Let A be 3 by 3 matrix over real numbers satisfying A⁻¹= I -2A. Then what will be the

determinant of the matrix A?

thanks in advance. regards.(Nod)

Printable View

- November 24th 2011, 02:42 AMsorv1986what will be the determinant of the matrix A
Let A be 3 by 3 matrix over real numbers satisfying A⁻¹= I -2A. Then what will be the

determinant of the matrix A?

thanks in advance. regards.(Nod) - November 24th 2011, 12:14 PMFernandoRevillaRe: what will be the determinant of the matrix A
- November 24th 2011, 02:47 PMHallsofIvyRe: what will be the determinant of the matrix A
There are two possible answers. First, the determinant of a matrix is the same as the product of all of its eigenvalues. Second, if A satisfies that equation, its eigenvalues must also. You can manipulate the equation to a quadratic that has two solutions. Since A is 3 by 3, it has three eigenvalues. Since only two numbers sastisfy that equation, one of them must be a duplicate eigenvalue. The determinant depends upon which of them is the duplicate.

- November 24th 2011, 05:48 PMsorv1986Re: what will be the determinant of the matrix A
- November 24th 2011, 06:34 PMCaramelCardinalRe: what will be the determinant of the matrix A
det(AB) = det(A)det(B)

Determinant is non-zero if it is invertible

det(A) = 1/(det(A^-1)) - November 25th 2011, 12:36 AMFernandoRevillaRe: what will be the determinant of the matrix A
But that is not possible. We have . That is , is an annihilator polynomial of being and . As , and . This means that the minimal polynomial of is just and are eigenvalues of as you said. The third one should be necessarily real and is diagonalizable in . So, is similar (in ) to and as a consequence:

Again, as you said or which implies (contradiction). - November 25th 2011, 03:40 AMFernandoRevillaRe: what will be the determinant of the matrix A
- November 25th 2011, 05:39 AMsorv1986Re: what will be the determinant of the matrix A
how [COLOR=rgb(0, 0, 0)]lambda3=lambda1 or lambda3= lambda2[/COLOR] is possible? lambda_3 must be real as the matrix is 3 by 3.Is that problem is wrong or something? Still i got no clue. 2A^2-A+I=0 cant be the minimal polynomial as every annihilating polynomial is multiple of the minimal polynomial. Please help me

- November 25th 2011, 05:44 AMsorv1986Re: what will be the determinant of the matrix A
- November 25th 2011, 07:00 AMFernandoRevillaRe: what will be the determinant of the matrix A
Plainly: does not exist.

Quote:

Still i got no clue. 2A^2-A+I=0 cant be the minimal polynomial as every annihilating polynomial is multiple of the minimal polynomial. Please help me