### 数学代写|复分析作业代写Complex function代考|MATH307

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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|复分析作业代写Complex function代考|Cyclic AFD for n-Best Rational Approximation

In core-AFD the parameters $a_1, \ldots, a_n, \ldots$ are selected in the one by one manner to obtain an optimal sequence of Blaschke forms to approximate the given function
$$\sum_{k=1}^n\left\langle f, B_{\left{a_1, \cdots, a_k\right}}\right\rangle B_{\left{a_1, \cdots, a_k\right}}(z) .$$
Now we change the question to the following: Given $f \in H^2(\mathbf{D})$ and a fixed positive integer $n$, find $n$ parameters $\tilde{a}1, \ldots, \tilde{a}_n$ such that the associated $n$-Blaschke form best approximates $f$, that is, \begin{aligned} & \left|f-\sum{k=1}^n\left\langle f, B_{\left{\tilde{a}1, \cdots, \tilde{a}_k\right}}\right\rangle B{\left{\tilde{a}1, \cdots, \tilde{a}_k\right}}(z)\right| \ = & \min \left{\left|f-\sum{k=1}^n\left\langle f, B_{\left{b_1, \cdots, b_k\right}}\right\rangle B_{\left{b_1, \cdots, b_k\right}}(z)\right|:\left{b_1, \cdots, b_n\right} \in \mathbf{D}^n\right} . \end{aligned}
This amounts an optimization with simultaneous selected $n$ parameters that is obviously better than one on selections of $n$ parameters in the one by one manner. Simultaneous selection of the parameters in an approximating $n$-Blaschke form is equivalent with the so-called optimal approximation by rational functions of degrees not larger than $n$. The latter problem was phrased as $n$-best rational approximation. It has been a long standing open problem, presented as follows.

Let $p$ and $q$ denote polynomials of one complex variable. We say that $(p, q)$ is an n-pair if $p$ and $q$ are co-prime, both of degrees less than or equal to $n$, and $q$ does not have zero in the unit disc. Denote by $\mathcal{R}_n$ the set of all such $n$-pairs. If $(p, q) \in \mathcal{R}_n$, then the rational function $p / q$ is said to be a rational function of degree less or equal $n$. Let $f$ be a function in the Hardy $H^2$ space in the unit disc. To find an $n$-best rational approximation to $f$ is to find an $n$-pair $\left(p_1, q_1\right)$ such that
$$\left|f-p_1 / q_1\right|=\min \left{|f-p / q|:(p, q) \in \mathcal{R}_n\right} .$$
Existence of such a minimum solution was proved many decades ago [4, 112], a practical algorithm to get a solution, however, has been an open problem till now. The best $n$-Blaschke form approximation is essentially equivalent with the $n$-best rational approximation. There are separate proofs for existence of the solution of optimization problem (15) $[75,84]$. By taking advantages of the explicit form and the orthogonality of Blaschke forms we get a practical algorithm for the classical $n$-best rational approximation problem.

## 数学代写|复分析作业代写Complex function代考|Pre-Orthogonal Adaptive Fourier Decomposition

The approximation theory and algorithm that were developed in the previous sections can be extended to more general contexts. To explain just the idea we restrict ourselves to the simplest cases, including the weighted Bergman spaces and weighted Hardy spaces, etc. Assume that Hilbert space $\mathcal{H}$ consists of functions defined in an open connected region $\mathcal{E}$ (can be unbounded) in the complex plane, and the reproducing kernel $k_a$ is an analytic function of the variable $a$ in $\mathcal{E}$ satisfying the relation
$$f^{(l)}(a)=\left\langle f,\left(\frac{\partial}{\partial \bar{a}}\right)^l k_a\right\rangle, \quad l=1,2, \cdots$$
Let $\left{a_1, \cdots, a_n, \cdots\right}$ be a finite or infinite sequence. For a fixed $n$ we define the multiple of $a_n$, denoted by $l\left(a_n\right)$, to be the repeating times of $a_n$ in the $n$-tuple $\left{a_1, \cdots, a_n\right}$. With this definition, for instance, the multiple of $a_1$ is just 1 , and the multiple of $a_2$ will depend on whether $a_2=a_1$. If yes, then $l\left(a_2\right)=2$, and, if not, $l\left(a_2\right)=1$, and so on. Note that it is a little abuse of notation for it is not dependent on the value of $a_n$ but on the repeating times of $a_n$ in the corresponding $n$-tuple. We accordingly define
$$\tilde{k}{a_n} \triangleq\left[\left(\frac{\partial}{\partial \bar{a}}\right)^{l\left(a_n\right)-1} k_a\right]{a=a_n} \triangleq\left(\frac{\partial}{\partial \bar{a}}\right)^{l\left(a_n\right)-1} k_{a_n} .$$
We further assume the following boundary vanishing condition, implying the maximal selection principle in every individual context, as follows: Let $a_1, \cdots, a_{n-1}$ be previously given, and $\left{B_1, \cdots, B_{n-1}\right}$ be the Gram-Schmidt orthonormalization of $\left{\tilde{k}{a_1}, \cdots, \tilde{k}{a_{n-1}}\right}$, then for every $f \in \mathcal{H}$, the pre-orthogonal system has the property
$$\lim _{a \rightarrow \partial \mathcal{E}}\left\langle f, B_n^a\right\rangle=0,$$ where $\left{B_1, \cdots, B_{n-1}, B_n^a\right}$ is the Gram-Schmidt orthonormalization of $\left{\tilde{k}{a_1}, \cdots, \tilde{k}{a_{n-1}}, k_a\right}$, with $a \neq a_k, k=1, \cdots, n-1$. We note (1) if $a \rightarrow \partial \mathcal{E}$, then $a$ is different from any already selected $a_k, k=1, \cdots, n-1$; and (2) in any case the limit $a \rightarrow \partial \mathcal{E}$ is in the sense of the topology of the one-point-compactification of the complex plane while the “one point” takes to be $\infty$. With boundary vanishing assumption we conclude the maximal selection principle of POAFD: Under the assumption (21), through a compact argument, there exists a sequence $\left{b_j\right}_{j=1}^{\infty}$ such that none of the $b_j$ ‘s take any values $a_1, \cdots, a_{n-1}$, and $\lim {j \rightarrow \infty} b_j \triangleq a_n \in \mathcal{E}$, and $$\lim {j \rightarrow \infty}\left|\left\langle f, B_n^{b_j}\right\rangle\right|=\max \left{\left|\left\langle f, B_n^a\right\rangle\right|: a \in \mathcal{E}\right}$$

# 复分析代写

## 数学代写|复分析作业代写Complex function代考|Pre-Orthogonal Adaptive Fourier Decomposition

$$f^{(l)}(a)=\left\langle f,\left(\frac{\partial}{\partial \bar{a}}\right)^l k_a\right\rangle, \quad l=1,2, \cdots$$

$$\tilde{k} a_n \triangleq\left[\left(\frac{\partial}{\partial \bar{a}}\right)^{l\left(a_n\right)-1} k_a\right] a=a_n \triangleq\left(\frac{\partial}{\partial \bar{a}}\right)^{l\left(a_n\right)-1} k_{a_n} .$$

$$\lim {a \rightarrow \partial \mathcal{E}}\left\langle f, B_n^a\right\rangle=0,$$ 在哪里 Veft{B_1, \cdots, B{n-1}, B_n^a\right } } \text { 是 Gram-Schmidt 正交化 }
Veft{\tilde{k}{a_1}, \cdots, \tilde{k}{a_{n-1}}, k_a\right} ， 和 $a \neq a_k, k=1, \cdots, n-1$. 我们注意到 (1) 如 果 $a \rightarrow \partial \mathcal{E} ，$ 然后 $a$ 不同于任何已选择的 $a_k, k=1, \cdots, n-1$ ； (2) 在任何情况下限制 $a \rightarrow \partial \mathcal{E}$ 是在复 平面的一点紧化的拓扑意义上，而“一点”是 $\infty$. 通过边界消失假设，我们得出 POAFD 的最大选择原则： 在假设 (21) 下，通过坚凑的论证，存在一个序列 \eft{b_jright $\left.}_{-}{j=1}^{\wedge}{\backslash i n f t y}\right}$ 竝样就没有 $b_j$ 取任何值 $a_1, \cdots, a_{n-1}$ ， 和lim $j \rightarrow \infty b_j \triangleq a_n \in \mathcal{E} ，$ 和

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## MATLAB代写

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