向量 AB（AB上面有→）的长度叫做向量的模，记作|AB|(AB上有→）或|a|(a上有→)。
也就是说，如果朝该方向运动，函数值变动得最剧烈，那么相应的斜率就是梯度向量的模长。
OK, so if I go in that direction, which gives me the fastest increase, then the corresponding slope will be the length of the gradient.
也就是，曲线每一点上,垂直于曲线，模长为1的向量。
That means a vector that is at every point of the curve perpendicular to the curve and has length one.
讨论局部对称空间中具有平行平均曲率向量的子流形，得到其关于第二基本形式模长平方的积分不等式的相关定理。
This paper discusses submanifolds with parallel mean curvature vector in local symmetric Spaces and obtains integral invariants about the square of modulus-length.
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