2005-3.md

course	course_year	question_number	tags	title	year
Linear Algebra	IB	3	IB 2005 Linear Algebra	2.II.10C	2005

(i) Let $A=\left(a_{i j}\right)$ be an $n \times n$ matrix with entries in C. Define the determinant of $A$, the cofactor of each $a_{i j}$, and the adjugate matrix $\operatorname{adj}(A)$. Assuming the expansion of the determinant of a matrix in terms of its cofactors, prove that

$$\operatorname{adj}(A) A=\operatorname{det}(A) I_{n}$$

where $I_{n}$ is the $n \times n$ identity matrix.

(ii) Let

$$A=\left(\begin{array}{llll} 0 & 1 & 0 & 0 \\ 0 & 0 & 1 & 0 \\ 0 & 0 & 0 & 1 \\ 1 & 0 & 0 & 0 \end{array}\right)$$

Show the eigenvalues of $A$ are $\pm 1, \pm i$, where $i^{2}=-1$, and determine the diagonal matrix to which $A$ is similar. For each eigenvalue, determine a non-zero eigenvector.