The value of \( a \) for which the system \[ ax + y + z = 0,\; x + ay + z = 0,\; x + y + z = 0 \] has non-zero solutions is
If \( A = \begin{bmatrix} 1 & 0 & 0 \\ 0 & 1 & 0 \\ a & b & -1 \end{bmatrix} \), then \( A^2 \) is equal to