在散度定理中的约定是,将曲面的定向取为外法线的方向。
The convention in the divergence theorem is that we orient the surface with a normal vector pointing always outwards.
当我在曲线上时，我是在曲面的边上的，所以一个是沿着曲面的边指，那个就是曲线的定向，另外一个就要指向曲面里面的那个方向。
And when I am on the curve I am on the boundary of the surface, so there is a direction along the surface that is the curve and the other one is pointing into the surface.
引入积分绝对平均曲率来描述可定向闭曲面的平均弯曲程度。
Integral absolute mean curvature is introduced to describe average curving of an orientable closed surface.
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