Mathematics Archives - Page 35 of 60 - Shawn Zhong

Math 541 – 1/24

$Introduction • Course Plan ○ Frist 2/3 - Groups ("Sets with a multiplication rule") ○ Last 1/3 - Rings ("Set with notions of addition and multiplication") • Office Hours ○ Tuesdays 2:30 p.m. - 4:00 p.m. ○ Wednesdays 12:30 p.m. - 2:00 p.m. Notations • "□(≔) " means "equals, by definition" • Z≔{0,±1,±2,±3,…} the set of integers • Q≔{a/b│a,b∈Zb≠0} • R≔ the set of all real numbers • ℂ≔{a+bi│a,b∈Ri^2=−1} • Z(≥0)≔{a∈Za≥0} • S∖{x}≔{s∈S│s≠x} • Denote a function f from a set A to set B by f:A→B • Denote the image of f by im(f)≔{b∈B│∃a∈A s.t. f(a)=b} Injective, Surjective and Bijective • Definition ○ Let f:A→B be a function, then ○ f is injective if ∀a,a^′∈A, a≠a^′, f(a)≠f(a′) ○ f is surjective if ∀b∈B, ∃a∈A s.t. f(a)=b (i.e. im(f)=B) ○ f is bijective if f is both injective and surjective. • Example 1 ○ For f:Z→Z, f(a)=2a ○ f is injective § Let a,a^′∈Z § Suppose f(a)=f(a′) § ⇒2a=2a^′ § ⇒2a−2a^′=0 § ⇒2(a−a^′ )=0 § ⇒a−a^′=0 § ⇒a=a^′ § Therefore f is injective ○ f is not surjective § Because the image of f does not contain any odd integers § im(f)={even integer} • Example 2 ○ Let f:Q→Q is given by f(a)=2a ○ f is injective § By the same proof as before ○ f is surjective § Let b∈Q, then b/2∈Q § And f(b/2)=2(b/2)=b∈Q § Therefore f is surjective ○ f is bijective § Since f is both injective and surjective. Divides • Definition ○ If x,y∈Z, and x≠0 ○ We say x divides y and write x|y, if ∃q∈Z s.t. xq=y • Examples ○ ∀x∈Z\\{0}, x|0, since x⋅0=0 ○ ∀x∈Z, 1|x, since 1⋅x=x ○ ∀x∈Z, −1|x, since (−1)⋅(−x)=x Equivalence Relations • Product of Two Sets ○ If A and B are sets, then the product of A and B is ○ A×B≔{(a,b)│a∈A, b∈B} • Relations ○ A relation on a set A is a subset R of A×A ○ We write a~a′ if (a,a^′ )∈R • Equivalence Relations ○ A equivalence relation is a relation R on A such that R is ○ Reflexive § if a∈A, a~a § i.e. (a,a)∈R ○ Symmetric § if a~a′, then a^′~a § i.e. (a,a′)∈R⇒(a′,a)∈R ○ Transitive § if a~a^′, a^′~a^′′, then a~a′′ § i.e. if (a,a′)∈R and (a^′,a^′′ )∈R, then (a,a^′′ )∈R • Example 1 ○ Let R be a relation on set A such that R≔{(a,a)│a∈A} ○ Then R is an equivalence relation (a~a^′⟺a=a^′) ○ Reflexive § If a∈A,(a,a)∈R ○ Symmetric § If a~a′,a=a^′ § Thus a^′=a, hence a^′~a ○ Transitive § If a~a^′,a^′~a′′ then a=a′ and a=a′′ § Thus a=a^′′, hence a~a^′′ • Example 2 ○ Let n be a positive integer ○ Then R≔{(a,b)∈ZZn|(a−b) } is an equivalence relation ○ Reflexive § Recall that n|0 § Thus, n|(a−a), ∀a∈Z § It follows that a~a,∀a∈Z ○ Symmetric § Let a,b∈Z, and suppose n|(a−b) (i.e. a~b) § Choose q∈Z s.t. nq=a−b § Then n(−q)=−(a−b)=b−a § Thus, n|(b−a), and so b~a ○ Transitive § Suppose a,b,c∈Z, and we have a~b, b~c § Then n|(a−b) and n|(b−c) § Choose q,q^′∈Z s.t. nq=a−b, nq^′=b−c § Then n(q+q^′ )=(a−b)+(b−c)=a−c § Thus, n|(a−c), and so a~c$

Math 521 – 1/29

Proposition 1.14 • Given a field F, for x,y,z∈F (1) If x+y=x+z, then y=z (2) If x+y=x, then y=0 (3) If x+y=0, then y=−x (4) −(−x)=x • Proof (1) ○ x+y=x+z ○ (x+y)+(−x)=(x+z)+(−x) by (A5) ○ x+y+(−x)=x+z+(−x) by (A3) ○ x+(−x)+y=x+(−x)+z by (A2) ○ 0+y=0+z by (A6) ○ y=z∎ by (A4) • Proof (2) ○ x+y=x=x+0 ○ ⇒y=0∎ • Proof (3) ○ x+y=0=x+(−x) ○ ⇒y=−x∎ • Proof (4) ○ (−x)+(−(−x))=0 ○ x+(−x)+(−(−x))=x+0 ○ 0+(−(−x))=x+0 ○ −(−x)=x∎ Proposition 1.15 • Given a field F, for x,y,z∈F (1) If x≠0 and xy=xz, then y=z (2) If x≠0 and xy=x, then y=1 (3) If x≠0 and xy=1, then y=1/x (4) If x≠0, then 1/(1/x)=x • Proof similar to Proposition 1.14 Proposition 1.16 • Given a field F, for x,y∈F (1) 0x=0 (2) If x≠0 and y≠0, then xy≠0 (3) (−x)y=−(xy)=x(−y) (4) (−x)(−y)=xy • Proof (1) ○ 0+0=0 ○ (0+0)x=0x ○ 0x+0x=0x ○ 0x+0x+(−(0x))=0x+(−(0x)) ○ 0x=0∎ • Proof (2) ○ Suppose x≠0, y≠0, but xy=0 ○ x≠0, so 1/x exists ○ 1/x (xy)=1/x⋅0 ○ (1/x⋅x)y=1/x⋅0 ○ 1⋅y=0 ○ y=0 ○ This is a contradiction, so xy≠0∎ • Proof (3) ○ (−x)y+xy=((−x)+x)y=0⋅y=0 ○ (−x)y+xy+(−xy)=0+(−xy) ○ (−x)y=−xy ○ And the rest is similar • Proof (4) ○ Use (3), (−x)(−y)=−(x(−y))=−(−xy)=xy∎ Order • Intuition ○ The real number line • Definition ○ Let S be a set. ○ An order on S is a relation, denoted by ,with the following two properties: § If x∈S and y∈S, then one and only one of the statements xy, x=y, yx is true § If x,y,z∈S, if xy and yz, then xz (Transitivity) ○ x≤y means either xy or x=y ○ x≥y means either xy or x=y ○ An ordered set is a set for which an order is defined. • Example ○ Q is an ordered set under the definition that ○ For r,s∈Q, rs, if and only if s−r is positive Upper Bound and Lower Bound • Definition ○ Suppose S is an ordered set and E⊂S. ○ If there exists β∈S such that x≤β, ∀x∈E ○ We say that x is bounded above and call β an upper bound for E ○ Similarly, if x≥β, ∀x∈E. ○ We say that x is bonded below by β, and β is a lower bound for E

Math 521 – 1/26

$Sets • Contains ○ If A is a set and x is an element of A (an object of A), then we write x∈A ○ Otherwise, we write x∉A • Set ○ The empty set or null set is a set with no elements, and is denoted as ∅ ○ If a set has at least one element, it is called nonempty • Subset ○ If A and B are sets and every element of A is an element of B ○ Then A is a subset of B ○ Rubin write this A⊂B, or B⊃A ○ A⊂A for all sets A • Proper subset ○ If B contain something not in A, then A is a proper subset of B • Equal ○ If A⊂B and B⊂A then A=B. ○ Otherwise A≠B √2 is Not Rational • We proved that √2 is not rational • i.e. there is no rational number p such that p^2=2 • Let A={p∈Qp^2<2}, B={p∈Qp^2>2} • Prove: A has no largest element, and B has no smallest element ○ Let p∈Q, and p>0 ○ Let q≔p−(p^2−2)/(p+2)=(2p+2)/(p+2),then q^2−2=((2p+2)/(p+2))^2−2=2(p^2−2)/(p+2)^2 ○ If p∈A § then p^2−2<0 § ⇒q^2−2=2(p^2−2)/(p+2)^2 <0 § ⇒q^2<2 § ⇒q∈A § ⇒q>p § i.e. A has no largest element ○ If p∈B § then p^2−2>0 § ⇒q^2−2=2(p^2−2)/(p+2)^2 >0 § ⇒q^2>2 § ⇒q∈B § ⇒q</p> <p § i.e. B has no smallest element What is R? • The real numbers are an example of field. • A field is a set F with two binary operations called addition and multiplication that satisfy that following axioms: • Axioms for addition (+) ○ (A1) If x∈F and y∈F, then x+y∈F ○ (A2) Addition is communicate: x+y=y+x,∀x,y∈F ○ (A3) Addition is associative: (x+y)+z=x+(y+z),∀x,y,z∈F ○ (A4) There exists 0∈F s.t. x+0=x, ∀x∈F ○ (A5) ∀x∈F, there exists an additive inverse −x∈F s.t. x+(−x)=0 • Axioms for multiplication (× or ⋅) ○ (M1) If x∈F and y∈F, then xy∈F ○ (M2) Addition is communicate: xy=yx,∀x,y∈F ○ (M3) Addition is associative: (xy)z=x(yz),∀x,y,z∈F ○ (M4) F contains an element 1≠0 s.t. 1⋅x=x, ∀x∈F ○ (M5) If x∈F and x≠0, then there exists 1/x∈F s.t. x⋅1/x=1 • (D) The distributive law: x(y+z)=xy+xz, ∀x,y,z∈F$

Math 521 – 1/24

$Course Overview • The real number system • Metric spaces and basic topology • Sequences and series • Continuity • Topics from differential and integral calculus Grading Homework assignments 20% Quiz (Feb. 9) 5% Midterm 1 (Mar. 9) 20% Midterm 2 (Apr. 13) 20% Final (May. 10 7:45-9:45 AM) 35% A ≥90% B ≥80% C ≥70% D ≥60% Tutoring • Tom Stone @VV B205 • Monday 2:30 - 4:30 PM • Tuesday 2:00 - 4:00 PM What is Analysis • Proof • How calculus works • Fundamental axioms Number Systems • Natural Numbers: N={1,2,3,…} • Integers: Z={0,±1,±2,±3,…} • Rational Numbers: Q={a/b│a,b∈Zb≠0} • Real numbers R: fill the "holes" in the rational numbers √2 is not rational • There is no rational number p such that p^2=2 • Proof by contradiction • Assume there is a rational number p such that p^2=2 • Then p=m/n , where m,n∈Z, n≠0, and m,n have no common factor • (m/n)^2=2⇒m^2/n^2 =2⇒m^2=2n^2 • So m is even • m=2k (k∈Z⇒(2k)^2=2n^2⇒4k^2=2n^2⇒2k^2=n^2 • So n is also even • m,n are both division by 2 • This contradicts the fact that m,n have no common factor • So no such p exists$

第1讲集合的定义

$集合 • 大致定义 ○ 一些对象，需要有方法判断对象是否属于集合 • 元素 ○ 即集合里的个体 ○ 将 x 属于集合 A 记作 x∈A • 例子 ○ 用大括号括起元素来表示集合 § {1,2,3,4} § 1∈{1,2,3,4} § 5∉{1,2,3,4} ○ 可以用省略号省略集合元素 § {1,2,3,4,…} ○ 常见的集合符号 § 整数集 Z § 自然数集 N § 有理数集 Q § 实数集 R ○ 空集 ∅ § 所有对空集进行的全称命题均为真 § ∀x∈∅, x≠x 真命题 ○ 用描述法表示集合 § {班上的同学} § {八大行星} 集合间的关系 • 包含 ○ 对于集合 A 与 B ○ 定义 A⊆B 当且仅当 ∀x∈A, x∈B • 相等 ○ 对于集合 A 与 B ○ 定义 A=B 当且仅当 A⊆B,且 B⊆A 空集 • 定理 1：对于任意集合A ○ 因为所有对空集进行的全称命题均为真 ○ 所以 ∀x∈∅, x∈A 为真命题 ○ 即 ∅⊆A • 定理 2：只有一个∅ ○ 假设有两个空集∅, ∅^′ ○ 那么根据定理1，我们得到 ∅⊆∅^′,∅^′⊆∅ ○ 即 ∅=∅′$

Shawn Zhong

Shawn Zhong

Mathematics

Math 541 – 1/24

Math 521 – 1/29

Math 521 – 1/26

Math 521 – 1/24

第1讲集合的定义

Search

WeChat Account

Mathematics

Math 541 – 1/24

Math 521 – 1/29

Math 521 – 1/26

Math 521 – 1/24

第1讲 集合的定义

Search

WeChat Account

第1讲集合的定义