Solving for Invertibility of (B+I) Given B=B^2

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In summary, the conversation discussed the invertibility of (B+I) given a matrix B, where B = B2. It was determined that if det(B) = 1, then B+I is invertible, and if det(B) = 0, then B must be a diagonal matrix with entries of 1 or 0. The conversation also mentioned that knowing the minimal polynomial of B can provide information about its eigenvalues.
  • #1
transcendency
2
0

Homework Statement


Given a matrix B, if B = B2, is (B+I) invertible?

2. The attempt at a solution

det(B) = 0 or 1

rref(rref(B) + I) is I, so (rref(B) + I) is invertible

if det(B) = 1:
let E1E2...En = B
then E1E2...En(rref(B) + I) = B + E1E2...En

I'm not sure if what I did is even useful =(
 
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  • #2
What can you say about det(B + I)?
 
  • #3
If det(B) = 1, then B-1B = B-1B2, B = I
so det(B+I) = det(2I) != 0, so B+I is invertible.

I'm still stuck on if det(B) = 0..

I'm quite sure that if B = B2, then B must be a diagonal matrix with entries being either 1 or 0, but I don't know how to prove it.
 
  • #4
If B= B2 then B2- B= B(B- I)= 0.
 
  • #5
transcendency said:
I'm quite sure that if B = B2, then B must be a diagonal matrix with entries being either 1 or 0, but I don't know how to prove it.

That follows from the fact you know its minimal poly divides X^2-X, hence you know all the possible eigenvalues (0 and 1), asndyou know the geometric multiplicity is 0 or 1.

Alternatively remember that a definition of an eigenvalue is:t is an eigenvalue of X if and only if X-tI is not invertible.
 

1. How do you solve for invertibility of (B+I) given B=B^2?

To solve for invertibility of (B+I) given B=B^2, we can use the properties of matrices and apply the determinant rule. This involves finding the determinant of (B+I) and showing that it is non-zero, which indicates that the matrix is invertible.

2. What is the determinant rule?

The determinant rule states that for a square matrix A, if the determinant of A is non-zero, then A is invertible. In other words, a matrix is invertible if and only if its determinant is non-zero.

3. Why is it important to determine the invertibility of a matrix?

Determining the invertibility of a matrix is important because it tells us whether or not we can find an inverse matrix for the given matrix. Inverse matrices are useful in solving systems of equations and performing other mathematical operations.

4. Can (B+I) be invertible if B=B^2?

Yes, (B+I) can still be invertible even if B=B^2. This is because the determinant of (B+I) is not affected by the fact that B=B^2. As long as the determinant is non-zero, the matrix is invertible.

5. Are there any other methods for determining the invertibility of a matrix?

Yes, besides using the determinant rule, we can also use the row reduction method to determine the invertibility of a matrix. This involves performing row operations on the matrix and checking if it can be reduced to the identity matrix, which is an indication of invertibility.

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