The Minimum Rank of a 3 × 3 Partial Block Matrix

The minimum rank of the 3 × 3 partial block matrix $$ \left[ {\matrix{ {A_{11} } & {A_{12} } & ? \cr {A_{21} } & ? & {A_{23} } \cr ? & {A_{32} } & {A_{33} } \cr } } \right]$$ is determined when A ij (1 h i , j h 3) are all nonsingular matrices of the same size. This solves partially a problem that was proposed in [N. Cohen, C.R. Johnson, L. Rodman and H.J. Woerdeman (1989). Ranks of completions of partial matrices. Operator Theory: Advances and Applications , 40 , 165-185.].