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Franc Forstneric

Publications and source records attributed to Franc Forstneric.

At least 55 records · Page 3Linked to original sources
arXiv2021

Minimal surfaces in Euclidean spaces by way of complex analysis

This is an expanded version of my plenary lecture at the 8th European Congress of Mathematics in Portorož on 23 June 2021. The main part of the paper is a survey of recent applications of complex-analytic techniques to the theory of conformal minimal surfaces in Euclidean spaces. New results concern approximation, interpolation, and general position properties of minimal surfaces, existence of minimal surfaces with a given Gauss map, and the Calabi-Yau problem for minimal surfaces. To be accessible to a wide audience, the article includes a self-contained elementary introduction to the theory of minimal surfaces in Euclidean spaces.

math.DG↗
arXiv2021

Immersions of open Riemann surfaces into the Riemann sphere

In this paper we show that the space of holomorphic immersions from any given open Riemann surface, $M$, into the Riemann sphere $\mathbb{CP}^1$ is weakly homotopy equivalent to the space of continuous maps from $M$ to the complement of the zero section in the tangent bundle of $\mathbb{CP}^1$. We show in particular that this space has $2^k$ path components, where $H_1(M,\mathbb Z)={\mathbb Z}^k$. We also prove a parametric version of Mergelyan approximation theorem for maps from Riemann surfaces into any complex manifold, a result used in the proof of our main theorem.

math.CV↗
arXiv2021

Holomorphic families of Fatou-Bieberbach domains and applications to Oka manifolds

We construct holomorphically varying families of Fatou-Bieberbach domains with given centres in the complement of any compact polynomially convex subset $K$ of $\mathbb C^n$ for $n>1$. This provides a simple proof of the recent result of Yuta Kusakabe to the effect that the complement $\mathbb C^n\setminus K$ of any polynomially convex subset $K$ of $\mathbb C^n$ is an Oka manifold. The analogous result is obtained with $\mathbb C^n$ replaced by any Stein manifold with the density property.

math.CV↗
arXiv2021

Minimal surfaces for undergraduates

In this article we present an elementary introduction to the theory of minimal surfaces in Euclidean spaces $\mathbb R^n$ for $n\ge 3$ by using only elementary calculus of functions of several variables at the level of a typical second-year undergraduate analysis course for students of Mathematics at European universities. No prior knowledge of differential geometry is assumed.

math.DG↗
arXiv2020

Mergelyan approximation theorem for holomorphic Legendrian curves

In this paper, we prove a Mergelyan type approximation theorem for immersed holomorphic Legendrian curves in an arbitrary complex contact manifold $(X,ξ)$. Explicitly, we show that if $S$ is a compact admissible set in a Riemann surface $M$ and $f:S\to X$ is a $ξ$-Legendrian immersion of class $\mathscr{C}^{r+2}(S,X)$ for some $r\ge 2$ which is holomorphic in the interior of $S$, then $f$ can be approximated in the $\mathscr{C}^r(S,X)$ topology by holomorphic Legendrian embeddings from open neighbourhoods of $S$ into $X$. This result has numerous applications, some of which are indicated in the paper. In particular, by using Bryant's correspondence for the Penrose twistor map $\mathbb{CP}^3\to S^4$ we show that a Mergelyan approximation theorem and the Calabi-Yau property hold for superminimal surfaces in the $4$-sphere $S^4$.

math.CV↗
arXiv2020

H-principle for complex contact structures on Stein manifolds

In this paper we introduce the notion of a formal complex contact structure on an odd dimensional complex manifold. Our main result is that every formal complex contact structure on a Stein manifold $X$ is homotopic to a holomorphic contact structure on a Stein domain $Ω\subset X$ which is diffeotopic to $X$. We also prove a parametric h-principle in this setting, analogous to Gromov's h-principle for contact structures on smooth open manifolds. On Stein threefolds we obtain a complete homotopy classification of formal complex contact structures. Our methods also furnish a parametric h-principle for germs of holomorphic contact structures along totally real submanifolds of class $\mathscr C^2$ in arbitrary complex manifolds.

math.CV↗
arXiv2020

The Calabi-Yau property of superminimal surfaces in self-dual Einstein four-manifolds

In this paper, we show that if $(X,g)$ is an oriented four dimensional Einstein manifold which is self-dual or anti-self-dual then superminimal surfaces in $X$ of appropriate spin enjoy the Calabi-Yau property, meaning that every immersed surface of this type from a bordered Riemann surface can be uniformly approximated by complete superminimal surfaces with Jordan boundaries. The proof uses the theory of twistor spaces and the Calabi-Yau property of holomorphic Legendrian curves in complex contact manifolds.

math.DG↗
arXiv2020

Proper superminimal surfaces of given conformal types in the hyperbolic four-space

Let $H^4$ denote the hyperbolic four-space. Given a bordered Riemann surface, $M$, we prove that every smooth conformal superminimal immersion $\overline M\to H^4$ can be approximated uniformly on compacts in $M$ by proper conformal superminimal immersions $M\to H^4$. In particular, $H^4$ contains properly immersed conformal superminimal surfaces normalised by any given open Riemann surface of finite topological type without punctures. The proof uses the analysis of holomorphic Legendrian curves in the twistor space of $H^4$.

math.DG↗
arXiv2020

Holomorphic approximation: the legacy of Weierstrass, Runge, Oka-Weil, and Mergelyan

In this paper we survey the theory of holomorphic approximation, from the classical 19th century results of Runge and Weierstrass, continuing with the 20th century work of Oka and Weil, Mergelyan, Vitushkin and others, to the most recent ones on higher dimensional manifolds. The paper includes some new results and applications of this theory, especially to manifold-valued maps.

math.CV↗
arXiv2019

The Calabi-Yau problem for Riemann surfaces with finite genus and countably many ends

In this paper, we show that if $R$ is a compact Riemann surface and $M=R\setminus\,\bigcup_i D_i$ is a domain in $R$ whose complement is a union of countably many pairwise disjoint smoothly bounded closed discs $D_i$, then $M$ is the complex structure of a complete bounded minimal surface in $\mathbb R^3$. We prove that there is a complete conformal minimal immersion $X:M\to\mathbb R^3$ extending to a continuous map $X:\overline M\to\mathbb R^3$ such that $X(bM)=\bigcup_i X(bD_i)$ is a union of pairwise disjoint Jordan curves. This extends a recent result for bordered Riemann surfaces.

math.DG↗
arXiv2018

Mergelyan's and Arakelian's theorems for manifold-valued maps

In this paper we show that Mergelyan's theorem holds for maps from open Riemann surfaces to Oka manifolds. This is used to prove the analogue of Arakelian's theorem on uniform approximation of holomorphic maps from closed subsets of the plane $\mathbb C$ to any compact complex homogeneous manifold.

math.CV↗
arXiv2018

Proper holomorphic immersions into Stein manifolds with the density property

In this paper we prove that every Stein manifold $S$ admits a proper holomorphic immersion into any Stein manifold $X$ of dimension $2\mathrm{dim}S$ enjoying the density property or the volume density property. The case $\mathrm{dim}S=1$ was proved beforehand by Andrist and Wold (Ann. Inst. Fourier (Grenoble), 64(2):681-697, 2014). This result generalizes the classical theorem of Bishop and Narasimhan for immersions to $\mathbb{C}^n$ with $n=2\mathrm{dim}S$, and it complements the proper embedding theorem proved by Andrist et al. when $\mathrm{dim}X>2\mathrm{dim}S$ (J. Anal. Math., 130:135-150, 2016).

math.CV↗
arXiv2018

Holomorphic embeddings and immersions of Stein manifolds: a survey

In this paper we survey results on the existence of holomorphic embeddings and immersions of Stein manifolds into complex manifolds. Most results pertain to proper maps into Stein manifolds. We include a new result saying that every continuous map $X\to Y$ between Stein manifolds is homotopic to a proper holomorphic embedding provided that $\mathrm{dim} Y > 2\mathrm{dim}\, X$ and we allow a homotopic deformation of the Stein structure on $X$.

math.CV↗
arXiv2018

Runge tubes in Stein manifolds with the density property

In this paper we give a very simple proof of the existence and plenitude of Runge tubes in $\mathbb C^n$ $(n>1)$ and, more generally, in Stein manifolds with the density property. We show in particular that for any algebraic submanifold $A$ of codimension at least two in a complex Euclidean space $\mathbb C^n$, the normal bundle of $A$ in $\mathbb C^n$ admits a holomorphic embedding onto a Runge domain in $\mathbb C^n$ which agrees with the inclusion map $A\hookrightarrow \mathbb C^n$ on the zero section.

math.CV↗
arXiv2018

Minimal surfaces in minimally convex domains

In this paper, we prove that every conformal minimal immersion of a compact bordered Riemann surface $M$ into a minimally convex domain $D\subset \mathbb{R}^3$ can be approximated, uniformly on compacts in $\mathring M=M\setminus bM$, by proper complete conformal minimal immersions $\mathring M\to D$. We also obtain a rigidity theorem for complete immersed minimal surfaces of finite total curvature contained in a minimally convex domain in $\mathbb{R}^3$, and we characterize the minimal surface hull of a compact set $K$ in $\mathbb{R}^n$ for any $n\ge 3$ by sequences of conformal minimal disks whose boundaries converge to $K$ in the measure theoretic sense.

math.DG↗
arXiv2018

A properly embedded holomorphic disc in the ball with finite area and dense boundary curve

In this paper we construct a properly embedded holomorphic disc in the unit ball $\mathbb{B}^2$ of $\mathbb{C}^2$ having a surprising combination of properties: on the one hand, it has finite area and hence is the zero set of a bounded holomorphic function on $\mathbb{B}^2$; on the other hand, its boundary curve is everywhere dense in the sphere $b\mathbb{B}^2$.

math.CV↗
arXiv2018

Divisors defined by noncritical functions

In this paper we show that every complex hypersurface $A$ in a Stein manifold $X$ with $H^2(X;\mathbb Z)=0$ is the divisor of a holomorphic function $f$ on $X$ whose critical points are precisely the singular points of $A$. A similar result is proved for complete intersections of higher codimension.

math.CV↗