Using the Gauss-Bonnet theorem, Stephen Hawking proved that the topology of the event horizon of a (four dimensional) black hole is a 2-sphere.
使用高斯-博内定理， 史蒂芬· 霍金证明一个（四维）黑洞活动视界的拓扑结构是2个球体。
Precession movement of bonnet tool polishing is realized by a spatial mechanism with 2 degrees of freedom (DOF).
气囊抛光工具的进动由一类特殊的2自由度空间机构实现。
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