非可定向曲面上光滑函数的形变对称性
Deformational symmetries of smooth functions on non-orientable surfaces
AI总结:
本文研究非可定向曲面(Möbius带等)上光滑函数的形变对称性,计算稳定子边界固定子群的π₀,进而给出所有非可定向曲面(除Klein瓶和射影平面)轨道基本群的显式代数描述。
摘要:
给定一个紧致曲面 $M$,考虑 $M$ 的微分同胚群 $\mathcal{D}(M)$ 在 $\mathcal{C}^{\infty}(M,\mathbb{R})$ 上的自然右作用,其规则为:$(f,h)\mapsto f\circ h$,其中 $f\in \mathcal{C}^{\infty}(M,\mathbb{R})$,$h\in\mathcal{D}(M)$。记 $\mathcal{F}(M)$ 为 $\mathcal{C}^{\infty}(M,\mathbb{R})$ 的子集,由满足以下条件的函数 $f:M\to\mathbb{R}$ 组成:在 $\partial{M}$ 的每个连通分量上取常数值,在 $\partial{M}$ 上没有临界点,并且在每个临界点 $z$ 处,函数 $f$ 与某个无重因子的齐次多项式 $\mathcal{C}^{\infty}$ 等价。特别地,$\mathcal{F}(M)$ 包含所有 Morse 映射。另设 $\mathcal{O}(f) = \{ f\circ h \mid h\in\mathcal{D}(M) \}$ 为 $f$ 的轨道。此前已计算了所有 $f\in\mathcal{F}(M)$ 的 $\pi_1\mathcal{O}(f)$ 的代数结构,其中 $M$ 是任意不同于 $2$-球面的可定向紧致曲面。在本文中,我们计算群 $\pi_0\mathcal{S}(f,\partial\mathbb{M})$,其中 $\mathbb{M}$ 是 Möbius 带,$\mathcal{S}(f,\partial\mathbb{M}) = \{ h\in\mathcal{D}(\mathbb{M}) \mid f\circ h = f, \\ h|_{\partial \mathbb{M}} = \mathrm{id}_{\mathbb{M}}\}$ 是 $f$ 的相应稳定子中由在边界 $\partial \mathbb{M}$ 上固定的微分同胚构成的子群。作为推论,我们得到了所有不同于 Klein 瓶和射影平面的非可定向曲面的 $\pi_1\mathcal{O}(f)$ 的显式代数描述。
英文摘要:
Given a compact surface $M$, consider the natural right action of the group of diffeomorphisms $\mathcal{D}(M)$ of $M$ on $\mathcal{C}^{\infty}(M,\mathbb{R})$ defined by the rule: $(f,h)\mapsto f\circ h$ for $f\in \mathcal{C}^{\infty}(M,\mathbb{R})$ and $h\in\mathcal{D}(M)$. Denote by $\mathcal{F}(M)$ the subset of $\mathcal{C}^{\infty}(M,\mathbb{R})$ consisting of function $f:M\to\mathbb{R}$ taking constant values on connected components of $\partial{M}$, having no critical points on $\partial{M}$, and such that at each of its critical points $z$ the function $f$ is $\mathcal{C}^{\infty}$ equivalent to some homogenenous polynomial without multiple factors. In particular, $\mathcal{F}(M)$ contains all Morse maps. Let also $\mathcal{O}(f) = \{ f\circ h \mid h\in\mathcal{D}(M) \}$ be the orbit of $f$. Previously it was computed the algebraic structure of $π_1\mathcal{O}(f)$ for all $f\in\mathcal{F}(M)$, where $M$ is any orientable compact surface distinct from $2$-sphere. In the present paper we compute the group $π_0\mathcal{S}(f,\partial\mathbb{M})$, where $\mathbb{M}$ is a Möbius band, and $\mathcal{S}(f,\partial\mathbb{M}) = \{ h\in\mathcal{D}(\mathbb{M}) \mid f\circ h = f, \ h|_{\partial \mathbb{M}} = \mathrm{id}_{\mathbb{M}}\}$ is the subgroup of the corresponding stabilizer of $f$ consisting of diffeomorphisms fixed on the boundary $\partial \mathbb{M}$. As a consequence we obtain an explicit algebraic description of $π_1\mathcal{O}(f)$ for all non-orientable surfaces distinct from Klein bottle and projective plane.
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