Difference between revisions of "12-240/Classnotes for Tuesday September 25"

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==Properties ==
 
==Properties ==
  
 +
==Polynomials==
 +
'''Definition''' : Pn(F) = {all polynomials of degree less than or equal n with coefficients in F}
 +
                        = {anx^n + an-1x^n-1 + ... + a1x^1 + a1x^1}
 +
 +
0 = 0x^n + 0x^n-1 +...+ 0x^0
 +
 +
addition and multiplication: as you imagine
 +
 +
P(f) = {all polynomials with coefficients in F}
 +
 +
Take F= '''Z'''/2    |F| = 2
 +
 +
|P(F)| = infinite
 +
 +
in Pn('''Z'''/2)  x^3≠x^2
 +
                  x^3 = 1*x^3+0x^2+0x+O = f
 +
                  x^2 = 1*x^2+0x+0 = g
 +
                  yet f(0)= g(0) and f(1)=g(1)
 +
 +
==Theorem==
 +
1. Cancellation Laws
 +
  (a) x+z=y+z        ==> x=y
 +
  (b) ax+ay,a≠0      ==> x=y
 +
  (c) x≠0 of V, ax=bx ==> a=b
 +
 +
2. 0 of V is unique
 +
 +
3. Negatives are unique (so subtraction makes sense
 +
 +
4.(0 of F)x = 0 of V
 +
 +
5. a*0=0
 +
 +
6. (-a)x= - (ax) = a(-x)
 +
 +
7. a*v=0 <==> a=0 or v=0
 +
 +
==Proof==
 +
1.  (a) x+z=y+z
 +
        Find a w s.t. z+w=0 (V.S. 4)
 +
      (x+z)+w = (y+z)+w
 +
  Use VS2
 +
        x+(z+w) = y +(z+w)
 +
        x + 0  = y + o
 +
  Use VS3      x=y
  
 
==Scanned Notes by [[User:Richardm|Richardm]]==
 
==Scanned Notes by [[User:Richardm|Richardm]]==

Revision as of 13:49, 2 November 2012

Today's class dealt with the properties of vector spaces.


Contents

Definition

Let F is a field, a vector space V over F is a set V of vectors with special element O ( of V) and tow operations: (+): VxV->V, (.): FxV->V

VxV={(v,w): v,w \in\!\, V}

FxV={(c,v): c \in\!\, F, v \in\!\, V}

Then, (+): VxV->V is (v,w)= v+w; (.): FxV->V is (c,v)=cv

Such that

VS1 \forall\!\, x, y \in\!\, V: x+y = y+x

VS2 \forall\!\, x, y, z \in\!\, V: x+(y+z) = (x+y)+z

VS3 \forall\!\, x \in\!\, V: 0 ( of V) +x = x

VS4 \forall\!\, x \in\!\, V, \exists \!\, V \in\!\, V: v + x= 0 ( of V)

VS5 \forall\!\, x \in\!\, V, 1 (of F) .x = x

VS6 \forall\!\, a, b \in\!\, F, \forall\!\, x \in\!\, V: (ab)x = a(bx)

VS7 \forall\!\, a \in\!\, F, \forall\!\, x, y \in\!\, V: a(x + y)= ax + ay

VS8 \forall\!\, a, b \in\!\, F, \forall\!\, x \in\!\, V: (a + b)x = ax + bx


Examples

Properties

Polynomials

Definition : Pn(F) = {all polynomials of degree less than or equal n with coefficients in F}

                        = {anx^n + an-1x^n-1 + ... + a1x^1 + a1x^1}

0 = 0x^n + 0x^n-1 +...+ 0x^0

addition and multiplication: as you imagine

P(f) = {all polynomials with coefficients in F}

Take F= Z/2 |F| = 2

|P(F)| = infinite

in Pn(Z/2) x^3≠x^2

                  x^3 = 1*x^3+0x^2+0x+O = f
                  x^2 = 1*x^2+0x+0 = g
                  yet f(0)= g(0) and f(1)=g(1)

Theorem

1. Cancellation Laws

  (a) x+z=y+z         ==> x=y
  (b) ax+ay,a≠0       ==> x=y
  (c) x≠0 of V, ax=bx ==> a=b

2. 0 of V is unique

3. Negatives are unique (so subtraction makes sense

4.(0 of F)x = 0 of V

5. a*0=0

6. (-a)x= - (ax) = a(-x)

7. a*v=0 <==> a=0 or v=0

Proof

1. (a) x+z=y+z

       Find a w s.t. z+w=0 (V.S. 4)
      (x+z)+w = (y+z)+w
 Use VS2
       x+(z+w) = y +(z+w)
       x + 0   = y + o
 Use VS3      x=y

Scanned Notes by Richardm