Why is O（n;k）not connected，and it has four connection branches？ $O（n；k）=\{A\in GL(n+k，C)|A^{T}GA=G\}$ where $G=\begin{pmatrix} 1&&&&&\\ &\ddots& & & &\\ &&1&& &\\ && &-1& &\\ && & &\ddots &\\ && & & &-1 \end{pmatrix}$，it has n 1 and k -1.