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Simona Nistor

Publications and source records attributed to Simona Nistor.

14 recordsLinked to original sources
arXiv2026

Extrinsic characterizations of biconservative surfaces in the $4$-dimensional hyperbolic space

Biconservative submanifolds arise as a natural relaxation of the biharmonic condition and play an important role in the submanifold theory. In this paper, we study non-CMC biconservative surfaces with parallel normalized mean curvature vector field (PNMC surfaces) in the four-dimensional hyperbolic space $\mathbb{H}^4$, for which we consider the hyperboloid model. We provide a local extrinsic description of such surfaces, showing that they are generated by a directrix curve lying in a totally geodesic hypersurface $\mathbb{H}^3$ of $\mathbb{H}^4$, through a certain normal flow. This extrinsic classification of non-CMC, PNMC biconservative surfaces in $\mathbb{H}^4$ splits naturally into three cases according to the type of a certain vector field, which can be non-zero null, spacelike or timelike. Together with the previous results, the classification of non-CMC, PNMC surfaces in four-dimensional space forms is now completed, from intrinsic and extrinsic point of view.

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arXiv2026

On the conformal-biharmonic stability of the identity map of Einstein manifolds

The identity map of an Einstein manifold is a critical point of both the classical energy functional and the conformal-bienergy functional. In this paper, we investigate the conformal-biharmonic stability of the identity map of compact Einstein manifolds of dimension at least four and with nonnegative scalar curvature, and we compare it with the harmonic stability, when the identity map is considered as a harmonic map. Somewhat surprisingly, we show that the conformal-biharmonic index coincides with the harmonic index, with a single notable exception: the four-dimensional Euclidean sphere. In this case, the identity map is unstable with respect to the energy functional, as shown independently by Mazet and Smith, whereas it is stable with respect to the conformal-bienergy functional.

math.DG↗
arXiv2024

Intrinsic characterizations of biconservative surfaces in the 4-dimensional hyperbolic space

In this paper, we extend the investigation of biconservative surfaces with parallel normalized mean curvature vector fields (PNMC) in the 4-dimensional space forms, focusing on the hyperbolic space \mathbb{H}^4, the last remaining case to explore. We establish that an abstract surface admits a PNMC biconservative immersion in \mathbb{H}^4 if and only if it satisfies a certain intrinsic condition; if such an immersion exists, it is unique. We further analyze these abstract surfaces, showing that they form a two-parameter family. Additionally, we provide three characterizations of the intrinsic condition to explore the geometric properties of these surfaces.

math.DG↗
arXiv2024

Gap results for biharmonic submanifolds in spheres

In this paper we determine a larger gap of the mean curvature for a class of proper biharmonic submanifolds with parallel mean curvature vector field in Euclidean spheres. When the bounds of the gap are reached, we obtain splitting results of the submanifold.

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arXiv2023

On conformal biharmonic maps and hypersurfaces

In this article we initiate a thorough geometric study of the conformal bienergy functional which consists of the standard bienergy augmented by two additional curvature terms. The conformal bienergy is conformally invariant in dimension four and its precise structure is motivated by the Paneitz operator from conformal geometry. The critical points of the conformal bienergy are called conformal biharmonic maps. Besides establishing a number of basic results on conformal biharmonic maps, we pay special attention to conformal biharmonic hypersurfaces in space forms. For hypersurfaces in spheres, we determine all conformal biharmonic hyperspheres and then we classify all conformal biharmonic generalized Clifford tori. Moreover, in sharp contrast to biharmonic hypersurfaces, we show that there also exist conformal biharmonic hypersurfaces of hyperbolic space, pointing out a fundamental difference between biharmonic and conformal biharmonic hypersurfaces. Finally, we also study the stability of the conformal biharmonic hyperspheres in spheres and explicitly compute their index and nullity. In particular, we obtain that the index of the equator $\mathbb{S}^4$ of $\mathbb{S}^5$ is zero, i.e., it is stable, while the index of the equator $\mathbb{S}^5$ of $\mathbb{S}^6$ is seven.

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arXiv2022

Biconservative surfaces in the $4$-dimensional Euclidean sphere

In this paper, we study biconservative surfaces with parallel normalized mean curvature vector field ($PNMC$) in the $4$-dimensional unit Euclidean sphere $\mathbb{S}^4$. First, we study the existence and uniqueness of such surfaces. We obtain that there exists a $2$-parameter family of non-isometric abstract surfaces that admit a (unique) $PNMC$ biconservative immersion in $\mathbb{S}^4$. Then, we obtain the local parametrization of these surfaces in the $5$-dimensional Euclidean space $\mathbb{E}^5$.

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arXiv2019

On the uniqueness of complete biconservative surfaces in $3$-dimensional space forms

Biconservative surfaces are surfaces with divergence-free stress-bienergy tensor. Simply connected, complete, non-$CMC$ biconservative surfaces in $3$-dimensional space forms were constructed working in extrinsic and intrinsic ways. Then, one raises the question of the uniqueness of such surfaces. In this paper we give a positive answer to this question.

math.DG↗
arXiv2019

Complete biconservative surfaces in the hyperbolic space $\mathbb{H}^3$

We construct simply connected, complete, non-$CMC$ biconservative surfaces in the $3$-dimensional hyperbolic space $\mathbb{H}^3$ in an intrinsic and extrinsic way. We obtain three families of such surfaces, and, for each surface, the set of points where the gradient of the mean curvature function does not vanish is dense and has two connected components. In the intrinsic approach, we first construct a simply connected, complete abstract surface and then prove that it admits a unique biconservative immersion in $\mathbb{H}^3$. Working extrinsically, we use the images of the explicit parametric equations and a gluing process to obtain our surfaces. They are made up of circles (or hyperbolas, or parabolas, respectively) which lie in $2$-affine parallel planes and touch a certain curve in a totally geodesic hyperbolic surface $\mathbb{H}^2$ in $\mathbb{H}^3$.

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arXiv2017

On the uniqueness of complete biconservative surfaces in $\mathbb{R}^3$

We study the uniqueness of complete biconservative surfaces in the Euclidean space $\mathbb{R}^3$, and prove that the only complete biconservative regular surfaces in $\mathbb{R}^3$ are either $CMC$ or certain surfaces of revolution. In particular, any compact biconservative regular surface in $\mathbb{R}^3$ is a round sphere.

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arXiv2017

On biconservative surfaces

We study in a uniform manner the properties of biconservative surfaces in arbitrary Riemannian manifolds. Biconservative surfaces being characterized by the vanishing of the divergence of a symmetric tensor field $S_2$ of type $(1,1)$, their properties will follow from general properties of a symmetric tensor field of type $(1,1)$ with free divergence. We find the link between the biconservativity, the property of the shape operator $A_H$ to be a Codazzi tensor field, the holomorphicity of a generalized Hopf function and the quality of the surface to have constant mean curvature. Then we determine the Simons type formula for biconservative surfaces and use it to study their geometry.

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arXiv2017

Global properties of biconservative surfaces in $\mathbb{R}^3$ and $\mathbb{S}^3$

We survey some recent results on biconservative surfaces in $3$-dimensional space forms $N^3(c)$ with a special emphasis on the $c=0$ and $c=1$ cases. We study the local and global properties of such surfaces, from extrinsic and intrinsic point of view. We obtain all non-$CMC$ complete biconservative surfaces in $\mathbb{R}^3$ and $\mathbb{S}^3$.

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arXiv2016

Complete biconservative surfaces in $\mathbb{R}^3$ and $\mathbb{S}^3$

In this paper we consider the complete biconservative surfaces in Euclidean space $\mathbb{R}^3$ and in the unit Euclidean sphere $\mathbb{S}^3$. Biconservative surfaces in 3-dimensional space forms are characterized by the fact that the gradient of their mean curvature function is an eigenvector of the shape operator, and we are interested in studying local and global properties of such surfaces with non-constant mean curvature function. We determine the simply connected, complete Riemannian surfaces that admit biconservative immersions in $\mathbb{R}^3$ and $\mathbb{S}^3$. Moreover, such immersions are explicitly described.

math.DG↗
arXiv2015

On biconservative surfaces in 3-dimensional space forms

We consider biconservative surfaces $\left(M^2,g\right)$ in a space form $N^3(c)$, with mean curvature function $f$ satisfying $f>0$ and $\nabla f\neq 0$ at any point, and determine a certain Riemannian metric $g_r$ on $M$ such that $\left(M^2,g_r\right)$ is a Ricci surface in $N^3(c)$. We also obtain an intrinsic characterization of these biconservative surfaces.

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