u曲率线是圆弧的充要条件

Curvature of the Line is Necessary and Sufficient Condition for Arc

从圆管曲面(有一主曲率是常数)是单参数等半径球面的包络面,推广到曲面∑上,有一族曲率线是圆弧,则该曲面仍是单参数球面族的包络面.

From the tube surface （a main curvature is constant） is the envelope face of the single parameter and the same radius. If it is extended to ∑ the surface, a curvature of the line is the curve, then the surface is still single parameter spherical surface of the envelope.