If $P$ is a $3 \times 3$ matrix such that $P^{T}=2 P+I$, where $P^{T}$ is the transpose of $P$ and $I$ is…

If $P$ is a $3 \times 3$ matrix such that $P^{T}=2 P+I$, where $P^{T}$ is the transpose of $P$ and $I$ is the $3 \times 3$ identity matrix, then there exists a column matrix $\quad X=\left[\begin{array}{l}x \\ y \\ z\end{array}\right] \neq\left[\begin{array}{l}0 \\ 0 \\ 0\end{array}\right]$ such that
  1. $P X=\left[\begin{array}{l}0 \\ 0 \\ 0\end{array}\right]$
  2. $P X=X$
  3. $P X=2 X$
  4. $P X=-X$

Solution

$P^{T}=2 P+I$ $\begin{array}{l} \Rightarrow P=2 P^{T}+I \Rightarrow P=2(2 P+I)+I \\ \Rightarrow P=4 P+3 I \Rightarrow P+I=0 \\ \Rightarrow P X+X=0 \Rightarrow P X=-X \end{array}$

Asked in: JEE Advanced 2012 (Paper 2)

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