Given a matrix $\mathbf{A}$ of size $n\times n$. Let $I=\{i_1,\ldots,i_k\}\subseteq\{1,\ldots,n\}$ for some $k\leqslant n$.
How to call the sub-matrix of $\mathbf{A}$ that has its indices in $I$? (I guess sub-matrix induced by $I$?)
Example. \begin{align} \mathbf{A}=\begin{bmatrix} a_{11} & a_{12} & a_{13} & a_{14}\\ a_{21} & a_{22} & a_{23} & a_{24}\\ a_{31} & a_{32} & a_{33} & a_{34}\\ a_{41} & a_{42} & a_{43} & a_{44} \end{bmatrix}. \end{align} $I=\{2, 4\}$.
Hence, the sub-matrix is
\begin{align} \begin{bmatrix} a_{22} & a_{24}\\ a_{42} & a_{44} \end{bmatrix}. \end{align}