已知 $\boldsymbol{\alpha}_1=\left(\begin{array}{c}a \\ 1 \\ 1\end{array}\right), \boldsymbol{\alpha}_2=\left(\begin{array}{c}1 \\ a \\ -1\end{array}\right), \boldsymbol{\alpha}_3=\left(\begin{array}{c}1 \\ -1 \\ a\end{array}\right), \boldsymbol{\alpha}_4=\left(\begin{array}{l}1 \\ 1 \\ a\end{array}\right)$ ，若 $\boldsymbol{\alpha}_1, \boldsymbol{\alpha}_2, \boldsymbol{\alpha}_3$ 线性相关，而 $\boldsymbol{\alpha}_2, \boldsymbol{\alpha}_3, \boldsymbol{\alpha}_4$ 线性无关，则 $a=$