Real Vector Spaces




Concept and calculation rules

Introduction

Many math structures are different at first sight, but looking deeper the resemblance is astonishing.
The benefits of the study of an abstract structure is that all the properties can be applied to all the representations of that structure.
The concept of a 'real vector space' is an abstract structure in that way.

Concept

We start with a set V and the field of real numbers R. We define the concept 'vector space' by means of postulates.
We say V is a vector space if and only if
  1. There is an addition '+' in V such that V,+ is a commutative group.
  2. Any element v in V and any r in R determine a scalar product rv in V.
    This scalar product has the following properties for any r,s in R and any v,w in V.
  3. r(sv) = (rs)v
  4. r(v + w) = rv + rw
  5. (r + s)v = rv + sv
  6. 1v = v
Any element of a vector space is called a vector.
The identity element of the group V,+ is called the vector 0.
The symmetric element of v is called the opposite vector -v.
The subtraction v - v' is defined by v - v' = v + (-v') .
Examples of real vector spaces are

Calculation rules

Deducing from the postulates of a vector space, one can prove the following calculation rules. They hold for each vector u,v in V, and for each r,s in R.

Subspaces

definition

M is a subspace of a vector space V
if and only if

Criterion

Theorem:
A non-void subset M of a vector space V, is a vector space
if and only if
rx + sy is in M for any r,s in R and any x,y in M.

Part 1: first we prove that
If M is a vector space
Then rx + sy is in M for any r,s in R and any x,y in M.

Well, if M is a vector space then, from the postulates, rx and sy are in M and therefore rx + sy is in M .

Part 2: we prove that
If rx + sy is in M for any r,s in R and any x,y in M,
Then M is a vector space.

Example 1
V is the vector space of all polynomials in x. M is the set of all the polynomials in x of second degree or lower.
We investigate whether or not M is a subspace of V. To this end we choose r, s at random in R and ax2+bx+c and dx2+ex+f are random elements in M.

 
   M is subspace of V
<=>
   r(ax2+bx+c) + s(dx2+ex+f)  is  in M
<=>
    (ra+sd)x2 + (rb+ se)x + (rc+sf)  is in M
Since the last claim is true, M is a subspace of V.

Example 2
V is the vector space of all polynomials in x. M is the set of all the polynomials in x divisible by (x-2).
We investigate whether or not M is a subspace of V. To this end we choose r, s at random in R and (x-2)p(x) and (x-2)q(x) are random elements in M.

 
    M is subspace of V
<=>
    r(x-2)p(x) + s.(x-2)q(x) is in M
<=>
    (x-2).( r.p(x) + s.q(x) ) is in M
Since the last claim is true, M is a subspace of V.

Example 3
V is the vector space of all couples of real numbers.
M = { (x,y) | x,y in R and x + y = 1}
One can now follow the method as in the previous examples, but it can shorter. Since (0,0) is not in M, M is not a vector space. So, M is not a subspace of V.

Example 4
V is the vector space of all 2x2 matrices. M is the set of all the regular 2x2 matrices. We investigate whether or not M is a subspace of V. To this end we choose r, s at random in R and A and B are regular 2x2 matrices.

 
    M is subspace of V
<=>
   r A + s B  is in M
The last claim is false because for r=s=0 we have 0.A + 0.B = 0 and the 0-matrix is not in M.

Intersection of two spaces

Theorem:
The intersection of two subspaces M and N of V, is itself a subspace of V.

Proof:
Since 0 is in M and in N, 0 is in the intersection.
For any r,s in R and any x, y in the intersection of M and N, we can state:
(rx + sy is in M) and (rx + sy is in N); so it is in the intersection.
Appealing on previous criterion the intersection of M and N is a subspace of V.

Example
V is the vector space of all polynomials in x.
M is the subspace of all the polynomials in x divisible by (x-2).
N is the subspace of all the polynomials in x divisible by (x-1).
The intersection I of M and N is the set of all the polynomials in x divisible by (x-1)(x-2). I is a subspace of V.

Generators of a vector space

Linear combinations of vectors

Take from a vector space V, the vectors a,b,c,d ... ,l .
The number of vectors in this set has no importance. Take as much real numbers r,s,t, ... ,z .
Then we say that the vector ra + sb + tc + ... + zl is a linear combination of the vectors a,b,c,d ... ,l.

Example
R3 is the vector space of ordered triples of real numbers. From this space, we take the vectors (1,2,0) and (0,0,1). The following three vectors are linear combinations of (1,2,0) and (0,0,1).

 
    3.(1,2,0) + 4.(0,0,1) = (3,6,4)
   -1.(1,2,0) + 0.(0,0,1) = (-1,-2,0)
    0.(1,2,0) + 0.(0,0,1) = (0,0,0)
But the vector (1,1,0) is not a linear combinations of (1,2,0) and (0,0,1), because there are no real numbers r and s such that r.(1,2,0) + s.(0,0,1) = (1,1,0).

Subspace and Linear combinations of vectors

Take some vectors {a,b,c,d} from a subspace M of V. Since M itself is a vector space, it has all the properties of it. So all real multiples of a or b or c or d belong to M. Then the sum of these multiples belongs to M and all linear combinations of a, b, c and d belong to M.

Conclusion:
Each subspace M of V contains all linear combinations of an arbitrarily set of its vectors.

Generating a vector space

D = { a,b,c,d ... ,l } is a fixed set of vectors from V.
Let M be the set of all possible linear combinations of the vectors of D.
We'll show that M is a vector space.
Indeed, appealing on previous criterion, take two vectors u,v from M.
For any r,s in R, we have that ru + sv is a linear combination of two linear combinations of a,b,c,d ... ,l. So ru + sv is itself a linear combination of a,b,c,d ... ,l and therefore ru + sv is in M.

Since each vector space containing the vectors a,b,c,d ... ,l must contain each linear combination of these vectors, M is the 'smallest' vector space generated by D.

Conclusions and definitions:
All linear combinations of vectors of D = { a,b,c,d ... ,l } generate a vector space M.
The elements of D are called, generators of M.
M is called the vector space spanned by D.
The vector space spanned by the vectors a,b,c,d ... ,l is denoted span(D).
It is the smallest vector space containing the set D.

Example 1
V is the vector space R3 = R x R x R .
Choose D = { (2,3,0) , (-1,4,0) }.
Now, we make all linear combinations of these 2 vectors. This gives us all the vectors of the form (x,y,0). (show this as an exercise)
M = span(D) = { (x,y,0) | x,y in R }.

Example 2
V is the vector space of all row matrices [a,b,c] with a,b,c in R.
D = { [1,1,1] }.
M = span(D) = { [r,r,r] | r in R}

Properties of a generating set

Say D is a subset of vector space V and M = span(D). It is easy to see that: Examples:
V is the vector space of all row matrices [a,b,c,d] with a,b,c,d in R.
D = { [1,0,0,0] , [0,1,0,0] , [2,3,1,0] , [2,4,1,0] }
M = span(D) = span( [1,0,0,0] , [0,1,0,0] , [2,3,1,0] , [2,4,1,0] )

3 [1,0,0,0] + 4 [0,1,0,0] = [3,4,0,0] is in M, So,
M = span( [1,0,0,0] , [0,1,0,0] , [2,3,1,0] , [2,4,1,0] , [3,4,0,0] )

[2,4,1,0] is a linear combination of the other vectors in D because
[2,4,1,0] = 0 [1,0,0,0] + [0,1,0,0] + [2,3,1,0]. So,
M = span( { [1,0,0,0] , [0,1,0,0] , [2,3,1,0] }

M = span([17,0,0,0] , [0,1,0,0] , [2,3,1,0] , [2,4,1,0] }

M = span( [1,0,0,0] , [0,1,0,0] - [2,3,1,0] , [2,3,1,0] , [2,4,1,0] }

Linear dependent

Linear dependent vectors

A set D of vectors is called dependent, if there is at least one vector in D, that can be written as a linear combination of the other vectors of D.
A set of one vector is called dependent, if and only if it is the vector 0.

Example:
V is the vector space of all row matrices [a,b,c,d] with a,b,c,d in R.
D = { [1,0,0,0] , [0,1,0,0] , [2,3,1,0] , [-2,0,-1,0] }
The vectors in D are dependent because
[-2,0,-1,0] = 0 [1,0,0,0] + 3.[0,1,0,0] - [2,3,1,0]

Linear independent vectors

A set D of vectors is called independent, if and only if that set is not a dependent set. Such set is called a free set of vectors.

Example:
V is the vector space of all row matrices [a,b,c,d] with a,b,c,d in R.
D = { [1,0,0,0] , [0,1,0,0] , [2,3,1,0] }
The set D is linear independent because there is no vector in D that can be written as a linear combination of the other vectors of D.

Criterion for linear dependent vectors

Theorem:
Take a set D = {a,b,c,..,l} of (more than one) vectors from a vector space V. That set D is linear dependent
if and only if
there is a suitable set of real numbers r,s,t, ... ,z , not all zero,
such that ra + sb + tc + ... + zl = 0

Proof:
Part 1 :
If the set D is dependent, there is at least one vector in D, say b, who is a linear combination of the other vectors of D.
Then
 
   b = ra + tc + ... + zl
<=>
   ra + (-1)b + tc + ... + zl = 0

   So, there is a suitable set of  real numbers
   r, s = -1, t, ... , z , not all zero,
   such that

   ra + sb + tc + ... + zl = 0 
Part 2 :
If there is a suitable set of real numbers r,s,t, ... ,z , not all zero,
such that ra + sb + tc + ... + zl = 0 , then we can choose a nonzero coefficient, say s ,
and then we have ra + tc + ... + zl = -sb .
Dividing both sides by (-s), we see that b is a linear combination of the other vectors of D.
So, D is a linear dependent set.

Example:

 
   De vectors [-12, 17, 14]  [10, -7, 8]  [-11, 3, 24]
   are linear dependent
<=>
   There are real numbers r,s,t not all zero, such that
     r [-12, 17, 14] + s [10, -7, 8] + t [-11, 3, 24] = [0,0,0]
<=>
   There are real numbers r,s,t not all zero, such that
    -12 r + 10 s - 11t = 0
     17 r -  7 s +  3t = 0
     14 r +  8 s + 24t = 0
<=>
                   (Relying on the theory of systems)
    | -12  10  -11 |
    |  17  -7    3 | = 0
    |  14   8   24 |

<=>
      -3930 = 0
Since the last statement is false, the three vectors are not linear dependent.

Corollary:

Take a set D = {a,b,c,..,l} of (more than one) vectors from a vector space V.
That set D is linear independent
if and only if
ra + sb + tc + ... + zl = 0 => r = s = t = ... = z = 0

Example
 
   The vectors  [-12, 17, 14]  [10, -7, 8]  [-11, 3, 24]
   are linear independent
<=>
   If r [-12, 17, 14] + s [10, -7, 8] + t [-11, 3, 24] = [0,0,0]
   then we must have   r = s = t = 0
<=>
   The system
    -12 r + 10 s - 11t = 0
     17 r -  7 s +  3t = 0
     14 r +  8 s + 24t = 0
   has only the trivial solution r = s = t = 0

     (Relying on the theory of systems)
<=>

    | -12  10  -11 |
    |  17  -7    3 | is different from 0
    |  14   8   24 |
<=>
     -3930  is different from 0
Since the last statement is true, the three vectors [-12, 17, 14] [10, -7, 8] [-11, 3, 24] are linear independent.

Second Criterion for linear dependent vectors

Take an ordered set D = {a,b,c,..,l} of (more than one) vectors from a vector space V.
That set D is linear dependent
if and only if
there is at least one vector, who is a linear combination of the PREVIOUS vectors in D.

Proof:
Part 1:
If the set D is linear dependent, we know from the first criterion that there is a suitable set of real numbers r,s,t, ... ,z , not all zero, such that ra + sb + tc + ... + vi + wj + ... + zl = 0
Say w is the last non-zero coefficient, then
ra + sb + tc + ... + vi + wj = 0 <=> -wj = ra + sb + tc + ... + vi
Dividing both sides by (-w), we see that vector j is a linear combination of the PREVIOUS vectors of D.
Part 2:
If a vector of D is a linear combination of the PREVIOUS vectors of D, then it is a linear combination of the other vectors of D (with coefficients 0 for the following vectors). Thus, D is a linear dependent set.

Example:
We investigate whether the vectors [1, 0, -13] , [2, 17, 0] , [12, 7, 0] are independent. The second vector is not a linear combination of the previous one. So, the first two vectors are independent.
The third vector is not a linear combination of the previous vectors because

 
    r[1, 0, -13] + s [2, 17, 0] = [12, 7, 0]
<=>
    r + 2s = 12
       17s = 7
      -13r = 0
We see immediately that there is no solution for that system. The three vectors are linear independent. It is a free set.

Basis and dimension of a vector space

Minimal generating set and basis

Say M = span(D).
D is a generating set of M. If there is a vector in D, which is a linear combination of the other vectors in D, we can remove that vector from D and still we have M = span(D).
Now remove, one by one, the vectors from D who are a linear combination of the others. Let D'= the remaining set of vectors. We still have M = span(D'), but the vectors in D' are linear independent. D' is a free set that spans M.
Such a minimal generating set of M is called a basis of M.
In this introduction, we restrict the theory to vector spaces with a finite basis.

Example:
D = { (1,3) , (2,1) , (4,7) } is a generating set of the vector space R2.
(4,7) is a linear combination of the other vectors because (4,7) = 2.(1,3) + (2,1).
We remove (4,7).
(2,1) is no multiple of (1,3).
D' = { (1,3) , (2,1) } is free and generating set. It is a basis of R2.

Coordinates in a vector space

Say D = (a,b,c,..,l) is an ordered basis of M.
Each vector v in M can be written as a linear combination of the vectors in D. Assume that it is possible to write v in two different ways as a linear combination of the vectors in D, then we have
v = ra + sb + tc + ... + zl = r'a + s'b + t'c + ... + z'l and then
(r - r')a + (s - s')b + (t - t')c + ... + (z - z')l = 0
But, relying on the criterion of linear independent vectors, all the coefficients must be zero.
So, r = r' ; s = s' ...
Conclusion:
Each vector v of M is uniquely expressible as a linear combination of the vectors of the ordered basis D. The unique coefficients are called the coordinates of v relative to D.

We write co(v) = (r,s,t, ... ,z) or v(r,s,t, ... ,z).

Mind the difference:

v(2,4,-3) is the vector v with coordinates (2,4,-3).

But in v = (2,4,-3) means that the vector v is equal to the vector (2,4,-3)

Properties of coordinates

It is easy to verify that
co(a + b) = co(a) + co(b)
co(ra) = r.co(a)
with a,b in M and r in R.

Two bases of V have exactly the same number of elements

Suppose there are two bases, B1 and B2, with a different number of elements.
Assume B1 = {a,b,c,d} and B2 = {u,v,w}
Then, span(B2) = V . Now, we have V = span{d,u,v,w} and {d,u,v,w} is a linear dependent set. It contains at least one vector, say v, who is a linear combination of the previous vectors. We can omit this vector and then V = span{d,u,w} .
Again, V = span{c,d,u,w} and {c,d,u,w} is a linear dependent set. It contains at least one vector,say w, which is a linear combination of the previous vectors. That vector can't be d, because c and d are independent (as a part of a basis). We can omit this vector and then V = span{c,d,u} .
Again, V = span{b,c,d,u} and {b,c,d,u} is a linear dependent set. It contains at least one vector, which is a linear combination of the previous vectors. That vector can't be b, c or d, because b, c and d are independent (as a part of a basis). That vector must be u! We can omit this vector and then V = span{b,c,d} .
Again, V = span{a,b,c,d} and {a,b,c,d} is a linear dependent set. This is impossible because it is a basis.
From all this we see that is is impossible that two bases, B1 and B2, have a different number of elements.

Dimension of a vector space

Since a basis of a vector space contains a constant number of vectors, that number n is characteristic for that space and n is called the dimension of that space.
We write dim(V) = n. Note that if D spans V, the linear independent vectors of D form a basis of V.

Corollary

If dim(V) = n, then

Example
Let V = the vector space R3. An obvious basis is ((1,0,0) , (0,1,0) , (0,0,1)). Dim(V) = 3. Each basis consists of three vectors but three random vectors do not always constitute a basis. Take the three vectors ( (2+m,m,m) (n,2,n) (2, 1, -4) ).
We search for the necessary and sufficient condition for m and n such that these three vectors are not a basis of R3.

 
   (2+m,m,m) (n,2,n) (2, 1, -4)  are not a basis
<=>
   (2+m,m,m) (n,2,n) (2, 1, -4) are linear dependent
<=>
   There is an  r,s and  t , not all zero, such that
   r(2+m,m,m) + s(m,2,n) + t(2, 1, -4) = 0
<=>
    The following system has a solution different from (0,0,0)
    (2+m)r + n s + 2t = 0
       m r + 2 s +  t = 0
       m r + n s - 4t = 0
<=>
    | 2+m   n   2 |
    |  m    2   1 | = 0
    |  m    n  -4 |
<=>
    6 m n - 12 m - 2 n -16 = 0
The three vectors are not a basis of V if and only if the latter condition is fulfilled.

Example
(1,2,5) and (-1,1,3) are two vectors of R3. Choose another vector from R3 such that the three vectors form a basis of R3.

We try with the simple vector (1,0,0). As in the previous example we have:

 
   (1,0,0) (1,2,5) and (-1,1,3) constitute a basis
<=>
   (1,0,0) (1,2,5) and (-1,1,3) are linear independent
<=>
    ...
<=>
   | 1 0 0 |
   | 1 2 5 | is not zero
   |-1 1 3 |
When we unfold the determinant following the first row, we see immediately that the determinant is 1. So, (1,0,0) (1,2,5) and (-1,1,3) constitute a basis.

Vector spaces and matrices

Row space of a matrix

Say A is a m x n matrix. The rows of that matrix can be viewed as a set D of vectors, of the vector space of all n-tuples of real numbers.
The space generated by D is called the row space of A. The rows of A are a generating set of the row space.
From the properties of generating parts, we have :
The row space of A does not change if we So, such row transformations don't change the row space of A.
The dimension of the row space, is the number of independent rows of A.

Dimension of a row space

We know that it is possible to transform a matrix A, by suitable row transformations, to a row canonic matrix. Then the non-zero rows are linear independent and form a basis of the row space. But the number of these non-zero rows is the rank of A. Hence, we can say that the rank of A is the dimension of the row space of A.
It can be proved that the non-zero rows of the canonic matrix form a unique basis for the row space.
Corollary : the rank of A is the number of linear independent rows.

Example: We'll find the row space of a matrix A and the unique basis for that row space.

 
     [1  0  2  3]
 A = [1  2  0  1]
     [1  0  1  0]
The rank of A is 3. There are 3 linear independent rows.
In this example the three rows of A form a basis of the row space.
The row space is a three dimensional space with basis ((1 0 2 3),(1 2 0 1),(1 0 1 0)).
It is a subspace of R4.

Now, we simplify the matrix A, by means of row transformations until we reach the canonic matrix.

 
     [1  0  2  3]
     [1  2  0  1]
     [1  0  1  0]

R2-R1

     [1  0  2  3]
     [0  2 -2 -2]
     [1  0  1  0]

(1/2)R2

     [1  0  2  3]
     [0  1 -1 -1]
     [1  0  1  0]

R3 - R1

     [1  0  2  3]
     [0  1 -1 -1]
     [0  0 -1 -3]

(-1)R3

     [1  0  2  3]
     [0  1 -1 -1]
     [0  0  1  3]

R1-2.R3

     [1  0  0 -3]
     [0  1 -1 -1]
     [0  0  1  3]

R2 + R3

     [1  0  0 -3]
     [0  1  0  2]
     [0  0  1  3]

Now, we have the unique basis of the row space.

    ((1  0  0 -3),(0  1  0  2),(0  0  1  3))

Column space of a matrix

Say A is a m x n matrix. The columns of that matrix can be viewed as a set D of vectors of the vector space of all m-tuples of real numbers.
The space generated by D is called the column space of A. The columns of A are a generating set of the column space.
From the properties of generating parts, we have :
The column space of A does not change if we So, such column transformations do not change the column space of A.
The dimension of the column space, is the number of independent columns of A.

Dimension of a column space

We know that it is possible to transform a matrix A, by suitable column transformations, to a column canonic matrix. Then the non-zero columns are linear independent and form a basis of the column space. But the number of non-zero columns is the rank of A. Thus, we can say that the rank of A is the dimension of the column space of A. It can be proved that the non-zero columns of the canonic matrix form a unique basis for the column space. Corollary :

Exercise:
Take the matrix A from previous example and find the unique basis of the column space.

Example:
Find the m-values such that: (the dimension of the column space of A) = 3.

 
    [ m  1  2 ]
A = [ 3  1  0 ]
    [ 1 -2  1 ]

The dimension of the column space of A = rank A.
The rank A is 3 if and only if (the determinant of A is not zero).
The determinant of A is m-17.
Conclusion: (the dimension of the column space of A) = 3 if and only if m is different from 17.

Coordinates and changing a basis

We'll show the properties in a vector space with dimension 3, but it can be extended to vector spaces with dimension n.
Take an ordered basis (u,v,w) of V. Then each vector s has coordinates (x,y,z) relative to this basis. If we take another basis (u',v',w'), then s has other coordinates (x',y',z') relative to that new basis.
Now, we'll investigate the link between these two ordered sets of coordinates.
We know that s = xu + yv + zw = x'u' + y'v' + z'w'. But the vectors of the new basis (u',v',w'), also have coordinates relative to the old basis (u,v,w).
co(u') = (a,b,c) => u' = au + bv + cw
co(v') = (d,e,f) => v' = du + ev + fw
co(w') = (g,h,i) => w' = gu + hv + iw
Then
 
s = x' (au + bv + cw) + y' (du + ev + fw) + z' (gu + hv + iw)
  = (ax' + dy' + gz')u + (bx' + ey' + hz')v + (cx' + fy' + iz')w
but from above we have also
s = xu  +  yv  + zw
Therefore, the relation between the coordinates is
 
x = ax' + dy' + gz'
y = bx' + ey' + hz'
z = cx' + fy' + iz'
These relations can be written in matrix notation.
 
[x]   [a  d  g] [x']
[y] = [b  e  h].[y']
[z]   [c  f  i] [z']

[a  d  g]
[b  e  h] is called the transformation matrix.
[c  f  i]
The columns of the transformation matrix are the coordinates of the new basis relative to the old basis.

Example 1:
Say V is the vector space of the ordinary three dimensional space. In that space we take a standard basis e1, e2, e3. They are the unit vectors along x-axis, y-axis and z-axis.
We rotate the three basis vectors, around the z-axis, by an angle of 90 degrees. Thus we get a new basis u1, u2, u3.
The link between old and new basis is

 
  u1 =   e2    co(u1) = (0,1,0)
  u2 = - e1    co(u2) = (-1,0,0)
  u3 =   e3    co(u3) = (0,0,1)

The transformation matrix is

  [0  -1  0]
  [1   0  0]
  [0   0  1]

(x,y,z) are the coordinates of a vector v relative to the old basis.
(x',y',z') are the coordinates of the vector v relative to the new basis.
The connection is

[x]   [0 -1  0] [x']
[y] = [1  0  0].[y']
[z]   [0  0  1] [z']
Example 2:
V is the vector space of all the third degree or lower polynomials in x, with real coefficients .
In that space we take the natural basis (1,x,x2,x3). The vector 6 + 2x - x2 + 4x3 has coordinates (6,2,-1,4).

Now we pass to a new basis (1,x,3x2-1,5x3-3x)

The coordinates of the new basis vectors, relative to the old basis, are

 
  (1,0,0,0)
  (0,1,0,0)
  (-1,0,3,0)
  (0,-3,0,5)

The transformation matrix M is

  [1  0 -1  0]
  [0  1  0 -3]
  [0  0  3  0]
  [0  0  0  5]
Now, we calculate the coordinates (x',y',z',t') of the vector 6+2x-x2+4x3 relative to the new basis.
The relationship between the old and the new coordinates is
 
   [6]        [x']
   [2]        [y']
  [-1]  =  M  [z']
   [4]        [t']

<=>
   [x']          [6]
   [y'] =  M-1   [2]
   [z']          [-1]
   [t']          [4]

<=>

   [x']    [ 1, 0, 1/3,  0  ]  [6]
   [y'] =  [ 0, 1,  0,  3/5 ]  [2]
   [z']    [ 0, 0, 1/3,  0  ] [-1]
   [t']    [ 0, 0,  0,  1/5 ]  [4]

   [x']    [ 17/3 ]
   [y'] =  [ 22/5 ]
   [z']    [ -1/3 ]
   [t']    [ 4/5  ]
( 17/3 , 22/5 , -1/3, 4/5 ) are the coordinates of the vector 6+2x-x2+4x3 relative to the new basis.

Vector spaces and systems of linear equations

The following sections are based on the theory of systems of equations

Vector spaces and homogeneous systems

Take a homogeneous system of linear equations in n unknowns. Each solution of that system can be viewed as a vector from the vector space V of all the real n-tuples.
Each real multiple of that solution is a solution too, and the sum of two solutions is a solution too. Therefore, all the solutions of the system form a subspace M of V. It is called the solution space of the system.

Basis of a solution space

By means of an example, we show how a basis of a solution space can be found.
 
/ 2x + 3y - z + t = 0
\ x - y + 2z - t = 0
This is a system of the second kind.
x and y can be taken as main unknowns.
z and t are the side unknowns.
The solutions are
 
x = -z + (2/5)t
y =  z - (3/5)t
The set of solutions can we written as
(-z + (2/5)t , z - (3/5)t , z , t )  with z and t in R

<=>

z(-1,1,1,0) + t(2/5,-3/5,0,1)    with z and t in R

Hence, all solutions are linear combinations of the linear independent vectors (-1,1,1,0) and (2/5,-3/5,0,1).
These two vectors constitute a basis of the solution space.

Solutions of a non homogeneous system

We can denote such system shortly as AX = B, with coefficient matrix A, the column matrix B of the known terms and X is the column matrix of the unknowns.
Consider also the corresponding homogeneous system AX = 0 with the same A and X as above.
If X' is a fixed solution of AX = B then AX' = B .
If X" is a arbitrary solution of AX = 0 then AX" = 0 .
Then,
 
    AX' + AX" = B
<=>
    A(X' + X") = B
<=>
    X' + X" is a solution of AX = B.
Conclusion:
If we add an arbitrary solution X" of AX = 0 to a fixed solution X' of AX = B
then X' + X" is a solution of AX = B.

Furthermore:
If X' is a fixed solution of AX = B then AX' = B .
If X" is a arbitrary solution of AX = B then AX" = B .
Then,

 
    AX" - AX' = 0
<=>
    A(X" - X') = 0
<=>
    X" - X' is a solution of AX = 0
<=>
    X"  =  X' + (a solution of AX = 0)
Conclusion:
Any arbitrary solution of AX = B is the sum of a fixed solution of AX = B and a solution of AX = 0

If we have a fixed solution of AX = B and we add to this solution all the solutions of the corresponding homogeneous system one after another, then we get all solutions AX = B.

Example:

 
  / 2x + 3y - z + t = 0
  \ x - y + 2z - t = 0

  Above we have seen that the solutions are z(-1,1,1,0) + t(2/5,-3/5,0,1)
  with z and t in R


  / 2x + 3y - z + t = 5
  \ x - y + 2z - t = 0

   has a solution (1,1,0,0)

   All solutions of the last system are  (1,1,0,0) + z(-1,1,1,0) + t(2/5,-3/5,0,1)
   with z and t in R

Sum of two vector spaces

Say A and B are subspaces of a vector space V.

We define the sum of A and B as the set

{ a + b with a in A and b in B }

We write this sum as A + B.


The sum as subspace

Theorem: The sum A+B, as defined above, is a subspace of V.

Proof:

For all a1 and a2 in A and all b1 and b2 in B and all r, s in R we have

 
r(a1 + b1) + s(a2 + b2)  = (r a1 + s a2) + (r b1 + s b2)

   is in A + B.

Direct sum of two vector spaces

The sum A+B, as defined above, is a direct sum if and only if the vector 0 is the only vector common to A and B.

Example :

 
In the space R3
    A = span{ (3,2,1) }
    B = span{ (2,1,4) ; (0,1,3) }
Investigate if A+B is a direct sum

 
Say r,s,t are real numbers, then
each vector in space A is of the form r.(3,2,1) and
each vector in space B is of the form s.(2,1,4) + t.(0,1,3) .

For each common vector, there is a suitable r,s,t such that

    r.(3,2,1) = s.(2,1,4) + t.(0,1,3)

<=>

    r.(3,2,1) - s.(2,1,4) - t.(0,1,3) = (0,0,0)

<=>
    / 3r - 2s     = 0
    | 2r -  s - t = 0
    \ r  - 4s -3t = 0


Since |3  -2   0|
      |2  -1  -1| is  not 0,
      |1  -4  -3|

the previous system has only the solution r = s = t = 0.

The vector (0,0,0) is the only common vector of A and B.
Thus, A+B is a direct sum.

Property of direct sum

If A + B is a direct sum, then each vector v in A+B can be written, in just one way, as the sum of an element of A and an element of B.

Proof:

 
Suppose v = a1 + b1 = a2 + b2  with ai in A and bi in B.

Then   a1 - a2 = b2 - b1 and a1 - a2 is in A and b2 - b1 is in B

Therefore a1 - a2 = b2 - b1 is a common element of A and B.

But the only common element is 0.

So,  a1 - a2 = 0 and b2 - b1 = 0

and   a1 = a2  and b2 = b1

Example :

 
In the space   R3  we define
    A = span{ (3,2,1) }
    B = span{ (2,1,4) ; (0,1,3) }
We know from previous example that A+B is a direct sum.

We'll write the vector (7,3,6) as a sum of an element from A and an element from B.


We calculate (r,s,t) such that (7,3,6) = r(3,2,1) +s(2,1,4) + t(0,1,3)

 
   3r + 2s + 0t = 7
   2r + 1s + 1t = 3
   1r + 4s + 3t = 6
This is a Cramer system. It has exactly 1 solution.
We find: (1, 2, -1)
So, the unique notation of (7,3,6) as a sum of an element from A and an element from B is
 
    (7,3,6) = (3,2,1) + (4,1,5)

Supplementary vector spaces

Say that vector space V is the direct sum of A and B, then
 
A is the supplementary vector space of B relative to V.

B is the supplementary vector space of A relative to V.

A and B are supplementary vector spaces relative to V.


Basis and direct sum

Theorem:
Say V is the direct sum of the spaces M and N.

If {a,b,c,..,l } is a basis of M and {a',b',c',..,l' } is a basis of N,
then {a,b,c,..,l,a',b',c',..,l' } is a basis of M+N.


Proof:
Each vector v of V can be written as m + n with m in M and n in N.
Then m = ra + sb + tc + ... + zl and n = r'a' + s'b' + t'c' + ... + z'l' , with r,s,t,...z,r',s',t',...z' real coefficients.

Thus each vector v = ra + sb + tc + ... + zl + r'a' + s'b' + t'c' + ... + z'l'
Therefore the set {a,b,c,..,l,a',b',c',..,l'} generates V.

If ra + sb + tc + ... + zl + r'a' + s'b' + t'c' + ... + z'l' = 0 , then ra + sb + tc + ... + zl is a vector m of M and
r'a' + s'b' + t'c' + ... + z'l' is a vector n of N.

From a previous theorem we know that we can write the vector 0 in just one way, as the sum of an element of M and an element of N. That way is 0 = 0 + 0 with 0 in M and 0 in N.

From this we see that necessarily m = 0 and n = 0 and thus ra + sb + tc + ... + zl = 0 and
r'a' + s'b' + t'c' + ... + z'l' = 0

Since all vectors in these expressions are linear independent, it is necessarily that all coefficients are 0 and from this we know that the generating vectors {a,b,c,..,l,a',b',c',..,l'} are linear independent.

Dimension of a direct sum

From previous theorem it follows that
dim(A+B) = dim(A) + dim(B)

Converse theorem

If {a,b,c,..,l } is a basis of M and {a',b',c',..,l' } is a basis of N, and {a,b,c,..,l,a',b',c',..,l' } are linear independent,
then M+N is a direct sum.

Proof:
Each element m of M can be written as ra + sb + tc + ... + zl .
Each element n of N can be written as r'a' + s'b' + t'c' + ... + z'l' .
For a common element we have
 
  ra + sb + tc + ... + zl  = r'a' + s'b' + t'c' + ... + z'l'

<=>
 ra + sb + tc + ... + zl - r'a' - s'b' - t'c' - ... - z'l' = 0
Since all vectors are linear independent, all coefficients must be 0. The only common vector is 0.

Direct sum criterion

From the two previous theorems we deduce that:
If {a,b,c,..,l } is a basis of M and {a',b',c',..,l' } is a basis of N, then
 
M+N is a direct sum   <=> {a,b,c,..,l,a',b',c',..,l' } are linear independent


Example :
V is the vector space of all polynomials in x with real coefficients.
M is the subspace of V with basis (12, 7x + 2x2)
N is the subspace of V with basis (3x, 4x + 5x2)
Examine whether M+N is a direct sum.

 
         M+N is a direct sum
<=>
     The vectors (12, 7x + 2x2, 3x, 4x + 5x2) are linear independent
So, all linear combinations of the four vectors constitute a subspace of the space of all second degree or lower polynomials.
The dimension of this space is three because (1,x,x2) is a basis of this space.
So, the dimension of M+N is three or lower.
Therefore it is impossible that the four vectors are linear independent.
M+N is no direct sum.

Projection in a vector space

Choose two supplementary subspaces M and N relative to the space V. Each vector v of V can be written in exactly one way as the sum of an element m of M and an element n of N.

Then v = m + n .

Now we can define the transformation

 
p: V --> V : v --> m

We define this transformation as

        the projection of V on M relative to N
Example :

V is the space of all polynomials with a degree not greater than 3.
We define two supplementary subspaces
M = span { 1, x }
N = span { x2, x3 }
Each vector of V is the sum of exactly one vector of M and of N.
e.g. 2x3 - x2 + 4x - 7 = (2x3 - x2) + (4x - 7)

Say p is the projection of V on M relative to N, then

 
    p(2x3 - x2 + 4x - 7 ) = 4x - 7
Say q is the projection of V on N relative to M, then
 
    q(2x3 - x2 + 4x - 7 ) = 2x3 - x2
To create the matrix of a projection see chapter: linear transformations.

Similarity transformation of a vector space

Let r = any constant real number.
In a vector space V we define the transformation
 
   h : V --> V : v --> r.v
We say that h is a similarity transformation of V with factor r.

Important special values of r are 0, 1 and -1.

Reflection in a vector space

Choose two supplementary subspaces M and N relative to the space V.
Each vector v of V is the sum of exactly one vector m of M and n of N.

Now we define the transformation

 
    s : V --> V : v --> m - n
We say that s is the reflection of V in M relative to the N.

This definition is a generalization of the ordinary reflection in a plane. Indeed, if you take the ordinary vectors in a plane and if M and N are one dimensional supplementary subspaces, then you'll see that with the previous definition, s becomes the ordinary reflection in M relative to the direction given by N.

Example of a reflection

 
Take V = R4.

M = span{(0,1,3,1);(1,0,-1,0)}

N = span{(0,0,0,1);(3,2,1,0)}
It is easy to show that M and N have only the vector 0 in common. (This is left as an exercise.) So, M and N are supplementary subspaces.

Now we'll calculate the image of the reflection of vector v = (4,3,3,1) in M relative to N.

First we write v as the sum of exactly one vector m of M and n of N.

 
 (4,3,3,1) = x.(0,1,3,1) + y.(1,0,-1,0) + z.(0,0,0,1) + t.(3,2,1,0)
The solution of this system gives x = 1; y = 1; z = 0; t = 1. The unique representation of v is
 
(4,3,3,1) = (1,1,2,1) + (3,2,1,0)
The image of the reflection of vector v = (4,3,3,1) in M relative to N is vector v' =
 
   (1,1,2,1) - (3,2,1,0) = (-2,-1,1,1)
To create the matrix of a reflection see chapter: linear transformations.

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