Exercise 15.  Solve the difference equation      with initial condition   .

15 (a).  Use the z-transform and tables to find the solution.    Hint.  Get  .  

15 (b).  Use residues to find the solution.

Solution 15.

See text and/or instructor's solution manual.

Answer.   .  

Solution 15 (a).  

        Take the z-transform of both sides and use the initial condition  :  

                    ,  

                    .  
        
Solve for    and get  

                    ,  

then use Table 9.1 to find the inverse z-transform   

                      

Remark.  The details for the partial fraction expansion are at the bottom of the page.

We are done.   

Solution 15 (b).  

        Using residues we get

                      

                    and

                      

Thus

                    

Therefore,

                    .  

We are done.   

Aside.  We can let Mathematica double check our work.

Take the z-transform of both sides.

Find the inverse z-transform.

          The Maple commands are similar  

  

                                                            


  

                                                            


  

                                                            


  

                                                            

Aside.  We can use Mathematica's InverseZTransform subroutine.

          The Maple command is similar  

  

                                                            

We are really done.   

Aside.  We can use Mathematica's Rsolve subroutine.

          The Maple command is similar  

  

                                                            

We are really really done.   

Aside.  We can graph the solution.

                                                                                                                        The sequence   .   

We are really really done.   

The Details for the Partial Fractions.   

Aside.  How can we expand      into the proper partial fractions?

It is natural to try the command:

But this is not the desired form for using Table 9.1 of z-transforms.

Method (i).   Use the following algebra steps  

                      

Method (ii).   Find the linear combination of   ,  

                    .  

Equate the numerators   ,  

and solve the linear system  

                      

and get   .   

Therefore, the desired form is  

                    .  

Aside.   The Mathematica commands for Method (ii)  are

Method (iii).   The substitution      does not apply when there are multiple roots.

This solution is complements of the authors.

(c) 2008 John H. Mathews, Russell W. Howell