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Zhaohu Nie

Publications and source records attributed to Zhaohu Nie.

At least 19 recordsLinked to original sources
arXiv2026

Connected fundamental domains for congruence subgroups

We give explicit sets of right coset representatives for the congruence subgroups $Γ_0(N)$, $Γ_1(N)$ and $Γ(N)$, and prove that the corresponding unions of standard modular triangles are connected fundamental domains. The construction is based on a study of the projective line ${\mathbb P}^1({\mathbb Z}/N{\mathbb Z})$. For every residue class $j\in{\mathbb Z}/N{\mathbb Z}$, the number of representatives above $j$ is governed by the simple function \[W_j=\min\{m\in{\mathbb Z}_{>0}\mid mj-1\in ({\mathbb Z}/N{\mathbb Z})^*\}.\] We also include examples illustrating how the connected domains make cusp and boundary data visible.

math.NT↗
arXiv2026

Cusps and boundaries of connected fundamental domains for $Γ_0(N)$

For $N>1$, we constructed a canonical connected fundamental domain for $Γ_0(N)$ in [Nie, Parent], utilizing an interesting function $W: {\mathbb Z}/N\to {\mathbb N}$. In this paper, we further study the function $W$, prove some identities, and use it to match the cusps, with widths, produced by our connected fundamental domain with the known cusp classes of $Γ_0(N)$. Furthermore, we list the boundary arcs and the gluing patterns of our connected fundamental domain, a key step in understanding the modular curve $X_0(N)$ by this approach.

math.NT↗
arXiv2025

Total masses of solutions to general Toda systems with singular sources

In this article we obtain total masses of solutions to the Toda system associated to a general simple Lie algebra with singular sources at the origin. The determination of such total masses is one of the important steps towards establishing the a priori bound for solutions to the mean field type of Toda system on compact surfaces. The total mass is found to be related to the longest element $κ$ in the Weyl group of the corresponding Lie algebra. This is the foundation to future work relating the local blowup masses (from analysis) with the Weyl group. This work generalizes the previous works in Lin et al. (2012), Ao et al. (2015) and Nie (20160 for Toda systems of types $A, G_2$ and $B, C$. However, a more Lie-theoretic method is needed here for the general case, and the method relies heavily on the DPW method, Drinfeld-Sokolov gauge and the $W$-invariants. The last crucial step for the total masses is obtained by applying the work of Kostant (1979) on the one dimensional Toda lattice.

math.AP↗
arXiv2023

Affine Toda system of $\mathbf{A}$ and $\mathbf{C}^t$ type: compactness and affine Weyl group

The local mass is a fundamental quantized information that characterizes the blow-up solution to the Toda system and has a profound relationship with its underlying algebraic structure. In \cite{Lin-Yang-Zhong-2020}, it was observed that the associated Weyl group can be employed to represent this information for the $\mathbf{A}_n$, $\mathbf{B}_n$, $\mathbf{C}_n$ and $\mathbf{G}_2$ type Toda systems. The relationship between the local mass of blow-up solution and the corresponding affine Weyl group is further explored for some affine $\mathbf{B}$ type Toda systems in \cite{Cui-Wei-Yang-Zhang-2022}, where the possible local masses are explicitly expressed in terms of $8$ types. The current work presents a comprehensive study of the general affine $\mathbf{A}$ and $\mathbf{C}^t$ type Toda systems with arbitrary rank. At each stage of the blow-up process (via scaling), we can employ certain elements (known as "set chains") in the corresponding affine Weyl group to measure the variation of local mass. Consequently, we obtain the a priori estimate of the affine $\mathbf{A}$ and $\mathbf{C}^t$ type Toda systems with arbitrary number of singularities.

math.AP↗
arXiv2023

Accidental CR structures

We noticed a discrepancy between Élie Cartan and Sigurdur Helgason about the lowest possible dimension in which the simple exceptional Lie group ${\bf E}_8$ can be realized. This raised the question about the lowest dimensions in which various real forms of the exceptional groups ${\bf E}_\ell$ can be realized. Cartan claims that ${\bf E}_6$ can be realized in dimension 16. However Cartan refers to the complex group ${\bf E}_6$, or its split real form $E_I$. His claim is also valid in the case of the real form denoted by $E_{IV}$. We find however that the real forms $E_{II}$ and $E_{III}$ of ${\bf E}_6$ can not be realized in dimension 16 à la Cartan. In this paper we realize them in dimension 24 as groups of CR automorphisms of certain CR structures of higher codimension. As a byproduct of these two realizations, we provide a full list of CR structures $(M,H,J)$ and their CR embeddings in an appropriate ${\bf C}^N$, which satisfy the following conditions: (1) they have real codimension $k>1$, (2) the real vector distribution $H$ proper for the action of the complex structure $J$ is such that $[H,H]+H=TM$, (3) the local group $G_J$ of CR automorphisms of the structure $(M,H,J)$ is simple, acts transitively on $M$ and has isotropy $P$ being a parabolic subgroup in $G_J$, (4) the local symmetry group $G$ of the vector distribution $H$ on $M$ coincides with the group $G_J$ of CR automorphisms of $(M,H,J)$. Because all the CR structures from our list satisfy the last property we call them accidental. Our CR structures of higher codimension with the exceptional symmetries $E_{II}$ and $E_{III}$ are particular entries in this list.

math.CV↗
arXiv2023

Infinitesimal CR Symmetries of Accidental CR Structures

In this companion paper to our article {\em Accidental CR structures} (arxiv.org, January 2023), thought of as an appendix not submitted for publication, we provide complete explicit lists of infinitesimal CR automorphisms for the concerned CR models having respective Lie algebra structures: $${\bf E}_{II}, \qquad\ {\bf E}_{III}, \qquad\ \mathfrak{so}(\ell-1,\ell+1), \qquad\ \mathfrak{su}(p,q).$$ We start from our lists of {\em quadric} CR submanifolds $M^{2n+c} \subset \mathbb{C}^{n+c}$ of codimension $c >1$ which are shown to be {\em accidental}, in the sense that their CR symmetry groups are {\em equal to} (and not smaller than) the symmetry groups of the underlying real distribution structures -- after forgetting the complex structure. Thanks to intensive symbolic computer explorations, we then determine embedded vector field generators of these CR symmetries Lie algebras, and we express them in {\em extrinsic} holomorphic coordinates, because intrinsic formulas would be too extended to be shown.

math.CV↗
arXiv2020

On instability of Type (II) Lawson-Osserman Cones

We obtain the instability of Type (II) Lawson-Osserman cones in Euclidean spaces, and thus provide a family of (uncountably many) unstable solutions with singularity to the Dirichlet problem for minimal graphs of high codimension versus smooth unstable ones by Lawson-Osserman through a min-max technique. To our knowledge, these are the first examples of non-smooth unstable minimal graphs and unlikely detectible through the mean curvature flow or min-max theory.

math.DG↗
arXiv2016

Stable equivariant abelianization, its properties, and applications

Let $G$ be a finite group. For a based $G$-space $X$ and a Mackey functor $M$, a topological Mackey functor $X\widetilde\otimes M$ is constructed, which will be called the stable equivariant abelianization of $X$ with coefficients in $M$. When $X$ is a based $G$-CW complex, $X\widetilde\otimes M$ is shown to be an infinite loop space in the sense of $\mathcal{G}$-spaces. This gives a version of the $RO(G)$-graded equivariant Dold-Thom theorem. Applying a variant of Elmendorf's construction, we get a model for the Eilenberg-Mac Lane spectrum $HM$. The proof uses a structure theorem for Mackey functors and our previous results.

math.AT↗
arXiv2016

Toda systems and hypergeometric equations

This paper establishes certain existence and classification results for solutions to $SU(n)$ Toda systems with three singular sources at 0, 1, and $\infty$. First, we determine the necessary conditions for such an $SU(n)$ Toda system to be related to an $n$th order hypergeometric equation. Then, we construct solutions for $SU(n)$ Toda systems that satisfy the necessary conditions and also the interlacing conditions from Beukers and Heckman. Finally, for $SU(3)$ Toda systems satisfying the necessary conditions, we classify, under a natural reality assumption, that all the solutions are related to hypergeometric equations. This proof uses the Pohozaev identity.

math.AP↗
arXiv2016

Toda field theories and integral curves of standard differential systems

This paper establishes three relations between the Toda field theory associated to a simple Lie algebra and the integral curves of the standard differential system on the corresponding complete flag variety. The motivation comes from the viewpoint on the Toda field theories as Darboux integrable differential systems as developed in \cite{AFV}. First, we establish an isomorphism concerning regular functions on the jet space and on the unipotent subgroup in the setting of a simple Lie group. Using this result, we then show that in the sense of differential systems, after restricting one independent variable to a constant the Toda field theory becomes the system for integral curves of the standard differential system on a complete flag variety. Finally, we establish that, in terms of differential invariants, the Toda field theory is the quotient of the product of two such systems by a natural group action.

math.DG↗
arXiv2015

Classification of solutions to Toda systems of types $C$ and $B$ with singular sources

In this paper, the classification in [Lin,Wei,Ye] of solutions to Toda systems of type $A$ with singular sources is generalized to Toda systems of types $C$ and $B$. Like in the $A$ case, the solution space is shown to be parametrized by the abelian subgroup and a subgroup of the unipotent subgroup in the Iwasawa decomposition of the corresponding complex simple Lie group. The method is by studying the Toda systems of types $C$ and $B$ as reductions of Toda systems of type $A$ with symmetries. The theories of Toda systems as integrable systems as developed in [Leznov, Saveliev, Nie], in particular the $W$-symmetries and the iterated integral solutions, play essential roles in this work, together with certain characterizing properties of minors of symplectic and orthogonal matrices.

math.AP↗
arXiv2014

Intrinsic construction of invariant functions on simple Lie algebras

An algorithm for constructing primitive adjoint-invariant functions on a complex simple Lie algebra is presented. The construction is intrinsic in the sense that it does not resort to any representation. A primitive invariant function on the whole Lie algebra is obtained by lifting a coordinate function on a Kostant slice of the Lie algebra. Such an intrinsic construction of invariant functions is most useful for the bigger exceptional Lie algebras such as the E's. The Maple implementation of this algorithm is outlined at the end and will be applied to these exceptional Lie algebras in a future work.

math.RT↗
arXiv2014

Non-Rigid Parabolic Geometries of Monge Type

In this paper we study a novel class of parabolic geometries which we call parabolic geometries of Monge type. These parabolic geometries are defined by special gradings of simple Lie algebras, namely, gradings with the property that their -1 component contains a nonzero co-dimension 1 abelian subspace whose bracket with its complement is non-degenerate. We completely classify the simple Lie algebras with such gradings in terms of elementary properties of the defining set of simple roots. We then characterize those parabolic geometries of Monge type which are non-rigid in the sense that, apart from the flat models, they have nonzero harmonic curvatures in positive weights. Standard models of all non-rigid parabolic geometries of Monge type are then described by under-determined ODE systems. The full symmetry algebras for these under-determined ODE systems are explicitly calculated; surprisingly, these symmetries are all just prolonged point symmetries.

math.DG↗
arXiv2013

Solving Toda field theories and related algebraic and differential properties

Toda field theories are important integrable systems. They can be regarded as constrained WZNW models, and this viewpoint helps to give their explicit general solutions, especially when a Drinfeld-Sokolov gauge is used. The main objective of this paper is to carry out this approach of solving the Toda field theories for the classical Lie algebras. In this process, we discover and prove some algebraic identities for principal minors of special matrices. The known elegant solutions of Leznov fit in our scheme in the sense that they are the general solutions to our conditions discovered in this solving process. To prove this, we find and prove some differential identities for iterated integrals. It can be said that altogether our paper gives complete mathematical proofs for Leznov's solutions.

math-ph↗
arXiv2011

On transgression in associated bundles

We formulate and prove a formula for transgressing characteristic forms in general associated bundles following a method of Chern. As applications, we derive D. Johnson's explicit formula for such general transgression and Chern's first transgression formula for the Euler class.

math.DG↗
arXiv2010

Secondary Chern-Euler forms and the Law of Vector Fields

The Law of Vector Fields is a term coined by Gottlieb for a relative Poincaré-Hopf theorem. It was first proved by Morse and expresses the Euler characteristic of a manifold with boundary in terms of the indices of a generic vector field and the inner part of its tangential projection on the boundary. We give two differential-geometric proofs of this topological theorem, in which secondary Chern-Euler forms naturally play an essential role. In the first proof, the main point is to construct a chain away from some singularities. The second proof employs a detailed study of the secondary Chern-Euler form on the boundary, which may be of independent interest. More precisely, we show by explicitly constructing a primitive that, away from the outward and inward unit normal vectors, the secondary Chern-Euler form is exact up to a pullback form. It should be emphasized that we obtain this result in the general case without assuming the metric is locally product near the boundary. In either case, Stokes' theorem is used to complete the proof.

math.DG↗