### 数学代写|拓扑学代写Topology代考|MTH3130

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• Statistical Inference 统计推断
• Statistical Computing 统计计算
• (Generalized) Linear Models 广义线性模型
• Statistical Machine Learning 统计机器学习
• Longitudinal Data Analysis 纵向数据分析
• Foundations of Data Science 数据科学基础

## 数学代写|拓扑学代写Topology代考|Maps, Homeomorphisms, and Homotopies

The equivalence of two topological spaces is determined by how the points that comprise them are connected. For example, the surface of a cube can be deformed into a sphere without cutting or gluing it because they are connected the same way. They have the same topology. This notion of topological equivalence can be formalized via functions that send the points of one space to points of the other while preserving the connectivity.

This preservation of connectivity is achieved by preserving the open sets. A function from one space to another that preserves the open sets is called a continuous function or a map. Continuity is a vehicle to define topological equivalence, because a continuous function can send many points to a single point in the target space, or send no points to a given point in the target space. If the former does not happen, that is, when the function is injective, we call it an embedding of the domain into the target space. True equivalence is given by a homeomorphism, a bijective function from one space to another which has continuity as well as a continuous inverse. This ensures that open sets are preserved in both directions.

Definition 1.15. (Continuous function; Map) A function $f: \mathbb{T} \rightarrow \mathbb{U}$ from the topological space $\mathbb{T}$ to another topological space $\mathbb{U}$ is continuous if for every open set $Q \subseteq \mathbb{U}, f^{-1}(Q)$ is open. Continuous functions are also called maps.
Definition 1.16. (Embedding) A map $g: \mathbb{T} \rightarrow \mathbb{U}$ is an embedding of $\mathbb{V}$ into $\mathbb{U}$ if $g$ is injective.

A topological space can be embedded into a Euclidean space by assigning coordinates to its points so that the assignment is continuous and injective. For example, drawing a triangle on paper is an embedding of $\mathbb{S}^1$ into $\mathbb{R}^2$. There are topological spaces that cannot be embedded into a Euclidean space, or even into a metric space – these spaces cannot be represented by any metric.

Next we define a homeomorphism that connects two spaces that have essentially the same topology.

Definition 1.17. (Homeomorphism) Let $\mathbb{T}$ and $\mathbb{U}$ be topological spaces. A homeomorphism is a bijective map $h: \mathbb{T} \rightarrow \mathbb{U}$ whose inverse is continuous too.

Two topological spaces are homeomorphic if there exists a homeomorphism between them.

## 数学代写|拓扑学代写Topology代考|Manifolds

A manifold is a topological space that is locally connected in a particular way. A 1-manifold has this local connectivity looking like a segment. A 2manifold (with boundary) has the local connectivity looking like a complete or partial disk. In layman’s terms, a 2-manifold has the structure of a piece of paper or rubber sheet, possibly with the houndaries glued together to form a closed surface – a category that includes disks, spheres, tori, and Möbius bands.

Definition 1.22. (Manifold) A topological space $M$ is an m-manifold, or simply a manifold, if every point $x \in M$ has a neighborhood homeomorphic to $\mathbb{B}_o^m$ or $\mathbb{H}^m$. The dimension of $M$ is $m$.

Every manifold can be partitioned into boundary and interior points. Observe that these words mean very different things for a manifold than they do for a metric space or topological space.

Definition 1.23. (Boundary; Interior) The interior Int $M$ of an $m$-manifold $M$ is the set of points in $M$ that have a neighborhood homeomorphic to $\mathbb{B}_o^m$. The boundary $\mathrm{Bd} M$ of $M$ is the set of points $M \backslash \operatorname{Int} M$. The boundary $\operatorname{Bd} M$, if not empty, consists of the points that have a neighborhood homeomorphic to $\mathbb{H}^m$. If $\mathrm{Bd} M$ is the empty set, we say that $M$ is without boundary.

A single point, a 0 -ball, is a 0 -manifold without boundary according to this definition. The closed disk $\mathbb{B}^2$ is a 2-manifold whose interior is the open disk $\mathbb{B}_o^2$ and whose boundary is the circle $\mathbb{S}^1$. The open disk $\mathbb{B}_o^2$ is a 2-manifold whose interior is $\mathbb{B}_o^2$ and whose boundary is the empty set. This highlights an important difference between Definitions $1.13$ and $1.23$ of “boundary”: when $\mathbb{B}_o^2$ is viewed as a point set in the space $\mathbb{R}^2$, its boundary is $\mathbb{S}^1$ according to Definition 1.13; but viewed as a manifold, its boundary is empty according to Definition 1.23. The boundary of a manifold is always included in the manifold.

The open disk $\mathbb{B}_o^2$, the Euclidean space $\mathbb{R}^2$, the sphere $\mathbb{S}^2$, and the torus are all connected 2-manifolds without boundary. The first two are homeomorphic to each other, but the last two are not. The sphere and the torus in $\mathbb{R}^3$ are compact (bounded and closed with respect to $\mathbb{R}^3$ ) whereas $\mathbb{B}_o^2$ and $\mathbb{R}^2$ are not.

A $d$-manifold, $d \geq 2$, can have orientations whose formal definition we skip here. Informally, we say that a 2-manifold $M$ is non-orientable if, starting from a point $p$, one can walk on one side of $M$ and end up on the opposite side of $M$ upon returning to $p$. Otherwise, $M$ is orientable. Spheres and balls are orientable, whereas the Möbius band in Figure 1.7(a) is a non-orientable 2-manifold with boundary.

# 拓扑学代考

## 数学代写|拓扑学代写Topology代考|Manifolds

1.13；但作为流形来看，根据定义 1.23，它的边界是空的。流形的边界总是包含在流形中。

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## MATLAB代写

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