Mathematics for Applied Sciences (Osnabrück 2011-2012)/Part I/Exercise sheet 6



Warm-up-exercises

Compute the following product of matrices

 


Compute, over the complex numbers, the following product of matrices

 


Determine the product of matrices

 

where the  -th standard vector (of length  ) is considered as a row vector, and the  -th standard vector (also of length  ) is considered as a column vector.


Let   be an  - matrix. Show that the matrix product   of   with the  -th standard vector (regarded as a column vector) is the  -th column of  . What is  , where   is the  -th standard vector (regarded as a row vector)?


Compute the product of matrices

 

according to the two possible parentheses.


For the following statements we will soon give a simple proof using the relationship between matrices and linear maps.

Show that the multiplication of matrices is associative. More precisely: Let   be a field, and let   be an  -matrix,   an  -matrix, and   a  -matrix over  . Show that  .


For a matrix   we denote by   the  -th product of   with itself. This is also called the  -th power of the matrix.

Compute, for the matrix

 

the powers

 


Let   be a field, and let   and   be vector spaces over  . Show that the product set

 

is also a  -vector space.


Let   be a field, and   an index set. Show that

 

with pointwise addition and scalar multiplication, is a  -vector space.


Let   be a field, and let

 

be a system of linear equations over  . Show that the set of all solutions of this system is a linear subspace of  . How is this solution space related to the solution spaces of the individual equations?


Show that the addition and the scalar multiplication of a vector space   can be restricted to a linear subspace, and that this subspace with the inherited structures of   is a vector space itself.


Let   be a field, and let   be a  -vector space. Let   be linear subspaces of  . Prove that the union   is a linear subspace of   if and only if   or  .




Hand-in-exercises

Exercise (3 marks)

Compute, over the complex numbers, the following product of matrices

 


Exercise (3 marks)

We consider the matrix

 

over a field  . Show that the fourth power of   is  , that is,

 


Exercise (3 marks)

Let   be a field, and let   be a  -vector space. Show that the following properties hold (for   and  ).

  1. We have  .
  2. We have  .
  3. We have  .
  4. If   and  , then  .


Exercise (3 marks)

Give an example of a vector space   and of three subsets of   that satisfy two of the subspace axioms, but not the third.