Eigenvalues Problem: Show Inverse of Diagonalizable Matrix A

In summary, it is shown that for a diagonalizable matrix A with eigenvalues x1, x2, ..., xn, the characteristic polynomial p(x) = a1(x)^n + a2(x)^n-1 + ...+an+1. Using the Cayley-Hamilton theorem, it can be determined that A^-1 = q(A) for some polynomial q of degree less than n. This is derived from the fact that p(A) = 0, implying that a*A^n + b*A^(n-1) + c*A^(n-2)...+ I = 0. By multiplying both sides by the inverse of A, it can be shown that the characteristic polynomial is of degree < n,
  • #1
tc
7
0
let A be a diagonalizable matrix with eignvalues = x1, x2, ..., xn
the characteristic polynomial of A is
p (x) = a1 (x)^n + a2 (x)^n-1 + ...+an+1
show that inverse A = q (A) for some polynomial q of degree less than n
 
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  • #2
By cayley-hamilton p(A)=0, now what can you do with that?
 
  • #3
Yeah, you use cayley-hamilton theorem
so, you have p(A)=0...
That implies a*A^n + b*A^(n-1) + c*A^(n-2)...+ I= 0
(i've used a,b,c as coefficients)...then take the identity matrix to the other side. Multiply both sides by inverse of A. Then RHS becomes -A^(-1) the LHS shows that the characteristic polynomial is of degree < n.
 
  • #4
of course, we assuming that none of the eigenvalues is zero, too.
 
  • #5
o..thx matt and mansi
 

1. What is an eigenvalue?

An eigenvalue is a scalar value that represents the amount by which a matrix stretches or compresses a vector in a certain direction. It is a key concept in linear algebra and is often used in analyzing systems of linear equations.

2. What is a diagonalizable matrix?

A diagonalizable matrix is a square matrix that can be transformed into a diagonal matrix through a similarity transformation. This means that it can be expressed as a product of three matrices: A = PDP-1, where P is a matrix of eigenvectors and D is a diagonal matrix of eigenvalues.

3. How do you show the inverse of a diagonalizable matrix A?

To show the inverse of a diagonalizable matrix A, you can use the formula A-1 = PD-1P-1, where P is the matrix of eigenvectors and D is the diagonal matrix of eigenvalues. This formula works because it is a property of diagonalizable matrices that their inverse can be expressed using the inverse of their eigenvalues.

4. What is the significance of solving for the inverse of a diagonalizable matrix A?

Solving for the inverse of a diagonalizable matrix A is important because it allows us to efficiently solve systems of linear equations involving the matrix A. It also helps us understand the behavior of A and its effect on vectors in the system.

5. Are there any special cases where a diagonalizable matrix may not have an inverse?

Yes, if any of the eigenvalues of a diagonalizable matrix A are equal to zero, then A will not have an inverse. This is because the inverse of a diagonal matrix is only defined when all of its diagonal elements are non-zero.

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