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Nikolaos I. Katzourakis

Publications and source records attributed to Nikolaos I. Katzourakis.

10 recordsLinked to original sources
arXiv2012

Infinity-Minimal Submanifolds

We identify the Variational Principle governing inifinity-Harmonic maps, that is solutions to the Infinity-Laplacian. The system was first derived in the limit of the p-Laplacian as p->inifinity in [K2] and is recently studied in [K3]. Here we show that it is the "Euler-Lagrange PDE" of vector-valued Calculus of Variations in L-inifinity for the L-inifinity norm of the gradient. We introduce the notion of inifinity-Minimal Maps, whch are Rank-One Absolute Minimals of with inifinity-Minimal Area" of the range submanifold and prove they solve the inifinity-laplacian. The converse is true for immersions. We also establish a maximum principle for |Du| for solutions. We further characterize minimal surfaces of 3-space as those locally parameterizable by isothermal immersions with inifinity-minimal area and show that isothermal inifinity-Harmonic maps are rigid.

math.AP↗
arXiv2012

A Holder Continuous Nowhere Improvable Function with Derivative Singular Distribution

We present a class of functions $\mathcal{K}$ in $C^0(\R)$ which is variant of the Knopp class of nowhere differentiable functions. We derive estimates which establish $\mathcal{K} \sub C^{0,\al}(\R)$ for $0<\al<1$ but no $K \in \mathcal{K}$ is pointwise anywhere improvable to $C^{0,\be}$ for any $\be>\al$. In particular, all $K$'s are nowhere differentiable with derivatives singular distributions. $\mathcal{K}$ furnishes explicit realizations of the functional analytic result of Berezhnoi. Recently, the author and simulteously others laid the foundations of Vector-Valued Calculus of Variations in $L^\infty$ (Katzourakis), of $L^\infty$-Extremal Quasiconformal maps (Capogna and Raich, Katzourakis) and of Optimal Lipschitz Extensions of maps (Sheffield and Smart). The "Euler-Lagrange PDE" of Calculus of Variations in $L^\infty$ is the nonlinear nondivergence form Aronsson PDE with as special case the $\infty$-Laplacian. Using $\mathcal{K}$, we construct singular solutions for these PDEs. In the scalar case, we partially answered the open $C^1$ regularity problem of Viscosity Solutions to Aronsson's PDE (Katzourakis). In the vector case, the solutions can not be rigorously interpreted by existing PDE theories and justify our new theory of Contact solutions for fully nonlinear systems (Katzourakis). Validity of arguments of our new theory and failure of classical approaches both rely on the properties of $\mathcal{K}$.

math.CA↗
arXiv2012

L-Infininity Variational Problems for Maps and the Aronsson PDE System

By employing Aronsson's Absolute Minimizers of $L^\infty$ functionals, we prove that Absolutely Minimizing Maps $u:\R^n \larrow \R^N$ solve a "tangential" Aronsson PDE system. By following Sheffield-Smart \cite{SS}, we derive $\De_\infty$ with respect to the dual operator norm and show that such maps miss information along a hyperplane when compared to Tight Maps. We recover the lost term which causes non-uniqueness and derive the complete Aronsson system which has \emph{discontinuous coefficients}. In particular, the Euclidean $\infty$-Laplacian is $\De_\infty u = Du \ot Du : D^2u\, +\, |Du|^2[Du]^\bot \De u$ where $[Du]^\bot$ is the projection on the null space of $Du^\top$. We exibit $C^\infty$ solutions having interfaces along which the rank of their gradient is discontinuous and propose a modification with $C^0$ coefficients which admits \emph{varifold solutions}. Away from the interfaces, Aronsson Maps satisfy a structural property of local splitting to 2 phases, an horizontal and a vertical; horizontally they possess gradient flows similar to the scalar case and vertically solve a linear system coupled by a scalar Hamilton Jacobi PDE. We also construct singular $\infty$-Harmonic local $C^1$ diffeomorphisms and singular Aronsson Maps.

math.AP↗
arXiv2011

Maximum Principles for Vectorial Approximate Minimizers of Nonconvex Functionals

We establish Maximum Principles which apply to vectorial approximate minimizers of the general integral functional of Calculus of Variations. Our main result is a version of the Convex Hull Property. The primary advance compared to results already existing in the literature is that we have dropped the quasiconvexity assumption of the integrand in the gradient term. The lack of weak Lower Semicontinuity is compensated by introducing a nonlinear convergence technique, based on the approximation of the projection onto a convex set by reflections and on the invariance of the integrand in the gradient term under the Orthogonal Group. Maximum Principles are implied for the relaxed solution in the case of non-existence of minimizers and for minimizing solutions of the Euler-Lagrange system of PDE.

math.AP↗
arXiv2011

Explicit Singular Viscosity Solutions of the Aronsson Equation

We establish that when n >= 2 and H is a C^1 Hamiltonian such that some level set contains a line segment, the Aronsson equation admits explicit entire viscosity solutions. They are superpositions of a linear part plus a Lipschitz continuous everywhere differentiable singular part which in general is non-C^1 and nowhere twice differentiable. In particular, we supplement the C^1 regularity result of Wang and Yu by deducing that strict level convexity is necessary for C^1 regularity of solutions.

math.AP↗
arXiv2005

The Riemann Surface of the Logarithm Constructed in a Geometrical Framework

The logarithmic Riemann surface Sigma_{log} is a classical holomorphic 1-manifold. It lives into R^4 and induces a covering space of C - 0 defined by exp. This paper suggests a geometric construction of it, derived as the limit of a sequence of vector fields extending exp suitably to embeddings of C into R^3, which turn to be helicoid surfaces living into C x R. In the limit we obtain a bijective complex exponential on the covering space in question, and thus a well-defined complex logarithm. In addition, the helicoids are diffeomorphic (not bi-holomorphic) copies of Sigma_{log} as smooth realizations living into R^3, without obstruction. Our approach is purely geometrical and does not employ any tools provided by the complex structure, thus holomorphy is no longer necessary to obtain constructively this Riemann surface Sigma_{log}. Moreover, the differential geometric framework we adopt affords explicit generalization on submanifolds of C^m x \R^m and certain corollaries are derived.

math.DG↗