Let $F$ be a proper rational map from the complex ball $\mathbb B ^n$ into $\mathbb B ^N$ with $n>7$ and $3n+1 \le N\le 4n-7$. Then $F$ is equivalent to a map $(G, 0, \dots , 0)$ where $G$ is a proper holomorphic map from $\mathbb B ^n$ into $\mathbb B ^{3n}$.

/pdf/on-the-third-gap-for-proper-holomorphic-maps-between-balls-3z3i3mt5hq.pdf

On the third gap for proper holomorphic maps between balls

In this paper, we study the gap rigidity phenomenon for proper holomorphic maps between balls of different dimension. We show that any $F\in prop_3({\mathbb{B}}^n, {\mathbb{B}} ^N)$, with $3n<N\leq 4n-7$ and $n\geq 7$, is equivalent to a map of the form $(G,0)$ with $G\in Rat ({\mathbb{B}}^n,{\mathbb{B}}^{3n})$. The main ingredients for the proof of our main theorem are the normal form obtained by Huang-Ji-Xu and a lemma of the first author.

On the Third Gap for Proper Holomorphic Maps between Balls

Super-rigidity for CR embeddings of real hypersurfaces into hyperquadrics☆

Our goal is to establish what seems to be the first rigidity result for CR embeddings between Shilov boundaries of bounded symmetric domains of higher rank. The result states that any such CR embedding is the standard linear embedding up to CR automorphisms. Our basic assumption extends precisely the well-known optimal bound for the rank one case. There are no other restrictions on the ranks, in particular, the difficult case when the target rank is larger than the source rank is also allowed.

/pdf/rigidity-of-cr-maps-between-shilov-boundaries-of-bounded-1kgpnm52s2.pdf

Rigidity of CR maps between Shilov boundaries of bounded symmetric domains

We prove and organize some results on the normal forms of Hermitian operators composed with the Veronese map. We apply this general framework to prove two specific theorems in CR geometry. First, extending a theorem of Faran, we classify all real-analytic CR maps between any hyperquadric in $\C^2$ and any hyperquadric in $\C^3$, resulting in a finite list of equivalence classes. Second, we prove that all degree-two CR maps of spheres in all dimensions are spherically equivalent to a monomial map, thus obtaining an elegant classification of all degree-two CR sphere maps.

Normal forms, Hermitian operators, and CR maps of spheres and hyperquadrics

https://www.intlpress.com/site/pub/files/_fulltext/journals/mrl/2006/0013/0004/MRL-2006-0013-0004-a002.pdf

A new gap phenomenon for proper holomorphic mappings from ${\BB}^n$ into ${\BB}^{N}$

In this paper (Math. Res. Lett. 13 (2006). No 4, 509-523), the authors established a pseudo-normal form for proper holomoprhic mappings between balls in complex spaces with degenerate rank. This then was used to give a complete characterization for all proper holomorphic maps with geometric rank one, which, in particular, includes the following as an immediate application: Theorem: Any rational holomorphic map from B^n into B^N with $4\le n\le N\le 3n-4$ is equivalent to the D'Angelo map $$F_{\theta}(z',w)=(z',(\cos\theta)w,(\sin\theta)z_1w, ..., (\sin\theta)z_{n-1}w, (\sin\theta)w^2, 0'), 0\le \theta\leq \pi/2.$$ It is a well-known (but also quite trivial) fact that any non-constant rational CR map from a piece of the sphere $\partial {B^n}$ into the sphere $\partial {B^N}$ can be extended as a proper rational holomoprhic map from $B^n$ into $B^N$ ($N\ge n\ge 2$). By using the rationality theorem that the authors established in [HJX05], one sees that the the above theorem (and also the main theorem of the paper) holds in the same way for any non-constant $C^3$-smooth CR map from a piece of $\partial {B^n}$ into $\partial{B^N}$. The paper [Math. Res. Lett. 13 (2006). No 4, 509-523] was first electronically published by Mathematical Research Letters several months ago at its home website: this http URL (The pdf file of the printed journal version can also be downloaded at this http URL).

Dekang Xu

Papers

A new gap phenomenon for proper holomorphic mappings from ${\BB}^n$ into ${\BB}^{N}$

A new gap phenomenon for proper holomorphic mappings from B^n into B^N