Difference between revisions of "12-240/Classnotes for Thursday September 27"

From Drorbn
Jump to: navigation, search
(Theorems)
(Subspaces)
 
(One intermediate revision by one user not shown)
Line 89: Line 89:
 
Definition: Let V be a vector space over a field F. A ''subspace''  W of V is a subset of V, has the operations inherited from V and 0_V of V, is itself a vector space.
 
Definition: Let V be a vector space over a field F. A ''subspace''  W of V is a subset of V, has the operations inherited from V and 0_V of V, is itself a vector space.
  
 +
=== Examples of subspaces ===
 +
Look at scanned notes for examples of subspaces!
  
Theorem: A subset W ⊂ V, W ≠ {},   is a subspace iff it is closed under the operations of V.
+
Theorem: A subset W ⊂ V, W ≠ ∅, is a subspace iff it is closed under the operations of V.
  
 
1. ∀ x, y ∈ W, x + y ∈ W
 
1. ∀ x, y ∈ W, x + y ∈ W

Latest revision as of 14:57, 30 September 2012

Vector Spaces

Contents

Reminders

- Tag yourself in the photo!

- Read along textbook 1.1 to 1.4

- Riddle: Professor in ring with lion around the perimeter. Consider this: http://mathforum.org/library/drmath/view/63421.html

Vector space axioms

(Quick recap)

VS1. x + y = y + x

VS2. (x + y) + z = x + (y + z)

VS3. 0 vector

VS4. + inverse → -

VS5. 1x = x

VS6. a(bx) = (ab)x

VS7. a(x + y) = ax + ay

VS8. (a+b)x = ax + bx

Theorems

1.a x + z = y + z ⇒ x = y

1.b ax = ay, a ≠ 0, ⇒ x = y

1.c ax = bx, x ≠ 0, ⇒ a = b


2. 0 is unique.


3. Additive inverse is unique.


4. 0_F ∙ x = 0_V


5. a ∙ 0_V = 0_V


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


7. cx = 0 ⇔ c = 0 or x = 0_V


Hints for proofs

1.a Same as for fields

1.b. Use similar proof as for fields, but use VS6 NOT F2b. F2b guarantees existence, but VS6 allows algebraic manipulation.

1.c Discussed after proof of 7, harder than you think at first glance.

2. Same as F.

3. Same as F

4. 0_F + 0_F = 0_F => by [VS8]: 0x + 0x = (0+0)x = 0x = 0x + 0 [VS3] = 0 + 0x [VS1] ⇒ 0x + 0x = 0 + 0x ⇒ [Cancellation property] 0x = 0

5. Same as 4 except using 0_V + 0_V = 0_V and using VS7

6. Skip

7. Prove both ways: Easy way is to the left, show left is 0 if either on right is 0. To the right, Suppose c not= 0, then show x must equal 0.

1.c Add (-bx) to each side, use VS8 then VS6 -> (a-b)x =0, use property 7.

Subspaces

Definition: Let V be a vector space over a field F. A subspace W of V is a subset of V, has the operations inherited from V and 0_V of V, is itself a vector space.

Examples of subspaces

Look at scanned notes for examples of subspaces!

Theorem: A subset W ⊂ V, W ≠ ∅, is a subspace iff it is closed under the operations of V.

1. ∀ x, y ∈ W, x + y ∈ W

2. ∀ c ∈ F, ∀ x ∈ W, cx ∈ W

Scanned notes upload by Starash