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Tristan Rivière

Publications and source records attributed to Tristan Rivière.

70 records · Page 4Linked to original sources
arXiv2013

A frame energy for immersed tori and applications to regular homotopy classes

The paper is devoted to study the Dirichelet energy of moving frames on 2-dimensional tori immersed in the euclidean $3\leq m$-dimensional space. This functional, called Frame energy, is naturally linked to the Willmore energy of the immersion and on the conformal structure of the abstract underlying surface. As first result, a Willmore-conjecture type lower bound is established : namely for every torus immersed in $\R^m$, $m\geq 3$, and any moving frame on it, the frame energy is at least $2π^2$ and equalty holds if and only if $m\geq 4$, the immersion is the standard Clifford torus (up to rotations and dilations), and the frame is the flat one. Smootheness of the critical points of the frame energy is proved after the discovery of hidden conservation laws and, as application, the minimization of the Frame energy in regular homotopy classes of immersed tori in $\R^3$ is performed.

math.DG↗
arXiv2013

Critical weak immersed Surfaces within Sub-manifolds of the Teichmüller Space

We prove that the critical points of various energies such as the area, the Willmore energy, the frame energy for tori...etc among possibly branched immersions constrained to evolve within a smooth sub-manifold of the Teichmüller space satisfy the corresponding constrained Euler Lagrange equation. We deduce that critical points of the Willmore energy or the frame energy for tori are smooth analytic surfaces, away possibly from isolated branched points, under the condition that either the genus is at most 2 or if the sub-manifold does not intersect the subspace of hyper-elliptic points. Using a compactness result from a previous work of the author, we can conclude that each closed sub-manifold of the Teichmüller space, including points, under the previous assumptions, posses a possibly branched smooth Willmore minimizer satisfying the conformal-constrained Willmore equation.

math.DG↗
arXiv2013

Sequential Weak Approximation for Maps of Finite Hessian Energy

Consider the space $W^{2,2}(Ω;N)$ of second order Sobolev mappings $\ v\ $ from a smooth domain $Ω\subset\R^m$ to a compact Riemannian manifold $N$ whose Hessian energy $\int_Ω|\nabla^2 v|^2\, dx$ is finite. Here we are interested in relations between the topology of $N$ and the $W^{2,2}$ strong or weak approximability of a $W^{2,2}$ map by a sequence of smooth maps from $Ω$ to $N$. We treat in detail $W^{2,2}(\B^5,S^3)$ where we establish the \underline{sequential weak} $W^{2,2}$ density of $W^{2,2}(\B^5,S^3)\cap{\mathcal C}^\infty$. The strong $W^{2,2}$ approximability of higher order Sobolev maps has been studied in the recent preprint \cite{BPV} of P. Bousquet, A. Ponce, and J. Van Schaftigen. For an individual map $v\in W^{2,2}(\B^5,S^3)$, we define a number $L(v)$ which is approximately the total length required to connect the isolated singularities of a strong approximation $u$ of $v$ either to each other or to $\p\B^5$. Then $L(v)=0$ if and only if $v$ admits $W^{2,2}$ strongly approximable by smooth maps. Our critical result, obtained by constructing specific curves connecting the singularities of $u$, is the bound $\ L(u)\leq c\int_{\B^5}|\nabla^2 u|^2\, dx\ $. This allows us to construct, for the given Sobolev map $v\in W^{2,2}(\B^5,S^3)$, the desired $W^{2,2}$ weakly approximating sequence of smooth maps. To find suitable connecting curves for $u$, one uses the twisting of a $u$ pull-back normal framing of a suitable level surface of $u$

math.FA↗
arXiv2013

Immersed Spheres of Finite Total Curvature into Manifolds

We prove that a sequence of possibly branched, weak immersions of the two-sphere $S^2$ into an arbitrary compact riemannian manifold $(M^m,h)$ with uniformly bounded area and uniformly bounded $L^2-$norm of the second fundamental form either collapse to a point or weakly converges as current, modulo extraction of a subsequence, to a Lipschitz mapping of $S^2$ and whose image is made of a connected union of finitely many, possibly branched, weak immersions of $S^2$ with finite total curvature. We prove moreover that if the sequence belongs to a class $γ$ of $π_2(M^m)$ the limiting lipschitz mapping of $S^2$ realizes this class as well.

math.DG↗
arXiv2013

Embedded surfaces of arbitrary genus minimizing the Willmore energy under isoperimetric constraint

The isoperimetric ratio of an embedded surface in $R^3$ is defined as the ratio of the area of the surface to power three to the squared enclosed volume. The aim of the present work is to study the minimization of the Willmore energy under fixed isoperimetric ratio when the underlying abstract surface has fixed genus $g\geq 0$. The corresponding problem in the case of spherical surfaces, i.e. $g=0$, was recently solved by Schygulla with different methods.

math.DG↗
arXiv2013

Global gauges and global extensions in optimal spaces

We consider the problem of extending functions ϕ:\to S^n to functions u:B^{n+1}\to S^n for n=2,3. We assume ϕto belong to the critical space W^{1,n} and we construct a W^{1,(n+1,\infty)}-controlled extension u. The Lorentz-Sobolev space W^{1,(n+1,\infty)} is optimal for such controlled extension. Then we use such results to construct global controlled gauges for L^4-connections over trivial SU(2)-bundles in 4 dimensions. This result is a global version of the local Sobolev control of connections obtained by K. Uhlenbeck.

math.FA↗
arXiv2012

Conformally Invariant Variational Problems

Conformal invariance plays a significant role in many areas of Physics, such as conformal field theory, renormalization theory, turbulence, general relativity. Naturally, it also plays an important role in geometry: theory of Riemannian surfaces, Weyl tensors, $Q$-curvature, Yang-Mills fields, etc... We shall be concerned with the study of conformal invariance in analysis. More precisely, we will focus on the study of nonlinear PDEs arising from conformally invariant two dimensional variational problems (e.g. harmonic maps, prescribed mean curvature surfaces, Willmore and Constrained conformal surfaces, isothermic surfaces). The present manuscript are lecture notes of courses given by the author at several places including UBC Vancouver, SNS Pisa, IHP Paris, ICTP Trieste.

math.AP↗
arXiv2012

Willmore Spheres in Compact Riemannian Manifolds

The paper is devoted to the variational analysis of the Willmore, and other L^2 curvature functionals, among immersions of 2-dimensional surfaces into a compact riemannian m-manifold (M^m,h) with m>2. The goal of the paper is twofold, on one hand, we give the right setting for doing the calculus of variations (including min max methods) of such functionals for immersions into manifolds and, on the other hand, we prove existence results for possibly branched Willmore spheres under various constraints (prescribed homotopy class, prescribed area) or under curvature assumptions for M^m. To this aim, using the integrability by compensation, we develop first the regularity theory for the critical points of such functionals. We then prove a rigidity theorem concerning the relation between CMC and Willmore spheres. Then we prove that, for every non null 2-homotopy class, there exists a representative given by a Lipschitz map from the 2-sphere into M^m realizing a connected family of conformal smooth (possibly branched) area constrained Willmore spheres (as explained in the introduction, this comes as a natural extension of the minimal immersed spheres in homotopy class constructed by Sacks and Uhlembeck in \cite{SaU}, in situations when they do not exist). Moreover, for every A>0 we minimize the Willmore functional among connected families of weak, possibly branched, immersions of the 2-sphere having prescribed total area equal to A and we prove full regularity for the minimizer. Finally, under a mild curvature condition on (M^m,h), we minimize the sum of the area with the square of the L^2 norm of the second fundamental form, among weak possibly branched immersions of the two sphere and we prove the regularity of the minimizer.

math.AP↗
arXiv2012

Sequences of Smooth Global Isothermic Immersions

In the present work we study the behavior of sequences of smooth global isothermic immersions of a given closed surface and having a uniformly bounded total curvature. We prove that, if the conformal class of this sequence is bounded in the Moduli space of the surface, it weakly converges in W^{2,2} away from finitely many points, modulo extraction of a subsequence, to a possibly branched weak isothermic immersion of this surface. Moreover, if this limit happens to be smooth away from the branched points, we give an optimal description of the possible loss of strong compactness of such a subsequence by proving that, beside possibly finitely many atomic concentrations, the defect measure associated to the L^2 norm of the second fundamental form is "transported" along exceptional directions given by some holomorphic quadratic forms associated the limiting surface. We give examples where these losses of compactness, invariant along such exceptional directions, eventually happen.

math.AP↗
arXiv2011

Asymptotic Analysis of Branched Willmore Surfaces

We consider a closed Willmore surface properly immersed in ${\R}^m$ (m>2) with square-integrable second fundamental form, and with one point-singularity of finite arbitrary integer order. Using the "conservative" reformulation of the Willmore equation introduced in a previous paper by the second author, we show that, in an appropriate conformal parametrization, the gradient of the Gauss map of the immersion has bounded mean oscillations if the singularity has order one, and is bounded if the order is at least two. We develop around the singular point local asymptotic expansions for the immersion, its first and second derivatives, and for the mean curvature vector. Finally, we exhibit an explicit condition ensuring the removability of the point-singularity

math.AP↗
arXiv2011

Energy Quantization for Willmore Surfaces and Applications

We prove a bubble-neck decomposition together with an energy quantization result for sequences of Willmore surfaces into an arbitrary euclidian space with uniformly bounded energy and non-degenerating conformal type. We deduce the strong compactness of Willmore closed surfaces of a given genus modulo the Möbius group action, below some energy threshold

math.AP↗
arXiv2010

Weak closure of Singular Abelian $L^p$-bundles in $3$ dimensions

We prove the closure for the sequential weak $L^p$-topology of the class of vectorfields on $B^3$ having integer flux through almost every sphere. We show how this problem is connected to the study of the minimization problem for the Yang-Mills functional in dimension higher than critical, in the abelian case.

math.AP↗
arXiv2010

Variational Principles for immersed Surfaces with $L^2$-bounded Second Fundamental Form

In this work we present new fundamental tools for studying the variations of the Willmore functional of immersed surfaces into $R^m$. This approach gives for instance a new proof of the existence of a Willmore minimizing embedding of an arbitrary closed surface in arbitrary codimension. We explain how the same approach can solve constraint minimization problems for the Willmore functional. We show in particular that, for a given closed surface and a given conformal class for this surface, there is an immersion in $R^m$, away possibly from isolated branched points, which minimizes the Willmore energy among all possible Lipschitz immersions in $R^m$ having an $L^2-$bounded second fundamental form and realizing this conformal class. This branched immersion is either a smooth Conformal Willmore branched immersion or an isothermic branched immersion. We show that branched points do not exist whenever the minimal energy in the conformal class is less than $8π$ and that these immersions extend to smooth conformal Willmore embeddings or global isothermic embeddings of the surface in that case. Finally, as a by-product of our analysis, we establish that inside a compact subspace of the moduli space the following holds : weak limit of Palais Smale Willmore sequences are Conformal Willmore, that weak limits of Palais Smale sequences of Conformal Willmore are either Conformal Willmore or Global Isothermic and finally we observe also that weakly converging Palais Smale sequences of Global Isothermic Immersions are Global Isothermic. The analysis developped along the paper - in particular these last results - opens the door to the possibility of constructing new critical saddle points of the Willmore functional without or with constraints using min max methods

math.AP↗
arXiv2009

Sub-criticality of Schroedinger Systems with Antisymmetric Potentials

Let $m$ be an integer larger or equal to 3. We prove that Schroedinger systems on $B^m$ with $L^{m/2}-$antisymmetric potential $Ω$ of the form $$ -Δv=Ωv $$ can be written in divergence form and we deduce that solutions $v$ in $L^{m/(m-2)}$ are in fact $W^{2,q}_{loc}$ for any $q<m/2$.

math.AP↗
arXiv2003

Bubbling and regularity issues in geometric non-linear analysis

Numerous elliptic and parabolic variational problems arising in physics and geometry (Ginzburg-Landau equations, harmonic maps, Yang-Mills fields, Omega-instantons, Yamabe equations, geometric flows in general...) possess a critical dimension in which an invariance group (similitudes, conformal groups) acts. This common feature generates, in all these different situations, the same non-linear effect. One observes a strict splitting in space between an almost linear regime and a dominantly non-linear regime which has two major characteristics : it requires a quantized amount of energy and arises along rectifiable objects of special geometric interest (geodesics, minimal surfaces, J-holomorphic curves, special Lagrangian manifolds, mean-curvature flows...).

math.AP↗