Geodesic
Some properties of surfaces with slowly varying negative extrinsic curvature in a Riemannian space
Sbornik. Mathematics, Tome 12 (1970) no. 2, pp. 313-324 Cet article a éte moissonné depuis la source Math-Net.Ru

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We consider surfaces of negative extrinsic curvature in a Riemannian space with nonpositive curvature. We prove that the following inequality holds on a surface which is complete in the sense of the intrinsic metric: $$ \sup_F\biggl\{\biggl|\operatorname{grad}\frac1k\biggr|+\frac{\Lambda-\lambda}{2k^2}\biggr\}=q>\frac1{\sqrt3}, $$ here $F$ is the surface being considered, $k=\sqrt{K_e}$ ($K_e$ is the extrinsic curvature of $F$) and $\Lambda$ and $\lambda$ are the maximum and minimum of the Riemannian curvature of the space $R$ at a given point. This theorem generalizes a theorem of Efimov concerning $q$-metrics. We give an example of a surface for which $q=4,5$. Bibliography: 8 titles. 
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     author = {I. S. Brandt},
     title = {Some properties of surfaces with slowly varying negative extrinsic curvature in {a~Riemannian} space},
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I. S. Brandt. Some properties of surfaces with slowly varying negative extrinsic curvature in a Riemannian space. Sbornik. Mathematics, Tome 12 (1970) no. 2, pp. 313-324. http://geodesic.mathdoc.fr/item/SM_1970_12_2_a9/

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