14-240/Classnotes for Monday September 15

DBN: Classes: 2014-15: 14-240 / Navigation Welcome to Math 240! (additions to this web site no longer count towards good deed points) # Week of... Notes and Links 1 Sep 8 About This Class, What is this class about? (PDF, HTML), Monday, Wednesday 2 Sep 15 HW1, Monday, Wednesday, TheComplexField.pdf,HW1_solutions.pdf 3 Sep 22 HW2, Class Photo, Monday, Wednesday, HW2_solutions.pdf 4 Sep 29 HW3, Wednesday, Tutorial, HW3_solutions.pdf 5 Oct 6 HW4, Monday, Wednesday, Tutorial, HW4_solutions.pdf 6 Oct 13 No Monday class (Thanksgiving), Wednesday, Tutorial 7 Oct 20 HW5, Term Test at tutorials on Tuesday, Wednesday 8 Oct 27 HW6, Monday, Why LinAlg?, Wednesday, Tutorial 9 Nov 3 Monday is the last day to drop this class, HW7, Monday, Wednesday, Tutorial 10 Nov 10 HW8, Monday, Tutorial 11 Nov 17 Monday-Tuesday is UofT November break 12 Nov 24 HW9 13 Dec 1 Wednesday is a "makeup Monday"! End-of-Course Schedule, Tutorial F Dec 8 The Final Exam Register of Good Deeds Add your name / see who's in!

Definition of Subtraction and Division

• Subtraction: if $a,b\in F,a-b=a+(-b)$.
• Division: if $a,b\in F,a/b=a\times b^{-1}$.

Basic Properties of a Field (cont'd)

8. $\forall a\in F$, $a\times 0=0$.

Proof of 8

By F3 , $a\times 0=a\times (0+0)$

By F5 , $a\times (0+0)=a\times 0+a\times 0$;

By F3 , $a\times 0=0+a\times 0$;

By Thm P1, $0=a\times 0$.

9. $\nexists b\in F$ s.t. $0\times b=1$;

$\forall b\in F$ s.t. $0\times b\neq 1$.

Proof of 9

By F3 , $\times b=0\neq 1$.

10. $(-a)\times b=a\times (-b)=-(a\times b)$.

11. $(-a)\times (-b)=a\times b$.

12. $a\times b=0\iff a=0$ or $b=0$.

Proof of 12

<= :

By P8 , if $a=0$ , then $a\times b=0\times b=0$;

By P8 , if $b=0$ , then $a\times b=a\times 0=0$.

=> : Assume $a\times b=0$ , if a = 0 we are done;

Otherwise , by P8 , $a\neq 0$ and we have $a\times b=0=a\times 0$;

by cancellation (P2) , $b=0$.

$(a+b)\times (a-b)=a^{2}-b^{2}$.

Proof

By F5 , Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle (a + b) \times (a - b) = a \times (a + (-b)) + b \times (a + (-b))}

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle = a^2 - b^2}

Theorem

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \exists! \iota : \Z \rightarrow F} s.t.

1. Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \iota(0) = 0 , \iota(1) = 1} ;

2. Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \forall m ,n \in \Z, \iota(m+n) = \iota(m) + \iota(n)} ;

3. Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \forall m ,n \in \Z, \iota(m\times n) = \iota(m) \times \iota(n)} .

Examples

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \iota(2) = \iota(1+1) = \iota(1) + \iota(1) = 1 + 1;} Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \iota(3) = \iota(2+1) = \iota(2) + \iota(1) = \iota(2) + 1;}

......

In F2: Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \begin{align} 27 ----> \iota(27) &= \iota(26 + 1)\\ &= \iota(26) + \iota(1)\\ &= \iota(26) + 1\\ &= \iota(13 \times 2) + 1\\ &= \iota(2) \times \iota(13) + 1\\ &= (1 + 1) \times \iota(13) + 1\\ &= 0 \times \iota(13) + 1\\ &= 1 \end{align} } http://drorbn.net/images/c/cd/MAT_240_lecture_3_%281_of_2%29.pdf (Lecture 3 notes by AM part 1 of 2) http://drorbn.net/images/6/6a/MAT240_lectuire_3_%282_of_2%29.pdf (Lecture 3 notes by AM part 2 of 2)