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arXiv 1002.3749math.DG
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$\R^4$中曲面上直线的不变量

Invariants of Lines on Surfaces in $\R^4$

Georgi Ganchev, Velichka Milousheva

摘要:

考虑四维欧几里得空间中曲面的一点的切平面,我们得到了该平面中两条切线对的一个不变量。如果这个不变量为零,则称这两条切线共轭。当两条切线与一条给定的切线重合时,我们就得到这条切线的法曲率。渐近切线(曲线)的特征是法曲率为零。考虑到给定切线与其正交切线对的不变量,我们引入了该切线的测地挠率。我们得到了主切线(曲线)是零测地挠率的特征。引入不变量$\varkappa$和$k$作为两个主法线曲率的对称函数。主法线曲率的半和$\varkappa$的几何意义等于(直到一个符号)曲面的法线连接的曲率。曲面上一点的渐近切线的数目由不变量$k$的符号决定。在$k=0$情形中,存在一个单参数的渐近线族,它们是主线。我们发现这样的表面($k=0$)在类的一般旋转曲面(在摩尔的意义)的例子。这些曲面上的主渐近线是四维欧几里得空间中的螺旋线。

英文摘要:

Considering the tangent plane at a point to a surface in the four-dimensional Euclidean space, we find an invariant of a pair of two tangents in this plane. If this invariant is zero, the two tangents are said to be conjugate. When the two tangents coincide with a given tangent, then we obtain the normal curvature of this tangent. Asymptotic tangents (curves) are characterized by zero normal curvature. Considering the invariant of the pair of a given tangent and its orthogonal one, we introduce the geodesic torsion of this tangent. We obtain that principal tangents (curves) are characterized by zero geodesic torsion. The invariants $\varkappa$ and $k$ are introduced as the symmetric functions of the two principal normal curvatures. The geometric meaning of the semi-sum $\varkappa$ of the principal normal curvatures is equal (up to a sign) to the curvature of the normal connection of the surface. The number of asymptotic tangents at a point of the surface is determined by the sign of the invariant $k$. In the case $k=0$ there exists a one-parameter family of asymptotic lines, which are principal. We find examples of such surfaces ($k=0$) in the class of the general rotational surfaces (in the sense of Moore). The principal asymptotic lines on these surfaces are helices in the four-dimensional Euclidean space.

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