The rank of the matrix\(\begin{bmatrix} 1 & 1 & 1 \\[0.3em] a & a^2 & a^3 \end{bmatrix}\) is ____ .
The mean of the density function is \(f(x) = \lambda e^{-\lambda x}, x > 0\) is ____ .
For the function \(f(x) = x^2 e^{-x},\) the maximum occurs when \( x \) is equal to ____ .