Science:Math Exam Resources/Courses/MATH152/April 2013/Question A 15
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Question A 15 

Assume that the matrix B was obtained by applying a sequence of elementary row operations to the matrix A. Circle all the correct statements that are true independent of what the given A and B are. (a) (b) If B is invertible then A is also invertible. (c) If is an eigenvector of A, then v is also an eigenvector of B. (d) If is an eigenvalue of A, then is also an eigenvalue of B. 
Make sure you understand the problem fully: What is the question asking you to do? Are there specific conditions or constraints that you should take note of? How will you know if your answer is correct from your work only? Can you rephrase the question in your own words in a way that makes sense to you? 
If you are stuck, check the hints below. Read the first one and consider it for a while. Does it give you a new idea on how to approach the problem? If so, try it! If after a while you are still stuck, go for the next hint. 
Hint 1 

Elementary row operations are
Do any of these change the value of the determinant? 
Hint 2 

A matrix is invertible if its determinant is nonzero. Can elementary row operations transform a matrix that had a nonzero determinant to a matrix with determinant zero? 
Hint 3 

Eigenvectors are very specific to a matrix. Multiplying a matrix A with e_{1} returns the first column of A. That is, if a_{j} is the jth column of A, then Since v = e_{1} is an eigenvector of the matrix A we know that Hence a_{1} = λ e_{1}. In other words, the only nonzero entry of the first column of A is in the first row. Can you perform elementary row operations to move that entry into another row, and hence find a counter example? 
Checking a solution serves two purposes: helping you if, after having used all the hints, you still are stuck on the problem; or if you have solved the problem and would like to check your work.

Solution 

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Please rate my easiness! It's quick and helps everyone guide their studies. (b) and (d) are the only correct statements independent of and . (a) Recall that scaling a row is an elementary row operation which scales the determinant by the same factor, so if the determinant of is nonzero, can fail to hold. (b) All elementary row operations can be represented by invertible matrices, so since is a product of these matrices, if is invertible, then so is . Alternatively, recall that elementary row operations can only scale the determinant of a matrix by a nonzero factor. Thus, if is nonzero (i.e. it is invertible) then is nonzero (so is invertible as well). (c) This is false, elementary row operations do change eigenvectors. For a simple example, let and . Subtracting the first row from the second row reduces B to A. However, , so v is an eigenvector of A, while which is not a multiple of v. Hence v is not an eigenvector of B. (d) If is an eigenvalue of A, then A is not invertible and the determinant of A is 0. As in (b), elementary row operations only scale the determinant by a constant nonzero factor. Therefore, the and hence B is not invertible. Hence is also an eigenvalue of B. Alternatively, applying an elementary row operation can be represented by multiplication of the original matrix by an elementary matrix (which is invertible). Suppose . Then if , we have , so 0 is an eigenvalue for as well (in this case, v is also an eigenvector for this special eigenvalue). 