# Finding ##A^{-1}## of a matrix given three submatrices

• ChiralSuperfields
In summary, the solution to the problem is to find ##A^{-1}## given,The solution is,However, in the first image, why are we allowed to put together the submatrices in random order?
ChiralSuperfields
Homework Statement
Relevant Equations
For this problem,
Find ##A^{-1}## given,

The solution is,

However, in the first image, why are we allowed to put together the submatrices in random order? In general does someone please know why we are allowed to decompose matrices like this?

Many thanks!

Last edited by a moderator:
ChiralSuperfields said:

why are we allowed to put together the submatrices in random order?
From three instances of ##Ax=b## for x and b vectors, you just put the three equations in a row of 3 to get the matrices B and C with ##AB=C##. You are thus given a matrix multiplication equation. Then you use the properties of this multiplication: ##AB=C## implies ##B^{-1}A^{-1}=C^{-1}## implies ##A^{-1}=BC^{-1}##. We might transform B and C with a column permutation by right-multiplying with a permutation matrix, in fact with any invertible matrix M. Then we get ##ABM=CM## implies ##A^{-1}=BM(CM)^{-1}=BMM^{-1}C^{-1}=BC^{-1}##

Last edited by a moderator:
ChiralSuperfields
So the point is that ##Ax=b## comma ##Ay=c## may also be written as ##A(x ~ y) = (b ~ c)##. This is a property of matrices

ChiralSuperfields
There are six ways of forming matrices as you say. Choose one of them,e.g.

When you multiply
$$\begin{pmatrix} 0 & 1 & 0 \\ 0 & 0 & 1 \\ 1 & 0 & 0 \\ \end{pmatrix}$$
from right on the both sides, you will find that the equation becomes one of another choice and you will know that the both choices are equivalent. You may be able to find all the six matrices to confirm that all the six choices are equivalent.

ChiralSuperfields
ChiralSuperfields said:
However, in the first image, why are we allowed to put together the submatrices in random order? In general does someone please know why we are allowed to decompose matrices like this?

Many thanks!
It is not "random order".
They did not decompose a matrix, rather a matrix is being constructed based upon the given information.

As is often the case:
Rather than investigating how that construction works, you just post a solution and ask others to explain the solution.

My suggestion:
Using matrix multiplication verify (for yourself) the first line of the solution.

You can not learn math or physics simply by looking at posted solutions.

ChiralSuperfields and PhDeezNutz
I’d imagine it has something to do with swapping rows and columns in a matrix (elementary row operations or column operations preserve the solution space so you can arrange them however you want)

ChiralSuperfields
I think no one has yet pointed out that the solution (2nd image in Post #1) is wrong. It says:

##\begin {bmatrix}
0 & 1 & 0\\
0 & 2 & 1\\
-1 & 2 & 1
\end {bmatrix}
\begin {bmatrix}
\frac 1c & 0 & 0\\
0 & \frac 1a & 0\\
0 & 0 & \frac 1b
\end {bmatrix}
=
\begin {bmatrix}
0 & \frac 1a & 0\\
0 & \frac 2a & \frac 1c\\
-\frac 1c & \frac 2a & \frac 1c
\end{bmatrix} ##

This is clearly wrong as ##b## has disappeared into the aether! It should say:

##\begin {bmatrix}
0 & 1 & 0\\
0 & 2 & 1\\
-1 & 2 & 1
\end {bmatrix}
\begin {bmatrix}
\frac 1c & 0 & 0\\
0 & \frac 1a & 1\\
0 & 0 & \frac 1b
\end {bmatrix}
=
\begin {bmatrix}
0 & \frac 1a &0\\
0 & \frac 2a & \frac 1b\\
-\frac 1c & \frac 2a & \frac 1b
\end {bmatrix} ##

In addition to what has already been said, it appears that the ‘submatrices’ (columns) have not been put together in ‘random order’. They have been put together, for convenience, in the order which creates the diagonal matrix, ##diag(c, a, b)##. This is to make finding the inverse of the matrix easy.

ChiralSuperfields
Steve4Physics said:
It should say:

##\begin {bmatrix}
0 & 1 & 0\\
0 & 2 & 1\\
-1 & 2 & 1
\end {bmatrix}
\begin {bmatrix}
\frac 1c & 0 & 0\\
0 & \frac 1a & 1\\
0 & 0 & \frac 1b
\end {bmatrix}
=
\begin {bmatrix}
0 & \frac 1a &0\\
0 & \frac 2a & \frac 1b\\
-\frac 1c & \frac 2a & \frac 1b
\end {bmatrix} ##

In addition to what has already been said, it appears that the ‘submatrices’ (columns) have not been put together in ‘random order’. They have been put together, for convenience, in the order which creates the diagonal matrix, ##diag(c, a, b)##. This is to make finding the inverse of the matrix easy.
Exactly.

To flesh out what @Steve4Physics says above, it's important to have clear, concise equations of the work. It's also extremely important to post legible screen shots. I had a difficult time reading the entries in some of your matrices. The problem with illegible screen shots is why we discourage the use of images for most homework problems.

##A
\begin {bmatrix}
0 & 1 & 0\\
0 & 2 & 1\\
-1 & 2 & 1
\end {bmatrix} =
\begin {bmatrix}
c & 0 & 0\\
0 & a & 0\\
0 & 0 & b
\end {bmatrix}##
For the matrix equation above, multiply on the left by ##A^{-1}##. It's reasonable to assume this inverse exists, otherwise there's no point to the problem.

This results in the following matrix equation:
##
\begin {bmatrix}
0 & 1 & 0\\
0 & 2 & 1\\
-1 & 2 & 1
\end {bmatrix} = A^{-1}
\begin {bmatrix}
c & 0 & 0\\
0 & a & 0\\
0 & 0 & b
\end {bmatrix}##

Now multiply each side of the matrix equation on the right by the inverse of this diagonal matrix. This unnamed matrix on the right is very easy to invert. The entries on the main diagonal of its inverse are 1/c, 1/a, and 1/b, respectively.

##
\begin {bmatrix}
0 & 1 & 0\\
0 & 2 & 1\\
-1 & 2 & 1
\end {bmatrix}
\begin {bmatrix}
\frac 1c & 0 & 0\\
0 & \frac 1a & 0\\
0 & 0 & \frac 1b
\end {bmatrix} = A^{-1} I = A^{-1}##

I got the identity matrix in the next-to-last expression when I multiplied the diagonal matrix with entries c, a, and b by its inverse, which is another diagonal matrix with entries 1/c, 1/a, and 1/b.

All that's left to do is to multiply the two matrices on the left side of the equation above, which results in the matrix that @Steve4Physics showed.

Steve4Physics, ChiralSuperfields and PhDeezNutz

## 1. How do I find the inverse of a matrix given three submatrices?

To find the inverse of a matrix given three submatrices, you can use the submatrix method. First, you will need to calculate the determinant of the main matrix using the three submatrices. Then, you can use the formula A-1 = (1/det(A)) * adj(A) to find the inverse, where adj(A) is the adjugate of the main matrix.

## 2. What is the submatrix method for finding the inverse of a matrix?

The submatrix method is a technique used to find the inverse of a matrix given three submatrices. It involves calculating the determinant of the main matrix using the submatrices and then using this value to calculate the inverse using the formula A-1 = (1/det(A)) * adj(A).

## 3. Can I use the submatrix method to find the inverse of any matrix?

Yes, the submatrix method can be used to find the inverse of any matrix as long as the matrix is square and has a non-zero determinant. If the determinant is zero, the matrix is not invertible and the submatrix method cannot be used.

## 4. Are there any other methods for finding the inverse of a matrix?

Yes, there are other methods for finding the inverse of a matrix such as Gaussian elimination, Cramer's rule, and the LU decomposition method. However, the submatrix method is a commonly used and efficient method for finding the inverse of a matrix given three submatrices.

## 5. Can I use the inverse of a matrix to solve equations?

Yes, the inverse of a matrix can be used to solve systems of equations. By multiplying both sides of an equation by the inverse of the coefficient matrix, you can isolate the variables and solve for their values. This method is especially useful for solving systems of equations with more than two variables.

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