How Do You Find the Nearest Vector in the Row Space of a Matrix?

In summary, the conversation revolves around finding the vector in the row space A nearest to the vector (1,1,-1,1). The solution involves computing the orthogonal projection of (1,1,-1,1) onto Row A, which can be expressed as a linear combination of the rows of A. There is a discussion about the confusion in understanding the question and clarification is given on the correct approach to solving the problem.
  • #1
Lchan1
39
0

Homework Statement



I have a question about an old exam and i have no idea where to start. The question says:
Find the vector in the row space A=[1 0 1 1 /new row 0 1 -1 1] 2x4 matrix nearest to the vector (1,1,-1,1)
I have absolutely no clue how to start.

Homework Equations





The Attempt at a Solution


I have no clue how to start this...
my attempt was looking at it being confused for an hour
 
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  • #2
Try writing the vector (1,1,-1,1) as a sum of two vectors, one parallel to Row A and one orthogonal to Row A. Hint: what does an orthogonal projection tell you?
 
  • #3
I got an answer
(2/3,2/3,-1,1/3)
does that sound right to you?
I project the vector onto the row space.
and v-the projection gives me nearest vector.
 
  • #4
I arrived at a different answer. You may want to check and see if the vector you found is in Row(A). This is equivalent to seeing if you can find real constants a and b such that

[tex]a(1,0,1,1) + b(0,1,-1,1) = (\frac{2}{3}, \frac{2}{3}, -1, \frac{1}{3})[/tex]

I think you will find that the system of linear equations associated with this equation is inconsistent, so the vector you found does not satisfy the requirements of the problem. You may want to try and compute the orthogonal projection again.
 
  • #5
should i just project the vector to the matrix?
which i will get (1/3,1/3,0,2/3)
 
  • #6
Let r1 and r2 be the first and second rows of A respectively, and let v be the vector given in the problem.

The orthogonal projection you should be computing is:

[tex]\frac{v \cdot r_1}{r_1\cdot r_1}r_1 + \frac{v \cdot r_2}{r_2 \cdot r_2}r_2[/tex]

This guarantees the vector you find is in Row A. Do you see why?
 
  • #7
Yea as this projects v onto r.
I think i didnt understand the question correctly. that's why i got the other answer.
so they wanted the vector in A that is neartest to vector r which is projecting R onto A.
and the projection could be expressed by r1 and r2
 
  • #8
I think you are confused. The projection I gave you is the projection of (1,1,-1,1) onto Row A. We know that
Row A = span{(1,0,1,1), (0,1,-1,1)} = a(1,0,1,1)+b(0,1,-1,1). They are asking for the vector in Row A that is closest to the vector (1,1,-1,1), which is the same as asking, "what is the orthogonal projection of (1,1,-1,1) onto the subspace Row A?"

Do you need any further clarification?
 
  • #9

Related to How Do You Find the Nearest Vector in the Row Space of a Matrix?

1. What is a vector in row space?

A vector in row space is a representation of a linear combination of the rows of a matrix. It can be thought of as a set of coefficients that, when multiplied with the corresponding rows of the matrix, will produce a linear combination that is equal to the original vector.

2. Why is it important to solve a vector in row space?

Solving a vector in row space is important because it allows us to find the coefficients that make up the linear combination of the rows of a matrix. This can be useful in various applications, such as solving systems of linear equations, finding eigenvectors and eigenvalues, and performing transformations.

3. What is the process for solving a vector in row space?

The process for solving a vector in row space involves using Gaussian elimination to reduce the matrix to its reduced row echelon form. Then, the coefficients of the pivot columns in the reduced matrix can be used to form the vector in row space.

4. Are there any limitations to solving a vector in row space?

Yes, there are limitations to solving a vector in row space. One limitation is that the matrix must have at least as many rows as columns in order for a solution to exist. Additionally, if the matrix is not invertible, there may be infinitely many solutions or no solution at all.

5. Can you provide an example of solving a vector in row space?

Sure, let's say we have the matrix A = [1 2 3; 4 5 6; 7 8 9]. To solve the vector [x y z] in row space, we would use Gaussian elimination to reduce the matrix to its reduced row echelon form: A = [1 0 -1; 0 1 2; 0 0 0]. This means that x - z = 0 and y + 2z = 0. Therefore, the vector [x y z] in row space would be [z -2z z], or [z -2z z] = z[1 -2 1].

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