Numerical Methods Qualification Exam Problems and Solutions (University of Maryland)/Jan07 667

Problem 4

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Let  , suppose   and assume   is nonsingular . Consider the following iteration

 

Problem 4a

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Derive the following error equation for  

 

Solution 4a

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Note the following identity

 


The error   is given by

 

Problem 4b

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Let   be a fixed matrix. Find conditions on B that guarentee local convergence. What rate of convergence do you expect and why?

Solution 4b

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Assume   is invertible,   is bounded, and   is Lipschitz.

 


This implies local superlinear convergence.

Problem 4c

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Find sufficient conditions on   for the convergence to be superlinear. What choice of   corresponds to the Newton method and what rate of convergence do you expect?

Solution 4c

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Super linear convergence

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  as  

Find condition for super linear convergence

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From part(b)


 


Since  , if


 


as  , we have super linear convergence i.e.


 

Problem 5

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Let   be uniformly Lipschitz with respect to  . Let   be the solution to the initial value problem :  . Consider the trapezoid method

 .

Problem 5a

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Find a condition on the stepsize   that ensures (1) can be solved uniquely for  .

Solution 5a

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The implicit method can be viewed as a fix point iteration:


 


We want  


which implies


 

Problem 5b

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Define a local truncation error and estimate it. Examine the additional regularity of   needed for this estimate.

Solution 5b

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Re-writing (1) and replacing   we have a formula for consistency of order p:


 


For uniform stepsize h

 

Expanding in Taylor Series about   gives

Problem 5c

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Prove a global error estimate for (1)

Solution 5c

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Problem 6

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Consider the 2-point boundary value problem

 ,

with   constants and  . Let   be a uniform partition of [0,1] with meshsize  .

Problem 6a

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Use centered finite differences to discretize (2). Write the system as

 

and identify  . Prove that A is nonsingular.

Solution 6a

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Using Taylor Expansions, we can approximate the second derivative as follows


 


We can eliminate two equations from the n+2 equations by substituting the initial conditions   into the equations for   and   respectively.


We then have the system


 


  is nonsingular since it is diagonally dominant.

Problem 6b

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Define truncation error and derive a bound for this method in terms of  . State without proof an error estimate of the form

 

and specify the order s.

Solution 6b

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Local Truncation Error

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Bound for Local Truncation Error

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Derive Bound for Max Error

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Let  ,  , and   is the local truncation error.


Then


 


Subtracting the two last equations gives


 


Hence,


 

 , that is the error has order 2.

Problem 6c

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Prove the following discrete monotonicity property: If   is the solution corresponding to a forcing  , for  , and   then   componentwise.

Solution 6c

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Note that   is a   matrix and hence the discrete maximum principle applies. (See January 05 667 test for definition of   matrix)


Discrete Maximum Principle

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If  , then  .


Specifically let  , then   which implies