## 数学代写|图论作业代写Graph Theory代考|MTH607

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## 数学代写|图论作业代写Graph Theory代考|Ramsey properties and connectivity

According to Ramsey’s theorem, every large enough graph $G$ has a very dense or a very sparse induced subgraph of given order, a $K^r$ or $\overline{K^r}$. If we assume that $G$ is connected, we can say a little more:

Proposition 9.4.1. For every $r \in \mathbb{N}$ there is an $n \in \mathbb{N}$ such that every connected graph of order at least $n$ contains $K^r, K_{1, r}$ or $P^r$ as an induced subgraph.

Proof. Let $d+1$ be the Ramsey number of $r$, let $n:=\frac{d}{d-2}(d-1)^r$, and let $G$ be a graph of order at least $n$. If $G$ has a vertex $v$ of degree at least $d+1$ then, by Theorem 9.1.1 and the choice of $d$, either $N(v)$ induces a $K^r$ in $G$ or ${v} \cup N(v)$ induces a $K_{1, r}$. On the other hand, if $\Delta(G) \leqslant d$, then by Proposition 1.3.3 $G$ has radius $>r$, and hence contains two vertices at a distance $\geqslant r$. Any shortest path in $G$ between these two vertices contains a $P^r$.

In principle, we could now look for a similar set of ‘unavoidable’ $k$-connected subgraphs for any given connectivity $k$. To keep thse ‘unavoidable sets’ small, it helps to relax the containment relation from ‘induced subgraph’ for $k=1$ (as above) to ‘topological minor’ for $k=2$, and on to ‘minor’ for $k=3$ and $k=4$. For larger $k$, no similar results are known.

Proposition 9.4.2. For every $r \in \mathbb{N}$ there is an $n \in \mathbb{N}$ such that every 2-connected graph of order at least $n$ contains $C^r$ or $K_{2, r}$ as a topological minor.

Proof. Let $d$ be the $n$ associated with $r$ in Proposition 9.4.1, and let $G$ be a 2-connected graph with at least $\frac{d}{d-2}(d-1)^r$ vertices. By Proposition 1.3.3, either $G$ has a vertex of degree $>d$ or $\operatorname{diam} G \geqslant \operatorname{rad} G>r$.

In the latter case let $a, b \in G$ be two vertices at distance $>r$. By Menger’s theorem (3.3.6), $G$ contains two independent $a-b$ paths. These form a cycle of length $>r$.

## 数学代写|图论作业代写Graph Theory代考|Simple sufficient conditions

What kind of condition might be sufficient for the existence of a Hamilton cycle in a graph $G$ ? Purely global assumptions, like high edge density, will not be enough: we cannot do without the local property that every vertex has at least two neighbours. But neither is any large (but constant) minimum degree sufficient: it is easy to find graphs without a Hamilton cycle whose minimum degree exceeds any given constant bound.
The following classic result derives its significance from this background:

Theorem 10.1.1. (Dirac 1952)
Every graph with $n \geqslant 3$ vertices and minimum degree at least $n / 2$ has a Hamilton cycle.

Proof. Let $G=(V, E)$ be a graph with $|G|=n \geqslant 3$ and $\delta(G) \geqslant n / 2$. Then $G$ is connected: otherwise, the degree of any vertex in the smallest component $C$ of $G$ would be less than $|C| \leqslant n / 2$.

Let $P=x_0 \ldots x_k$ be a longest path in $G$. By the maximality of $P$, all the neighbours of $x_0$ and all the neighbours of $x_k$ lie on $P$. Hence at least $n / 2$ of the vertices $x_0, \ldots, x_{k-1}$ are adjacent to $x_k$, and at least $n / 2$ of these same $k<n$ vertices $x_i$ are such that $x_0 x_{i+1} \in E$. By the pigeon hole principle, there is a vertex $x_i$ that has both properties, so we have $x_0 x_{i+1} \in E$ and $x_i x_k \in E$ for some $i<k$ (Fig. 10.1.1).

We claim that the cycle $C:=x_0 x_{i+1} P x_k x_i P x_0$ is a Hamilton cycle of $G$. Indeed, since $G$ is connected, $C$ would otherwise have a neighbour in $G-C$, which could be combined with a spanning path of $C$ into a path longer than $P$.

# 图论代考

## Matlab代写

MATLAB 是一种用于技术计算的高性能语言。它将计算、可视化和编程集成在一个易于使用的环境中，其中问题和解决方案以熟悉的数学符号表示。典型用途包括：数学和计算算法开发建模、仿真和原型制作数据分析、探索和可视化科学和工程图形应用程序开发，包括图形用户界面构建MATLAB 是一个交互式系统，其基本数据元素是一个不需要维度的数组。这使您可以解决许多技术计算问题，尤其是那些具有矩阵和向量公式的问题，而只需用 C 或 Fortran 等标量非交互式语言编写程序所需的时间的一小部分。MATLAB 名称代表矩阵实验室。MATLAB 最初的编写目的是提供对由 LINPACK 和 EISPACK 项目开发的矩阵软件的轻松访问，这两个项目共同代表了矩阵计算软件的最新技术。MATLAB 经过多年的发展，得到了许多用户的投入。在大学环境中，它是数学、工程和科学入门和高级课程的标准教学工具。在工业领域，MATLAB 是高效研究、开发和分析的首选工具。MATLAB 具有一系列称为工具箱的特定于应用程序的解决方案。对于大多数 MATLAB 用户来说非常重要，工具箱允许您学习应用专业技术。工具箱是 MATLAB 函数（M 文件）的综合集合，可扩展 MATLAB 环境以解决特定类别的问题。可用工具箱的领域包括信号处理、控制系统、神经网络、模糊逻辑、小波、仿真等。

## 数学代写|图论作业代写Graph Theory代考|Math780

statistics-lab™ 为您的留学生涯保驾护航 在代写图论Graph Theory方面已经树立了自己的口碑, 保证靠谱, 高质且原创的统计Statistics代写服务。我们的专家在代写图论Graph Theory代写方面经验极为丰富，各种代写图论Graph Theory相关的作业也就用不着说。

## 数学代写|图论作业代写Graph Theory代考|Ramsey numbers

Ramsey’s theorem may be rephrased as follows: if $H=K^r$ and $G$ is a graph with sufficiently many vertices, then either $G$ itself or its complement $\bar{G}$ contains a copy of $H$ as a subgraph. Clearly, the same is true for any graph $H$, simply because $H \subseteq K^h$ for $h:=|H|$.

However, if we ask for the least $n$ such that every graph $G$ of order $n$ has the above property – this is the Ramsey number $R(H)$ of $H$-then the above question makes sense: if $H$ has only few edges, it should embed more easily in $G$ or $\bar{G}$, and we would expect $R(H)$ to be smaller than the Ramsey number $R(h)=R\left(K^h\right)$.

A little more generally, let $R\left(H_1, H_2\right)$ denote the least $n \in \mathbb{N}$ such that $H_1 \subseteq G$ or $H_2 \subseteq \bar{G}$ for every graph $G$ of order $n$. For most graphs $H_1, H_2$, only very rough estimates are known for $R\left(H_1, H_2\right)$. Interestingly, lower bounds given by random graphs (as in Theorem 11.1.3) are often sharper than even the best bounds provided by explicit constructions.

The following proposition describes one of the few cases where exact Ramsey numbers are known for a relatively large class of graphs:

Proposition 9.2.1. Let $s, t$ be positive integers, and let $T$ be a tree of order $t$. Then $R\left(T, K^s\right)=(s-1)(t-1)+1$.

Proof. The disjoint union of $s-1$ graphs $K^{t-1}$ contains no copy of $T$, while the complement of this graph, the complete $(s-1)$-partite graph $K_{t-1}^{s-1}$, does not contain $K^s$. This proves $R\left(T, K^s\right) \geqslant(s-1)(t-1)+1$.
Conversely, let $G$ be any graph of order $n=(s-1)(t-1)+1$ whose complement contains no $K^s$. Then $s>1$, and in any vertex colouring of $G$ (in the sense of Chapter 5) at most $s-1$ vertices can have the same colour. Hence, $\chi(G) \geqslant\lceil n /(s-1)\rceil=t$. By Corollary $5.2 .3, G$ has a subgraph $H$ with $\delta(H) \geqslant t-1$, which by Corollary 1.5 .4 contains a copy of $T$.

## 数学代写|图论作业代写Graph Theory代考|Induced Ramsey theorems

Ramsey’s theorem can be rephrased as follows. For every graph $H=K^r$ there exists a graph $G$ such that every 2-colouring of the edges of $G$ yields a monochromatic $H \subseteq G$; as it turns out, this is witnessed by any large enough complete graph as $G$. Let us now change the problem slightly and ask for a graph $G$ in which every 2-edge-colouring yields a monochromatic induced $H \subseteq G$, where $H$ is now an arbitrary given graph.

This slight modification changes the character of the problem dramatically. What is needed now is no longer a simple proof that $G$ is ‘big enough’ (as for Theorem 9.1.1), but a careful construction: the construction of a graph that, however we bipartition its edges, contains an induced copy of $H$ with all edges in one partition class. We shall call such a graph a Ramsey graph for $H$.

The fact that such a Ramsey graph exists for every choice of $H$ is one of the fundamental results of graph Ramsey theory. It was proved around 1973 , independently by Deuber, by Erdős, Hajnal \& Pósa, and by Rödl.

Theorem 9.3.1. Every graph has a Ramsey graph. In other words, for every graph $H$ there exists a graph $G$ that, for every partition $\left{E_1, E_2\right}$ of $E(G)$, has an induced subgraph $H$ with $E(H) \subseteq E_1$ or $E(H) \subseteq E_2$.

We give two proofs. Each of these is highly individual, yet each offers a glimpse of true Ramsey theory: the graphs involved are used as hardly more than bricks in the construction, but the edifice is impressive.

# 图论代考

## Matlab代写

MATLAB 是一种用于技术计算的高性能语言。它将计算、可视化和编程集成在一个易于使用的环境中，其中问题和解决方案以熟悉的数学符号表示。典型用途包括：数学和计算算法开发建模、仿真和原型制作数据分析、探索和可视化科学和工程图形应用程序开发，包括图形用户界面构建MATLAB 是一个交互式系统，其基本数据元素是一个不需要维度的数组。这使您可以解决许多技术计算问题，尤其是那些具有矩阵和向量公式的问题，而只需用 C 或 Fortran 等标量非交互式语言编写程序所需的时间的一小部分。MATLAB 名称代表矩阵实验室。MATLAB 最初的编写目的是提供对由 LINPACK 和 EISPACK 项目开发的矩阵软件的轻松访问，这两个项目共同代表了矩阵计算软件的最新技术。MATLAB 经过多年的发展，得到了许多用户的投入。在大学环境中，它是数学、工程和科学入门和高级课程的标准教学工具。在工业领域，MATLAB 是高效研究、开发和分析的首选工具。MATLAB 具有一系列称为工具箱的特定于应用程序的解决方案。对于大多数 MATLAB 用户来说非常重要，工具箱允许您学习应用专业技术。工具箱是 MATLAB 函数（M 文件）的综合集合，可扩展 MATLAB 环境以解决特定类别的问题。可用工具箱的领域包括信号处理、控制系统、神经网络、模糊逻辑、小波、仿真等。

## 数学代写|图论作业代写Graph Theory代考|MATH7331

statistics-lab™ 为您的留学生涯保驾护航 在代写图论Graph Theory方面已经树立了自己的口碑, 保证靠谱, 高质且原创的统计Statistics代写服务。我们的专家在代写图论Graph Theory代写方面经验极为丰富，各种代写图论Graph Theory相关的作业也就用不着说。

## 数学代写|图论作业代写Graph Theory代考|The topological end space

In this last section we shall develop a deeper understanding of the global structure of infinite graphs, especially locally finite ones, that can be attained only by studying their ends. This structure is intrinsically topological, but no more than the most basic concepts of point-set topology will be needed.

Our starting point will be to make precise the intuitive idea that the ends of a graph are the ‘points at infinity’ to which its rays converge. To do so, we shall define a topological space $|G|$ associated with a graph $G=(V, E, \Omega)$ and its ends. ${ }^8$ By considering topological versions of paths, cycles and spanning trees in this space, we shall then be able to extend to infinite graphs some parts of finite graph theory that would not otherwise have infinite counterparts (see the notes for more examples). Thus, the ends of an infinite graph turn out to be more than a curious new phenomenon: they form an integral part of the picture, without which it cannot be properly understood.

To build the space $|G|$ formally, we start with the set $V \cup \Omega$. For every edge $e=u v$ we add a set $\dot{e}=(u, v)$ of continuum many points, making these sets $\ddot{e}$ disjoint from each other and from $V \cup \Omega$. We then choose for each $e$ some fixed bijection between $\dot{e}$ and the real interval $(0,1)$, and extend this bijection to one between $[u, v]:={u} \cup \grave{e} \cup{v}$ and $[0,1]$. This bijection defines a metric on $[u, v]$; we call $[u, v]$ a topological edge with inner points $x \in \dot{e}$. Given any $F \subseteq E$ we write $\stackrel{\circ}{F}:=\bigcup{\dot{e} \mid e \in F}$.

When we speak of a ‘graph’ $H \subseteq G$, we shall often also mean its corresponding point set $V(H) \cup \tilde{E}(H)$.

Having thus defined the point set of $|G|$, let us choose a basis of open sets to define its topology. For every edge $u v$, declare as open all subsets of $(u, v)$ that correspond, by our fixed bijection between $(u, v)$ and $(0,1)$, to an open set in $(0,1)$. For every vertex $u$ and $\epsilon>0$, declare as open the ‘open star around $u$ of radius $\epsilon$ ‘, that is, the set of all points on edges $[u, v]$ at distance less than $\epsilon$ from $u$, measured individually for each edge in its metric inherited from $[0,1]$. Finally, for every end $\omega$ and every finite set $S \subseteq V$, there is a unique component $C(S, \omega)$ of $G-S$ that contains a ray from $\omega$. Let $\Omega(S, \omega):=\left{\omega^{\prime} \in \Omega \mid C\left(S, \omega^{\prime}\right)=C(S, \omega)\right}$. For every $\epsilon>0$, write $E_\epsilon(S, \omega)$ for the set of all inner points of $S$ $C(S, \omega)$ edges at distance less than $\epsilon$ from their endpoint in $C(S, \omega)$. Then declare as open all sets of the form
$$\hat{C}\epsilon(S, \omega):=C(S, \omega) \cup \Omega(S, \omega) \cup \dot{E}\epsilon(S, \omega) .$$

## 数学代写|图论作业代写Graph Theory代考|Ramsey’s original theorems

In its simplest version, Ramsey’s theorem says that, given an integer $r \geqslant 0$, every large enough graph $G$ contains either $K^r$ or $\overline{K^r}$ as an induced subgraph. At first glance, this may seem surprising: after all, we need about $(r-2) /(r-1)$ of all possible edges to force a $K^r$ subgraph in $G$ (Corollary 7.1 .3 ), but neither $G$ nor $\bar{G}$ can be expected to have more than half of all possible edges. However, as the Turán graphs illustrate well, squeezing many edges into $G$ without creating a $K^r$ imposes additional structure on $G$, which may help us find an induced $\overline{K^r}$.

So how could we go about proving Ramsey’s theorem? Let us try to build a $K^r$ or $\overline{K^r}$ in $G$ inductively, starting with an arbitrary vertex $v_1 \in V_1:=V(G)$. If $|G|$ is large, there will be a large set $V_2 \subseteq V_1 \backslash\left{v_1\right}$ of vertices that are either all adjacent to $v_1$ or all non-adjacent to $v_1$. Accordingly, we may think of $v_1$ as the first vertex of a $K^r$ or $\overline{K^r}$ whose other vertices all lie in $V_2$. Let us then choose another vertex $v_2 \in V_2$ for our $K^r$ or $\overline{K^r}$. Since $V_2$ is large, it will have a subset $V_3$, still fairly large, of vertices that are all ‘of the same type’ with respect to $v_2$ as well: either all adjacent or all non-adjacent to it. We then continue our search for vertices inside $V_3$, and so on (Fig. 9.1.1).

How long can we go on in this way? This depends on the size of our initial set $V_1$ : each set $V_i$ has at least half the size of its predecessor $V_{i-1}$, so we shall be able to complete $s$ construction steps if $G$ has order about $2^s$. As the following proof shows, the choice of $s=2 r-3$ vertices $v_i$ suffices to find among them the vertices of a $K^r$ or $\overline{K^r}$.
Theorem 9.1.1. (Ramsey 1930)
For every $r \in \mathbb{N}$ there exists an $n \in \mathbb{N}$ such that every graph of order at least $n$ contains either $K^r$ or $\overline{K^r}$ as an induced subgraph.

Proof. The assertion is trivial for $r \leqslant 1$; we assume that $r \geqslant 2$. Let $n:=2^{2 r-3}$, and let $G$ be a graph of order at least $n$. We shall define a sequence $V_1, \ldots, V_{2 r-2}$ of sets and choose vertices $v_i \in V_i$ with the following properties:
(i) $\left|V_i\right|=2^{2 r-2-i} \quad(i=1, \ldots, 2 r-2)$;

(ii) $V_i \subseteq V_{i-1} \backslash\left{v_{i-1}\right} \quad(i=2, \ldots, 2 r-2)$;
(iii) $v_{i-1}$ is adjacent either to all vertices in $V_i$ or to no vertex in $V_i$ $(i=2, \ldots, 2 r-2)$.

# 图论代考

## Matlab代写

MATLAB 是一种用于技术计算的高性能语言。它将计算、可视化和编程集成在一个易于使用的环境中，其中问题和解决方案以熟悉的数学符号表示。典型用途包括：数学和计算算法开发建模、仿真和原型制作数据分析、探索和可视化科学和工程图形应用程序开发，包括图形用户界面构建MATLAB 是一个交互式系统，其基本数据元素是一个不需要维度的数组。这使您可以解决许多技术计算问题，尤其是那些具有矩阵和向量公式的问题，而只需用 C 或 Fortran 等标量非交互式语言编写程序所需的时间的一小部分。MATLAB 名称代表矩阵实验室。MATLAB 最初的编写目的是提供对由 LINPACK 和 EISPACK 项目开发的矩阵软件的轻松访问，这两个项目共同代表了矩阵计算软件的最新技术。MATLAB 经过多年的发展，得到了许多用户的投入。在大学环境中，它是数学、工程和科学入门和高级课程的标准教学工具。在工业领域，MATLAB 是高效研究、开发和分析的首选工具。MATLAB 具有一系列称为工具箱的特定于应用程序的解决方案。对于大多数 MATLAB 用户来说非常重要，工具箱允许您学习应用专业技术。工具箱是 MATLAB 函数（M 文件）的综合集合，可扩展 MATLAB 环境以解决特定类别的问题。可用工具箱的领域包括信号处理、控制系统、神经网络、模糊逻辑、小波、仿真等。