Find the values of λ for which the following system of equations has nontrivial solutions:
\begin{matrix}5x + 2y + z = λx \\ 2x + y \quad\quad = λy \\ x \ +\quad\quad z = λz \end{matrix} (∗)
The variables x, y, and z appear on both sides of the equations, so we start by putting the system into standard form:
\begin{matrix}\ \ (5 − λ)x +\quad \ \ \ \ \ \ \ \ \ \ \ \ 2y + \quad z = 0 \\ \quad \quad \ \ \ \ \ \ \ 2x + (1 − λ)y \quad\quad \ \ \ \ \ = 0 \\ \quad \quad \quad \quad x \quad \quad \quad \quad + (1 − λ)z = 0 \end{matrix}According to Theorem 16.8.2, this system has a nontrivial solution if and only if the coefficient matrix is singular:
\left|{\begin{array}{l l l}{5-\lambda}&{2}&{1}\\ {2}&{1-\lambda}&{0}\\ {1}&{0}&{1-\lambda}\end{array}}\right|=0The value of the determinant is found to be λ(1 − λ)(λ − 6). Hence, system (∗) has nontrivial solutions if and only if λ = 0, 1, or 6.^{10}
^{10} Using terminology explained in FMEA, this example asks us to find the eigenvalues of the coefficient matrix \begin{pmatrix} 5 & 2 &1 \\ 2 & 1 &0 \\1 & 0 &1 \end{pmatrix}