A$, the image of $X$ is the set $f(X)=\{f(x):x\in X\}\subset B$
• If $Y\subset B$, the preimage of $Y$ is the set $f^{-1}(Y)=\{f^{-1}(y):y\in Y\}\subset A$

## Useful Definitions and Facts

### Parity & Multiplicity

• An integer $n$ is even iff if $n=2a$ for some $a\in\mathbb Z$
• An integer $n$ is odd iff $n=2a+1$ for some $a\in\mathbb Z$
• Integers $k$ and $n$ have the same parity if $k$ and $n$ are both even or odd.
• An integer $n$ is a multiple of an integer $q$ iff $n=qk$ for some $k\in \mathbb Z$
• A positive integer $n$ is a prime number iff $n$ has exactly two positive divisors, namely $1$ and $n$.

### Well-Ordering Principle

• Given a subset $A\subset \mathbb N$, and $A\neq \emptyset$, then there is an element $x_0\in A$ such that $x_0$ is the smallest element in $A$
• Given an integer $b$, any non-empty subset $A\subset\{b, b+1, b+2,\dots\}$ is well-ordered

### Division Algorithm

• Given integers $a$ and $b > 0$, there exists integer $q$ and $r$ for which $a=gb+r$ and $0\leq r \lt b$

### Fundamental Theorem of Arithmetic

• Every integer greater than 1 is either a prime number itself or is the product of prime numbers

### Greatest Common Divisor & Least Common Multiple

Given $k,n\in (\mathbb Z-\{0\})$,

• The GCD of $k$ and $n$ is the largest integer that divides both $k$ and $n$
• The LCM of $k$ and $n$ is the smallest integer that is a multiple of both $k$ and $n$

#### Proposition 7.1

If $a,b\in \mathbb N$, then there exists integers $k$ and $l$ for which $\gcd(a,b)=ak+bl$

#### Proposition 10.1

Suppose there are $n\geq2$ integers $a_1, a_2, \dots, a_n$. If a prime number $p$ divides $a_1\cdot a_2\cdots a_n$, then $p$ divides at least one of the integers $a_i$.

### Congruence

• Given $n\in \mathbb N, a,b\in \mathbb Z$, $a$ is congruent to $b\mod n$, denoted by $a\equiv b\mod n$, iff $n\mid a-b$

Assuming $a\equiv b\mod n$ and $c\equiv d\mod n$ for $a,b,c,d\in \mathbb Z$, then:

• $a-c\equiv b-d\mod n$
• $a^2\equiv b^2\mod n$
• $ac\equiv bd \mod n$
• If $a\equiv b \mod n$ and $b \equiv c \mod n$ then

see above

### Image & Preimage

Theorem 12.4: Suppose $f: A\rightarrow B$ and $W,X\subset A, Y,Z\subset B$

• $f(W\cap X)\subset f(W)\cap f(X)$
• $f(W\cup X)=f(W)\cup f(X)$
• $f^{-1}(Y\cap Z)=f^{-1}(Y)\cap f^{-1}(Z)$
• $f^{-1}(Y\cup Z)=f^{-1}(Y)\cup f^{-1}(Z)$
• $X\subset f^{-1}(f(X))$
• $f(f^{-1}(Y))\subset Y$