Eigenvalues Adj A
1. The problem asks to find the eigenvalues of the matrix \( A = \begin{pmatrix} 3 & 2 & 1 \\ 0 & 4 & 2 \\ 0 & 0 & 1 \end{pmatrix} \) and then find the eigenvalues of the adjoint matrix \( \text{Adj } A \).
2. The adjoint \( \text{Adj } A \) of a matrix \( A \) is defined as the transpose of the cofactor matrix of \( A \).
3. Since \( A \) is an upper triangular matrix, its eigenvalues are the diagonal elements:
$$ \lambda_1 = 3, \quad \lambda_2 = 4, \quad \lambda_3 = 1 $$
4. The eigenvalues of \( \text{Adj } A \) are related to those of \( A \) by:
$$ \text{If } \lambda_i \text{ are eigenvalues of } A, \text{ then the eigenvalues of } \text{Adj } A \text{ are } \prod_{j \neq i} \lambda_j $$
(This holds for invertible matrices; \( A \) has eigenvalues 3,4,1 so invertible.)
5. Calculate the eigenvalues of \( \text{Adj } A \):
- For \( \lambda_1 = 3 \), adjoint eigenvalue is \( \lambda_2 \lambda_3 = 4 \times 1 = 4 \)
- For \( \lambda_2 = 4 \), adjoint eigenvalue is \( \lambda_1 \lambda_3 = 3 \times 1 = 3 \)
- For \( \lambda_3 = 1 \), adjoint eigenvalue is \( \lambda_1 \lambda_2 = 3 \times 4 = 12 \)
6. Therefore the eigenvalues of \( \text{Adj } A \) are:
$$ 4, 3, 12 $$
7. Summarizing:
The matrix \( A \) has eigenvalues \( 3,4,1 \).
The adjoint matrix \( \text{Adj } A \) has eigenvalues \( 4,3,12 \).