Let $\displaystyle B$ be the intersection of the sphere $\displaystyle S^2$ with the first octant $\displaystyle \{(x_1,x_2,x_3)|x_1\geq0,x_2\geq0,x_3\geq0\}$ of $\displaystyle \mathbb{R}^3$. The book says $\displaystyle B$ is homeomorphic to the ball $\displaystyle B^2$. But I'm not sure why that is, can anyone explain this please. Thank you very much.