arXiv subjects

Theodoros Vlachos

Publications and source records attributed to Theodoros Vlachos.

At least 19 recordsLinked to original sources

arXiv2026

Topological and geometric rigidity of nonnegatively curved submanifolds

We investigate the topology and geometry of compact submanifolds in space forms of nonnegative curvature satisfying a lower bound on the sectional curvature that depends only on the length of the mean curvature vector of the immersion. We show that this condition imposes strong constraints on either the topology or geometry of the submanifold. More precisely, we prove that, for dimensions $n\ge5$, the manifold is homeomorphic to a sphere, whereas in dimension four it is either diffeomorphic to $\mathbb S^4$ or isometric to $\mathbb CP^2$, in which case the immersion is the standard embedding into $\mathbb S^7$ followed by an umbilical inclusion. Additionally, we provide examples showing that the spherical conclusion cannot in general be strengthened to an extrinsic rigidity statement, since there exist many geometrically distinct spheres satisfying the pinching condition.

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arXiv2026

Geometric and topological rigidity of pinched submanifolds in Riemannian manifolds

We study the rigidity of compact submanifolds of Riemannian manifolds of arbitrary codimension that satisfy a sharp pinching condition involving the norm of the second fundamental form and the mean curvature. Without assuming that the ambient manifold is a space form, we show that this condition imposes strong geometric and topological restrictions on the submanifold. The resulting theorems are sharp and provide extensions of several known results in the literature, particularly sphere theorems, without requiring additional assumptions.

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arXiv2025

Geometric and topological rigidity of pinched submanifolds II

We continue the study of the geometry and topology of compact submanifolds of arbitrary codimension in space forms that satisfy a pinching condition involving the length of the second fundamental form and the mean curvature. Our primary focus is on four-dimensional submanifolds, where both the results obtained and the methods employed differ substantially and are considerably more intricate than in higher dimensions. This study relies critically on concepts from four-dimensional geometry, the theory of Riemannian manifolds with nonnegative isotropic curvature, and the Bochner technique, each playing an essential role. The results are sharp and extend previous work by several authors, without imposing additional assumptions on either the mean curvature or the fundamental group of the submanifold.

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arXiv2025

Ricci pinched compact submanifolds in spheres

We investigate the topology of the compact submanifolds in round spheres that satisfy a lower bound on the Ricci curvature depending only on the length of the mean curvature vector of the immersion. Just in special cases, the limited strength of the assumption allows some strong additional information on the extrinsic geometry of the submanifold.

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arXiv2025

Geometric and topological rigidity of pinched submanifolds

We investigate the geometry and topology of compact submanifolds of arbitrary codimension in space forms satisfying a certain pinching condition involving the length of the second fundamental form and the mean curvature. We prove that this pinching condition either forces homology to vanish in a range of intermediate dimensions, or completely determines the submanifold up to congruence. The results are sharp and extend previous results due to several authors without imposing any further assumption on the mean curvature.

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arXiv2025

Local Wintgen ideal submanifolds

This paper is dedicated to the local parametric classification of Wintgen ideal submanifolds in space forms. These submanifolds are characterized by the pointwise attainment of equality in the DDVV inequality, which relates the scalar curvature, the length of the mean curvature vector field and the normal curvature tensor.

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arXiv2024

Isometric Euclidean submanifolds with isometric Gauss maps

We investigate isometric immersions $f\colon M^n\to\R^{n+2}$, $n\geq 3$, of Riemannian manifolds into Euclidean space with codimension two that admit isometric deformations that preserve the metric of the Gauss map. In precise terms, the preservation of the third fundamental form of the submanifold must be ensured throughout the deformation. For minimal isometric deformations of minimal submanifolds this is always the case. Our main result is of a local nature and states that if $f$ is neither minimal nor reducible, then it is a hypersurface of an isometrically deformable hypersurface $F\colon\tilde{M}^{n+1}\to\R^{n+2}$ such that the deformations of $F$ induce those of $f$. Moreover, for a particular class of such submanifolds, a complete local parametric description is provided.

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arXiv2024

Ricci pinched compact hypersurfaces in spheres

We investigate the topology of the compact hypersurfaces in round spheres whose Ricci curvature satisfies an appropriate bound that only depends on the mean curvature of the submanifold. In this paper, the use of the Bochner technique allows same stronger results than the ones obtained by us in the case of submanifolds lying in any codimension.

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arXiv2023

Ricci pinched compact submanifolds in space forms

We investigate the compact submanifolds in Riemannian space forms of nonnegative sectional curvature that satisfy a lower bound on the Ricci curvature, that bound depending solely on the length of the mean curvature vector of the immersion. While generalizing the results, we give a positive answer to a conjecture by H. Xu and J. Gu in (2013, Geom. Funct. Anal. 23). Our main accomplishment is the elimination of the need for the mean curvature vector field to be parallel.

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arXiv2022

Homology vanishing theorems for pinched submanifolds

We investigate the geometry and topology of submanifolds under a sharp pinching condition involving extrinsic invariants like the mean curvature and the length of the second fundamental form. Several homology vanishing results are given. Moreover, an integral bound is provided for the Bochner operator of compact Euclidean submanifolds in terms of the Betti numbers.

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arXiv2020

Kaehler submanifolds of hyperbolic space

We present several local and global results on isometric immersions of Kaehler manifolds $M^{2n}$ into hyperbolic space $\Hy^{2n+p}$. For instance, a classification is given in the case of dimension $n\geq 4$ and codimension $p=2$. Moreover, as corollaries of general results, we conclude that there are no isometric immersion in codimension $p\leq n-2$ if the Kaehler manifold is of dimension $n\geq 4$ and either has a point of positive holomorphic sectional curvature or is compact.

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arXiv2019

Minimal surfaces in spheres and a Ricci-like condition

We deal with minimal surfaces in spheres that are locally isometric to a pseudoholomorphic curve in a totally geodesic $\mathbb{S}^{5}$ in the nearly K{ä}hler sphere $\mathbb{S}^6$. Being locally isometric to a pseudoholomorphic curve in $\mathbb{S}^5$ turns out to be equivalent to the Ricci-like condition $Δ\log(1-K)=6K,$ where $K$ is the Gaussian curvature of the induced metric. Besides flat minimal surfaces in spheres, direct sums of surfaces in the associated family of pseudoholomorphic curves in $\mathbb{S}^5$ do satisfy this Ricci-like condition. Surfaces in both classes are exceptional surfaces. These are minimal surfaces whose all Hopf differentials are holomorphic, or equivalently the curvature ellipses have constant eccentricity up to the last but one. Under appropriate global assumptions, we prove that minimal surfaces in spheres that satisfy this Ricci-like condition are indeed exceptional. Thus, the classification of these surfaces is reduced to the classification of exceptional surfaces that are locally isometric to a pseudoholomorphic curve in $\mathbb{S}^5.$ In fact, we prove, among other results, that such exceptional surfaces in odd dimensional spheres are flat or direct sums of surfaces in the associated family of a pseudoholomorphic curve in $\mathbb{S}^5$.

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arXiv2019

Conformally flat submanifolds with flat normal bundle

We prove that any conformally flat submanifold with flat normal bundle in a conformally flat Riemannian manifold is locally holonomic, that is, admits a principal coordinate system. As one of the consequences of this fact, it is shown that the Ribaucour transformation can be used to construct an associated large family of immersions with induced conformal metrics holonomic with respect to the same coordinate system.

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arXiv2018

On the moduli space of isometric surfaces with the same mean curvature in 4-dimensional space forms

We study the moduli space of congruence classes of isometric surfaces with the same mean curvature in 4-dimensional space forms. Having the same mean curvature means that there exists a parallel vector bundle isometry between the normal bundles that preserves the mean curvature vector fields. We prove that if both Gauss lifts of a compact surface to the twistor bundle are not vertically harmonic, then there exist at most three nontrivial congruence classes. We show that surfaces with a vertically harmonic Gauss lift possess a holomorphic quadratic differential, yielding thus a Hopf-type theorem. We prove that such surfaces allow locally a one-parameter family of isometric deformations with the same mean curvature. This family is trivial only if the surface is superconformal. For such compact surfaces with non-parallel mean curvature, we prove that the moduli space is the disjoint union of two sets, each one being either finite, or a circle. In particular, for surfaces in $\R^4$ we prove that the moduli space is a finite set, under a condition on the Euler numbers of the tangent and normal bundles.

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arXiv2017

Almost conformally flat hypersurfaces

We prove a universal lower bound for the $L^{n/2}$-norm of the Weyl tensor in terms of the Betti numbers for compact $n$-dimensional Riemannian manifolds that are conformally immersed as hypersurfaces in the Euclidean space. As a consequence, we determine the homology of almost conformally flat hypersurfaces. Furthermore, we provide a necessary condition for a compact Riemannian manifold to admit an isometric minimal immersion as a hypersurface in the sphere and extend a result due to Shiohama and Xu \cite{SX} for compact hypersurfaces in any space form.

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arXiv2017

Topological obstructions for submanifolds in low codimension

We prove integral curvature bounds in terms of the Betti numbers for compact submanifolds of the Euclidean space with low codimension. As an application, we obtain topological obstructions for $δ$-pinched immersions. Furthermore, we obtain intrinsic obstructions for minimal submanifolds in spheres with pinched second fundamental form.

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arXiv2016

A class of minimal submanifolds in spheres

We introduce a class of minimal submanfolds $M^n$, $n\geq 3$, in spheres $\mathbb{S}^{n+2}$ that are ruled by totally geodesic spheres of dimension $n-2$. If simply-connected, such a submanifold admits a one-parameter associated family of equally ruled minimal isometric deformations that are genuine. As for compact examples, there are plenty of them but only for dimensions $n=3$ and $n=4$. In the first case, we have that $M^3$ must be a $\mathbb{S}^1$-bundle over a minimal torus $T^2$ in $\mathbb{S}^5$ and in the second case $M^4$ has to be a $\mathbb{S}^2$-bundle over a minimal sphere $\mathbb{S}^2$ in $\mathbb{S}^6$. In addition, we provide new examples in relation to the well-known Chern-do Carmo-Kobayashi problem since taking the torus $T^2$ to be flat yields a minimal submanifolds $M^3$ in $\mathbb{S}^5$ with constant scalar curvature.

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arXiv2015

Exceptional minimal surfaces in spheres

We study a class of exceptional minimal surfaces in spheres for which all Hopf differentials are holomorphic. Extending results of Eschenburg and Tribuzy \cite{ET0}, we obtain a description of exceptional surfaces in terms of a set of absolute value type functions, the $a$-invariants, that determine the geometry of the higher order curvature ellipses and satisfy certain Ricci-type conditions. We show that the $a$-invariants determine these surfaces up to a multiparameter family of isometric minimal deformations, where the number of the parameters is precisely the number of non-vanishing Hopf differentials. We give applications to superconformal surfaces and pseudoholomorphic curves in the nearly Kähler sphere $S^{6}$. Moreover, we study superconformal surfaces in odd dimensional spheres that are isometric to their polar and show a relation to pseudoholomorphic curves in $S^{6}$

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