On the Remarkable Formula for Spectral Distance of Block Southeast Submatrix

‎‎‎This paper presents a remarkable formula for spectral distance of a given block normal matrix $G_{D_0} = begin{pmatrix}‎ ‎A & B \‎ ‎C & D_0‎ ‎end{pmatrix} $ to set of block normal matrix $G_{D}$ (as same as $G_{D_0}$ except block $D$ which is replaced by block $D_0$)‎, ‎in which $A in mathbb{C}^{ntimes n}$ is invertible‎, ‎$ B in mathbb{C}^{ntimes m}‎, ‎C in mathbb{C}^{mtimes n}$ and $D in mathbb{C}^{mtimes m}$ with $rm {Rank{G_D}} < n+m-1$‎ ‎and given eigenvalues of matrix $mathcal{M} = D‎ - ‎C A^{-1} B $ as $z_1‎, ‎z_2‎, ‎cdots‎, ‎z_{m}$ where $|z_1|ge |z_2|ge cdots ge |z_{m-1}|ge |z_m|$‎. Finally, an explicit formula is proven for spectral distance $G_D$ and $G_D_0$ which is expressed by the two last eigenvalues of $mathcal{M}$.