Exercise 9.  Use residues to find   .  

9 (d).  .                 Hint.   .  

Solution 9 (d).

See text and/or instructor's solution manual.

Answer.   .  

Aside.   We could use Table 9.1 of z-transforms and get the solution  

                      

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

Solution.   Using residues we get

                      
                    
                    and

                      

Thus,  

                      

Therefore,

                    .  

Aside.  The solution can be put into the form

                      

Therefore,

                    .  

We are done.   

Aside.  We can let Mathematica double check our work.

Aside.  We can use Mathematica's  Limit  and  Residue  subroutines.

          The Maple code using limits is similar  

  

                                                            


  

                                                            


  

                                                            

  

                                                            

          The Maple code using residues is similar  

  

                                                            


  

                                                            


  

                                                            

          We can use Maple's subroutine to find the inverse.  

  

                                                            

We are really done.   

Aside.  We can graph some of the terms in the sequence.

                                                                                                                        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  

                      

Now we have the desired form:

                    .

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 complex roots.

This solution is complements of the authors.

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