Let $(a_{ij}), i,j \in \{1,...,n\}$ be a matrix. What are the names for the following special square submatrices:
- for any set of indices $Jā{1,2,..,n}$, the submatrix $(a_{jk})j,k\in J$,
- a submatrix formed by removing the last m rows and last m columns;
- a submatrix formed by removing any m rows and any m columns.
Does a principal submatrice mean any of the above three kinds? Thanks!