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1 MAC 2103 Module 10 lnner Product Spaces I

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Page 1: 1 MAC 2103 Module 10 lnner Product Spaces I. 2 Rev.F09 Learning Objectives Upon completing this module, you should be able to: 1. Define and find the

1

MAC 2103

Module 10

lnner Product Spaces I

Page 2: 1 MAC 2103 Module 10 lnner Product Spaces I. 2 Rev.F09 Learning Objectives Upon completing this module, you should be able to: 1. Define and find the

2Rev.F09

Learning ObjectivesUpon completing this module, you should be able to:

1. Define and find the inner product, norm and distance in a given inner product space.

2. Find the cosine of the angle between two vectors and determine if the vectors are orthogonal in an inner product space.

3. Find the orthogonal complement of a subspace of an inner product space.

4. Find a basis for the orthogonal complement of a subspace of ⁿ spanned by a set of row vectors. ℜ

5. Identify the four fundamental matrix spaces of an m x n matrix A, and know the column rank of A, the row rank of A, and the rank of A.

6. Know the equivalent statements of an n x n matrix A.

http://faculty.valenciacc.edu/ashaw/ Click link to download other modules.

Page 3: 1 MAC 2103 Module 10 lnner Product Spaces I. 2 Rev.F09 Learning Objectives Upon completing this module, you should be able to: 1. Define and find the

3Rev.09

General Vector Spaces II

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Inner Product Spaces, Inner Products, Inner Product Spaces, Inner Products,

Norm, Distance, Fundamental Matrix Spaces, Norm, Distance, Fundamental Matrix Spaces, Rank, and OrthogonalityRank, and Orthogonality

The major topics in this module:

Page 4: 1 MAC 2103 Module 10 lnner Product Spaces I. 2 Rev.F09 Learning Objectives Upon completing this module, you should be able to: 1. Define and find the

4Rev.F09

Definition of an Inner Product and Orthogonal Vectors

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A. Inner Product: Let

be any function from into that satisfies the following conditions for any u, v, z in V:

and iff

Then defines an inner product for all u, v in V.

u and v in V are Orthogonal Vectors iff

⟨⋅,⋅⟩:V ×V → °

V ×V °

c) ⟨kru,

rv⟩ =k⟨

ru,

rv⟩,

a) ⟨ru,

rv⟩ =⟨

rv,

ru⟩,

d) ⟨rv,

rv⟩ ≥0, ⟨

rv,

rv⟩= 0

rv =0.

⟨ru,

rv⟩

⟨ru,

rv⟩= 0.

Page 5: 1 MAC 2103 Module 10 lnner Product Spaces I. 2 Rev.F09 Learning Objectives Upon completing this module, you should be able to: 1. Define and find the

5Rev.F09

Definition of a Norm of a Vector

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B. Norm: Let

be any function from into that satisfies the following conditions for any u, v in V:

Then defines a norm for all u in V.

The special norm that is induced by an inner product is

⋅ :V → ° + = [0,∞)

V °+

ru

a)ru ≥0, and

ru =0 iff

ru=

r0,

b) sru = s

ru , and

c)ru+

rv ≤

ru +

rv Triangle inequality.

ru = ⟨

ru,

ru⟩.

Page 6: 1 MAC 2103 Module 10 lnner Product Spaces I. 2 Rev.F09 Learning Objectives Upon completing this module, you should be able to: 1. Define and find the

6Rev.F09

Definition of the Distance Between Two Vectors

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C. Distance: Let

be any function from into that satisfies the following conditions for any u, v, w in V:

Then defines the distance for all u, v in V.

The special distance that is induced by a norm is

d(⋅,⋅) :V ×V → ° + =[0,∞)V ×V °

+

d(ru,

rv)

d(ru,

rv)=

ru−

rv = ⟨

ru−

rv,

ru−

rv⟩.

a) d(ru,

rv)≥0, and d(

ru,

rv) =0 iff

ru=

rv,

b) d(ru,

rv) =d(

rv,

ru), and

c) d(ru,

rv) ≤d(

ru,

rw) +d(

rw,

rv) Triangle inequality.

Page 7: 1 MAC 2103 Module 10 lnner Product Spaces I. 2 Rev.F09 Learning Objectives Upon completing this module, you should be able to: 1. Define and find the

7Rev.F09

How to Find an Inner Product, Norm, and Distance for Vectors in ⁿℜ ?

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For u and v in ⁿℜ , we define

and

Note: These are our usual norm and distance in ⁿℜ . ⁿℜ is an inner product space with the dot product as its inner product.

ru =⟨

ru,

ru⟩

12 = (

ru⋅

ru) = u1

2 +u22 + ...+un

2 ,

d(ru,

rv)=

ru−

rv

=⟨ru−

rv,

ru−

rv⟩

12 = (

ru−

rv)⋅(

ru−

rv)

= (u1 −v1)2 + (u2 −v2 )

2 + ... + (un −vn)2 .

Page 8: 1 MAC 2103 Module 10 lnner Product Spaces I. 2 Rev.F09 Learning Objectives Upon completing this module, you should be able to: 1. Define and find the

8Rev.F09

How to Find an Inner Product, Norm, and Distance for Vectors in ⁿℜ ? (Cont.)

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Example 1: Let u = (1,2,-1) and v = (2,0,1) in ³. Find the ℜinner product, norms, and distance.

⟨ru,

rv⟩=

ru ⋅

rv = u1v1 + u2v2 + u3v3 = (1)(2) + (2)(0) + (−1)(1) = 1,

ru = ⟨

ru,

ru⟩

12 = u1

2 + u22 + u3

2 = 12 + 22 + (−1)2 = 6,rv = ⟨

rv,

rv⟩

12 = v1

2 + v22 + v3

2 = 22 + 02 +12 = 5, and

d(ru,

rv) =

ru −

rv = ⟨

ru −

rv,

ru −

rv⟩

12

= (1− 2)2 + (2 − 0)2 + (−1−1)2 = 9 = 3.

Page 9: 1 MAC 2103 Module 10 lnner Product Spaces I. 2 Rev.F09 Learning Objectives Upon completing this module, you should be able to: 1. Define and find the

9Rev.F09

How to Find an Inner Product, Norm, and Distance for Vectors (Matrices) in M22?

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In M22, the vectors are 2 x 2 matrices. Thus, given two matrices U and V in M22, we define

U =u1 u2

u3 u4

⎣⎢⎢

⎦⎥⎥,V =

v1 v2v3 v4

⎣⎢⎢

⎦⎥⎥.

Recall: tr(A) = sum of the entries on the main diagonal of A.

⟨U,V⟩= tr(UTV ) = tr(V TU ) = ⟨V ,U⟩= u1v1 + u2v2 + u3v3 + u4v4 .

U =⟨U,U⟩12 = u12 +u2

2 +u32 +u4

2 , and

d(U,V) = U −V =⟨U −V,U −V⟩12 = ⟨U −V,U −V⟩

= (u1 −v1)2 + (u2 −v2 )

2 + (u3 −v3)2 + (u4 −v4 )

2 .

Page 10: 1 MAC 2103 Module 10 lnner Product Spaces I. 2 Rev.F09 Learning Objectives Upon completing this module, you should be able to: 1. Define and find the

10

Rev.F09

How to Find an Inner Product, Norm, and Distance for Vectors (Matrices) in M22? (Cont.)

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Example 2: Let

be matrices in the vector space M22. Find the inner product, norms and distance for U and V in M22.

U =u1 u2

u3 u4

⎣⎢⎢

⎦⎥⎥= 2 −1

3 1⎡

⎣⎢

⎦⎥,V =

v1 v2v3 v4

⎣⎢⎢

⎦⎥⎥= 0 9

4 6⎡

⎣⎢

⎦⎥

⟨U,V⟩= tr(UTV ) = tr(V TU ) = u1v1 + u2v2 + u3v3 + u4v4

= (2)(0) + (3)(4) + (−1)(9) + (1)(6) = 9.Recall: tr(A) = sum of the entries on the main diagonal of A.

Page 11: 1 MAC 2103 Module 10 lnner Product Spaces I. 2 Rev.F09 Learning Objectives Upon completing this module, you should be able to: 1. Define and find the

11

Rev.F09

How to Find an Inner Product, Norm, and Distance for Vectors (Matrices) in M22? (Cont.)

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U =⟨U,U⟩12 = u12 +u2

2 +u32 +u4

2 = 22 + (−1)2 + 32 +12 = 15,

V =⟨V,V⟩12 = u12 +u2

2 +u32 +u4

2 = 02 + 92 + 42 + 62 = 133, and

d(U,V) = U −V =⟨U −V,U −V⟩12 = ⟨U −V,U −V⟩

= (u1 −v1)2 + (u2 −v2 )

2 + (u3 −v3)2 + (u4 −v4 )

2

= (2 −0)2 + ((−1)−9)2 + (3−4)2 + (1−6)2

= 4 +100 +1+ 25 = 130.

Page 12: 1 MAC 2103 Module 10 lnner Product Spaces I. 2 Rev.F09 Learning Objectives Upon completing this module, you should be able to: 1. Define and find the

12

Rev.F09

How to Find an Inner Product, Norm, and Distance for Vectors (Polynomials) in P2?

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Let P2 be the set of all polynomials of degree less than or equal to two. For two polynomials

we define

p(x)=a0x0 + a1x

1 + a2x2 ,

q(x) =b0x0 +b1x

1 +b2x2

rp,

rq ∈P2 ,

⟨rp,

rq⟩= a0b0 + a1b1 + a2b2 ,

rp = ⟨

rp,

rp⟩

12 = a2

0 + a21 + a2

2 , and

d(rp,

rq) =

rp −

rq = ⟨

rp −

rq,

rp −

rq⟩

12

= (a0 − b0 )2 + (a1 − b1)12 + (a3 − b3)

2 .

Page 13: 1 MAC 2103 Module 10 lnner Product Spaces I. 2 Rev.F09 Learning Objectives Upon completing this module, you should be able to: 1. Define and find the

13

Rev.F09

How to Find an Inner Product, Norm, and Distance for Vectors (Polynomials) in P2? (Cont.)

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Example 3: Find the inner product, norms, and distance for p and q in P2, where

⟨rp,

rq⟩= a0b0 + a1b1 + a2b2 = (8)(1) + (−3)(−2) + (5)(7) = 8 + 6 + 35 = 49,

rp = ⟨

rp,

rp⟩

12 = a0

2 + a21 + a2

2 = 82 + (−3)2 + 52 = 98 = 7 2,

rq = ⟨

rq,

rq⟩

12 = b0

2 + b12 + b2

2 = 12 + (−2)2 + 72 = 54 = 3 6, and

d(rp,

rq) =

rp −

rq = ⟨

rp −

rq,

rp −

rq⟩

12 = (a0 − b0 )2 + (a1 − b1)1

2 + (a3 − b3)2

= (8 −1)2 + ((−3) − (−2))2 + (5 − 7)2 = 49 +1+ 4 = 54 = 3 6.

p(x)=8−3x+ 5x2 and q(x) =1−2x+ 7x2 .

Page 14: 1 MAC 2103 Module 10 lnner Product Spaces I. 2 Rev.F09 Learning Objectives Upon completing this module, you should be able to: 1. Define and find the

14

Rev.F09

How to Find an Inner Product, Norm, and Distance for Vectors (Functions) in C[a,b]?

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⟨rf ,

rg⟩= f (x)g(x)dx

a

b

∫ ,

rf = ⟨

rf ,

rf ⟩

12 = [ f (x)]2 dx

a

b

∫⎛

⎝⎜⎞

⎠⎟

12

= f 2 (x)dxa

b

∫ ,

and d(rf ,

rg) =

rf −

rg = ⟨

rf −

rg,

rf −

rg⟩

12 = [ f (x) − g(x)]2 dx

a

b

∫⎛

⎝⎜⎞

⎠⎟

12

.

Let C[a,b] be the set of all continuous functions on [a,b].

For all f and g in C[a,b], we define

Page 15: 1 MAC 2103 Module 10 lnner Product Spaces I. 2 Rev.F09 Learning Objectives Upon completing this module, you should be able to: 1. Define and find the

15

Rev.F09

How to Find an Inner Product, Norm, and Distance for Vectors (Functions) in C[a,b]? (Cont.)

http://faculty.valenciacc.edu/ashaw/ Click link to download other modules.

Example 4: Let f = f(x) = 1 and g = g(x) = -x. Find the inner product, norms, and distance on C[2,4].

⟨rf ,

rg⟩= f (x)g(x)dx

2

4

∫ = (1)(−x)dx2

4

∫ = −x2

22

4

= (−8) − (−2) = −6,

rf = ⟨

rf ,

rf ⟩

12 = [ f (x)]2 dx

2

4

∫⎛

⎝⎜⎞

⎠⎟

12

= f 2 (x)dx2

4

∫ = dx2

4

∫ == x2

4= 2,

rg = ⟨

rg,

rg⟩

12 = [g(x)]2 dx

2

4

∫⎛

⎝⎜⎞

⎠⎟

12

= g2 (x)dx2

4

∫ = (−x)2 dx2

4

∫ ==x3

32

4

= 214

3,

and d(rf ,

rg) =

rf −

rg = ⟨

rf −

rg,

rf −

rg⟩

12 = [ f (x) − g(x)]2 dx

2

4

∫⎛

⎝⎜⎞

⎠⎟

12

= (1+ x)2 dx2

4

∫ = (1+ 2x + x2 )dx2

4

∫ = (x + x2 +x3

3)

2

4

= 72

3.

Page 16: 1 MAC 2103 Module 10 lnner Product Spaces I. 2 Rev.F09 Learning Objectives Upon completing this module, you should be able to: 1. Define and find the

16

Rev.F09

The Cauchy-Schwarz Inequality

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Cauchy-Schwarz Inequality:

for all f, g in an inner product space V, with the norm induced by

the inner product. We evaluate the Cauchy-Schwarz inequality

for the previous examples.

⟨rf ,

rg⟩ ≤

rf

rg

1. ⟨ru,

rv⟩ =1≤ 6 5 =

ru

rv from Example 1.

2 ⟨U,V⟩ =9 ≤ 15 133 = U V from Example 2.

3. ⟨rp,

rq⟩ =49 ≤(7 2)(3 6) =

rp

rq from Example 3.

4. ⟨rf ,

rg⟩ =6 ≤( 2)(2

143) =

rf

rg from Example 4.

Page 17: 1 MAC 2103 Module 10 lnner Product Spaces I. 2 Rev.F09 Learning Objectives Upon completing this module, you should be able to: 1. Define and find the

17

Rev.F09

Orthogonality for Vectors (Functions) in C[a,b]

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Example 5: Let f = f(x) = 1 and g = g(x) = ex. Find the inner

product on C[0,2].

Since the inner product is not zero, f and g are not orthogonal

functions in C[0,2].

Example 6: Let f = f(x) = 5 and g = g(x) = cos(x). Find the

inner product on C[0,π].

Since the inner product is zero, f and g are orthogonal functions

in C[0,π].

⟨rf ,

rg⟩= f (x)g(x)dx

0

2

∫ = (1)(ex )dx0

2

∫ = e2 − e0 = e2 −1 ≠ 0

⟨rf ,

rg⟩= f (x)g(x)dx

0

π

∫ = 5cos(x)dx0

π

∫ == 5 cos(x)dx0

π

∫= 5[sin(π ) − sin(0)] = 5[0 − 0] = 0

Page 18: 1 MAC 2103 Module 10 lnner Product Spaces I. 2 Rev.F09 Learning Objectives Upon completing this module, you should be able to: 1. Define and find the

18

Rev.F09

Orthogonality for Vectors (Functions) in C[a,b] (Cont.)

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Example 7: Let f = f(x) = 4x and g = g(x) = x2. Find the inner product on C[-1,1].

Since the inner product is zero, f and g are orthogonal functions in C[-1,1].

⟨rf ,

rg⟩= f (x)g(x)dx

−1

1

∫ = (4x)(x2 )dx−1

1

∫ == 4 x3 dx−1

1

∫ = x4( )−1

1= 1−1 = 0

Page 19: 1 MAC 2103 Module 10 lnner Product Spaces I. 2 Rev.F09 Learning Objectives Upon completing this module, you should be able to: 1. Define and find the

19

Rev.F09

Inner Product and Orthogonality for Vectors (Functions) in C[a,b] (Cont.)

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Example 8: Let f = f(x) = 1 and g = g(x) = sin(2x). Find the inner product on C[-π,π].

Since the inner product is zero, f and g are orthogonal functions in C[-π,π].

⟨rf ,

rg⟩= f (x)g(x)dx

−π

π

∫ = (1)(sin(2x))dx−π

π

∫ = sin(2x)dx−π

π

∫ = −1

2cos(2x)

−π

π

= −1

2cos(2π )⎡

⎣⎢⎤⎦⎥

− −1

2cos(−2π )⎡

⎣⎢⎤⎦⎥

= −1

2cos(2π ) +

1

2cos(2π ) = 0,

cos(2x) is even.

Page 20: 1 MAC 2103 Module 10 lnner Product Spaces I. 2 Rev.F09 Learning Objectives Upon completing this module, you should be able to: 1. Define and find the

20

Rev.F09

The Generalized Theorem of Pythogoras

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Generalized Theorem of Pythogoras: If f and g are orthogonal vectors in an inner product space V, then

Proof: Let f and g be orthogonal in V, then

Thus,rf +

rg

2=⟨

rf +

rg,

rf +

rg⟩ =⟨

rf ,

rf +

rg⟩ + ⟨

rg,

rf +

rg⟩

=⟨rf ,

rf ⟩ + ⟨

rf ,

rg⟩ + ⟨

rg,

rf ⟩ + ⟨

rg,

rg⟩

=rf

2+ 2⟨

rf ,

rg⟩ +

rg 2

=rf

2+

rg 2 .

⟨rf ,

rg⟩= 0.

Page 21: 1 MAC 2103 Module 10 lnner Product Spaces I. 2 Rev.F09 Learning Objectives Upon completing this module, you should be able to: 1. Define and find the

21

Rev.F09

The Generalized Theorem of Pythogoras (Cont.)

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The Generalized Theorem of Pythogoras

We evaluate the theorem for the previous examples. From example 6, we have

rf +

rg

2= [5 + cos(x)]2 dx

0

π

∫ = (25 + (2)(5)cos(x) + cos2 (x))dx0

π

= 25dx0

π

∫ + 2 (5)cos(x)dx0

π

∫ + cos2 (x)dx0

π

=rf

2+ 2⟨5,cos(x)⟩ +

rg 2 =

rf

2+ 2(0) +

rg 2 =

rf

2+

rg 2 .

Page 22: 1 MAC 2103 Module 10 lnner Product Spaces I. 2 Rev.F09 Learning Objectives Upon completing this module, you should be able to: 1. Define and find the

22

Rev.F09

The Generalized Theorem of Pythogoras (Cont.)

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From Example 7, we have

Notice that we have used orthogonality to obtain our results without directly computing the integrals.

Next, we will compute the integrals directly to obtain the result.

rf +

rg

2= [4x+ x2 ]2 dx

−1

1

∫ = (16x2 + (2)(4x)x2 + x4 )dx−1

1

= 16x2 dx−1

1

∫ + 2 4x3 dx−1

1

∫ + x4 dx−1

1

=rf

2+ 2⟨4x,x2 ⟩ +

rg 2 =

rf

2+ 2(0) +

rg 2 =

rf

2+

rg 2 .

Page 23: 1 MAC 2103 Module 10 lnner Product Spaces I. 2 Rev.F09 Learning Objectives Upon completing this module, you should be able to: 1. Define and find the

23

Rev.F09

The Generalized Theorem of Pythogoras (Cont.)

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Thus,

rf

2=⟨

rf ,

rf ⟩ = f 2 (x)dx

−1

1

∫ = [ f (x)]2 dx−1

1

∫ = (4x)(4x)dx−1

1

∫ ==16 x2 dx−1

1

∫ =(16)x3

3−1

1

=323

rg 2 =⟨

rg,

rg⟩ = g2 (x)dx

−1

1

∫ = [g(x)]2 dx−1

1

∫ = (x2 )(x2 )dx−1

1

∫ == x4 dx−1

1

∫ =x5

5−1

1

=25

rf +

rg

2=⟨

rf +

rg,

rf +

rg⟩ = [ f (x) + g(x)]2 dx

−1

1

∫ = (4x+ x2 )(4x+ x2 )dx−1

1

= (16x2 + 8x3 + x4 )dx−1

1

∫ =16x3

3+8x4

4+

x5

5⎛

⎝⎜⎞

⎠⎟−1

1

=16615

=323+25=

rf

2+

rg 2 .

Page 24: 1 MAC 2103 Module 10 lnner Product Spaces I. 2 Rev.F09 Learning Objectives Upon completing this module, you should be able to: 1. Define and find the

24

Rev.F09

How to Find the Cosine of the Angle Between Two Vectors in an Inner Product Space?

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Example 9: Let ⁵ have the Euclidean inner product. ℜ

Find the cosine of the angle between

u = (2,1,0,-1,5) and v = (1,-1,-2,3,1).

cos(θ) =⟨ru,

rv⟩

ru

rv=

u1v1 +u2v2 +u3v3 + .u4v4 + +u5v5u12 +u2

2 +u32 +u4

2 +u52 v1

2 + v22 + v3

2 + v42 + v5

2

=(2)(1) + (1)(−1) + (0)(−2) + (−1)(3) + (5)(1)

22 +12 + 0 + (−1)2 + 52 12 + (−1)2 + (−2)2 + (3)2 +12

=3

4 31.

Page 25: 1 MAC 2103 Module 10 lnner Product Spaces I. 2 Rev.F09 Learning Objectives Upon completing this module, you should be able to: 1. Define and find the

25

Rev.F09

How to Find the Cosine of the Angle Between Two Vectors in an Inner Product Space? (Cont.)

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Example 10: Let P2 have the previous inner product.

Find the cosine of the angle between p and q.

cos(θ) =⟨rp,

rq⟩

rp

rq=

a0b0 + a1b1 + a2b2a02 + a1

2 + a22 b0

2 +b12 +b2

2

=(2)(−2) + (6)(−1) + (3)(4)

22 + 62 + 32 (−2)2 + (−1)2 + (4)2

=2

7 21.

rp =p(x) =a0x

0 + a1x1 + a2x

2 =2 + 6x+ 3x2 ,rq=q(x) =b0x

0 +b1x1 +b2x

2 =−2−x+ 4x2

Page 26: 1 MAC 2103 Module 10 lnner Product Spaces I. 2 Rev.F09 Learning Objectives Upon completing this module, you should be able to: 1. Define and find the

26

Rev.F09

Quick Review on a Basis for a Vector Space and the Dimension of a Vector Space

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Let be nonzero vectors in V. Then for , we have that

W = span(S) = {all linear combinations of } is a subspace of V.

If S contains some linearly dependent vectors, then the dim(W) < n.

If S is a linearly independent set of vectors, then S is a basis for the vector space W. W is a proper subspace of V, if dim(W) < dim(V). If dim(W) = dim(V) = n, then S is a basis for V.

A non-trivial vector space V always have two subspaces:

the trivial vector space, {0} and V.

rv1,

rv2 ,...,

rvn

S ={rv1,

rv2 ,...,

rvn}

rv1,

rv2 ,...,

rvn

Page 27: 1 MAC 2103 Module 10 lnner Product Spaces I. 2 Rev.F09 Learning Objectives Upon completing this module, you should be able to: 1. Define and find the

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Rev.F09

Quick Review on a Basis for a Vector Space and the Dimension of a Vector Space

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So, we need two conditions for the set S to be a basis of a vector space, it must be a linearly independent set and it must span the vector space. The number of basis vectors in S is the dimension of the vector space.

For example: dim( ²ℜ )=2, dim( ³ℜ )=3, dim( ⁵ℜ )=5,

dim (P2)=3, dim(M22)=4, and dim(C[a,b]) is infinite.

Page 28: 1 MAC 2103 Module 10 lnner Product Spaces I. 2 Rev.F09 Learning Objectives Upon completing this module, you should be able to: 1. Define and find the

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Rev.F09

Rank, Row Rank, and Column Rank of a Matrix

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The dimensions of the row space of A, column space of A, and nullspace of A are also called the row rank of A, column rank of A, and nullity of A, respectively.

The rank of A = the row rank of A = the column rank of A, and is the number of linearly independent columns in the matrix A.

The nullity of A = dim(null(A)) and is the number of free variables in the solution space.

Page 29: 1 MAC 2103 Module 10 lnner Product Spaces I. 2 Rev.F09 Learning Objectives Upon completing this module, you should be able to: 1. Define and find the

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Rev.F09

The Orthogonal Complement of a Subspace W

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The orthogonal complement of W, is the set of all vectors in a finite dimensional inner product space V that are orthogonal to every vector in W.

Both W and are subspaces of V where

dim(W) + dim( ) = dim(V).

The only vector common to W and is 0, and W ⊥

W ⊥

W ⊥W ⊥

W =(W⊥ )⊥ .

Page 30: 1 MAC 2103 Module 10 lnner Product Spaces I. 2 Rev.F09 Learning Objectives Upon completing this module, you should be able to: 1. Define and find the

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Rev.F09

The Four Fundamental Matrix Spaces

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Let A be an m x n matrix, then:

1.The null(A) and the row(A) are orthogonal complements in ⁿ with respect to the Euclidean inner product. ℜ

2.The null(AT) and the col(A) are orthogonal complements in ℜᵐ with respect to the Euclidean inner product.

Note that row(A) = col(AT) and col(A) = row(AT).

The four fundamental matrix spaces are:

• row(A) = col(AT),

• col(A) = row(AT),

• null(A), and

• null(AT).

Page 31: 1 MAC 2103 Module 10 lnner Product Spaces I. 2 Rev.F09 Learning Objectives Upon completing this module, you should be able to: 1. Define and find the

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Rev.F09

How to Find a Basis for the Orthogonal Complement of the Subspace of ⁿℜ Spanned by the Vectors?

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Example 11: Find a basis for the orthogonal complement of the subspace of ⁿℜ spanned by the vectors v1, v2, and v3.

Let V = span({v1, v2, v3}). Then, V is a subspace of of ⁿℜ if v1, v2, v3 ∈ ⁿℜ . Let

From the last module, we know that the null(A) is the orthogonal complement to the row(A) = col(AT), and that the null(A) is the solution space of the homogeneous system, Ax = 0. Let v1, v2, v3 be the row vectors of the matrix A. Then, row(A) is the orthogonal complement to null(A) which is the solution space to Ax = 0. We shall use Gauss Elimination to reduce A to a row echelon form to find the basis for null(A).

rv1 =(1,−1,3),

rv2 =(5,−4,−4),

rv3 =(7,−6,2).

Page 32: 1 MAC 2103 Module 10 lnner Product Spaces I. 2 Rev.F09 Learning Objectives Upon completing this module, you should be able to: 1. Define and find the

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Rev.F09

How to Find a Basis for the Orthogonal Complement of the Subspace of ⁿℜ Spanned by the Vectors? (Cont.)

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r1r2r3

157

−1−4−6

3−42

⎢⎢⎢

⎥⎥⎥

=A

r1−5r1+ r2−7r1+ r3

100

−111

3−19−19

⎢⎢⎢

⎥⎥⎥

r1r2

−r2 + r3

100

−110

3−190

⎢⎢⎢

⎥⎥⎥

Since the null(A) is the solution space of the homogeneous system, Ax = 0, the general solution of the system is:

x3 =s,x2 −19x3 =0,x2 −19s=0,x2 =19s.x1 −x2 + 3x3 =0,x1 −19s+ 3s=0,x1 =16s.

Note that column 1 and column 2, with the red leading 1’s, are linearly independent. The corresponding columns of A form a basis for the col(A). Then, dim(col(A)) = 2.

Page 33: 1 MAC 2103 Module 10 lnner Product Spaces I. 2 Rev.F09 Learning Objectives Upon completing this module, you should be able to: 1. Define and find the

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Rev.F09

How to Find a Basis for the Orthogonal Complement of the Subspace of ⁿℜ Spanned by the Vectors? (Cont.)

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The nonzero rows with the leading 1’s are linearly independent. These rows form a basis for row(A). Then, dim(row(A)) = 2.

Note that the row space and column space are both two dimensional, so rank(A) = 2. The rank(A) is the common dimension of the row space of A and the column space of A.

So, rank(A) = rowrank(A) = colrank(A) = dim(col(A)) =dim(row(A)) is always true for any m x n matrix A.

Page 34: 1 MAC 2103 Module 10 lnner Product Spaces I. 2 Rev.F09 Learning Objectives Upon completing this module, you should be able to: 1. Define and find the

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Rev.F09

How to Find a Basis for the Orthogonal Complement of the Subspace of ⁿℜ Spanned by the Vectors? (Cont.)

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Thus, the solution vector is:

and the vector

{w1} forms a basis for the null(A). In other words, {w1} = {(16,19,1)} forms a basis for the orthogonal complement of row(A).

x1

x2

x3

⎢⎢⎢

⎥⎥⎥=

16s19ss

⎢⎢⎢

⎥⎥⎥

=s16191

⎢⎢⎢

⎥⎥⎥

rw1 =

16191

⎢⎢⎢

⎥⎥⎥

Page 35: 1 MAC 2103 Module 10 lnner Product Spaces I. 2 Rev.F09 Learning Objectives Upon completing this module, you should be able to: 1. Define and find the

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Rev.F09

How to Find a Basis for the Orthogonal Complement of the Subspace of ⁿℜ Spanned by the Vectors? (Cont.)

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Since B = {w1} is a basis for null(A), span(B) = null(A), and

dim(null(A)) = 1 is the nullity of A. In this example, rank(A) is 2 and the nullity of A is 1, and the number of columns of A is 3.

For any m x n matrix A, rank(A) + nullity(A) is n, which is the number of columns of A.

Page 36: 1 MAC 2103 Module 10 lnner Product Spaces I. 2 Rev.F09 Learning Objectives Upon completing this module, you should be able to: 1. Define and find the

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Rev.F09

Let’s Review Some Equivalent Statements

If A is an n x n matrix, and if TA: ⁿ→ ⁿ is multiplication by A, ℜ ℜthan the following are equivalent. (See Theorem 6.2.7)

• A is invertible.• Ax = 0 has only the trivial solution • The reduced row-echelon form of A is In.• A is expressible as a product of elementary matrices. • Ax = b is consistent for every n x 1 matrix b. • Ax = b has exactly one solution for every n x 1 matrix b. • det(A) ≠ 0.• The range of TA is ⁿℜ .• TA is one-to-one.

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Page 37: 1 MAC 2103 Module 10 lnner Product Spaces I. 2 Rev.F09 Learning Objectives Upon completing this module, you should be able to: 1. Define and find the

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Rev.F09

Let’s Review Some Equivalent Statements (Cont.)

• The column vectors of A are linearly independent.• The row vectors of A are linearly independent.• The column vectors of A span ⁿℜ . • The row vectors of A span ⁿℜ . • The column vectors of A form a basis for ⁿℜ . • The row vectors of A form a basis for ⁿℜ .• A has rank n = row rank n = column rank n = dim(col(A)).• A has nullity 0 = dim(null(A)).• The orthogonal complement of the nullspace of A,

null(A), is ⁿℜ .• The orthogonal complement of the row space of A,

row(A), is {0}.

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Page 38: 1 MAC 2103 Module 10 lnner Product Spaces I. 2 Rev.F09 Learning Objectives Upon completing this module, you should be able to: 1. Define and find the

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Rev.F09

What have we learned?We have learned to:

1. Define and find the inner product, norm and distance in a given inner product space.

2. Find the cosine of the angle between two vectors and determine if the vectors are orthogonal in an inner product space.

3. Find the orthogonal complement of a subspace of an inner product space.

4. Find a basis for the orthogonal complement of a subspace of ⁿ ℜspanned by a set of row vectors.

5. Identify the four fundamental matrix spaces of an m x n matrix A, and know the column rank of A, the row rank of A, and the rank of A.

6. Know the equivalent statements of an n x n matrix A.

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Page 39: 1 MAC 2103 Module 10 lnner Product Spaces I. 2 Rev.F09 Learning Objectives Upon completing this module, you should be able to: 1. Define and find the

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Rev.F09

Credit

Some of these slides have been adapted/modified in part/whole from the following textbook:

• Anton, Howard: Elementary Linear Algebra with Applications, 9th Edition

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