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Total curvature of foliations. (English) Zbl 0791.57019

Let \(\mathcal F\) be a one-dimensional foliation (with isolated singularities and tangent to the boundary) of a 2-dimensional Riemannian manifold \(M\) (possibly with the boundary \(\partial M)\), \(K(\mathcal F)\) – the integral over \(M\) of the (absolute value of) geodesic curvature of the leaves of \(\mathcal F\), and \(\mathcal F^ \perp\) – the foliation of \(M\) by curves orthogonal to \(\mathcal F\).
Among some other results, the following estimates are obtained: (1) if \(M\) is a planar topological disc, then \(K(\mathcal F) \geq I(\partial M) - 2d\), where \(d\) is the diameter of \(M\) with respect to the distance \(d(x,y)\) defined as the largest lower bound for \(I(\gamma)\), \(\gamma\) being a curve in \(M\) joining \(x\) to \(y\), (2) if \(M\) is a planar topological annulus and \(\mathcal F\) has no singularities, then \(K(\mathcal F) \geq I(\partial M) - 2I(C')\), \(C'\) being the boundary or the convex hull of the bounded component of \(\mathbb{R}^ 2 \setminus M\), (3) if \(M\) is a closed hyperbolic surface, then \(K(\mathcal F) + K({\mathcal F}^ \perp) \geq (2 \pi^ 2/5I) \cdot | \chi(M) |\), \(\chi(M)\) being the Euler characteristic and \(I\) – the length of the side of a regular pentagon in the hyperbolic plane, with interior angles \(\pi /2\).

MSC:

57R30 Foliations in differential topology; geometric theory
51M99 Real and complex geometry
57N05 Topology of the Euclidean \(2\)-space, \(2\)-manifolds (MSC2010)
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