### 经济代写|计量经济学代写Econometrics代考|Find2022

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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

## 经济代写|计量经济学代写Econometrics代考|Sequences, Limits, and Convergence

The concept of infinity is one of unending fascination for mathematicians. One noted twentieth-century mathematician, Stanislaw Ulam, wrote that the continuing evolution of various notions of infinity is one of the chief driving forces behind research in mathematics (Ulam, 1976). However that may be, seemingly impractical and certainly unattainable infinities are at the heart of almost all valuable and useful applications of mathematics presently in use, among which we may count econometrics.

The reason for the widespread use of infinity is that it can provide workable approximations in circumstances in which exact results are difficult or impossible to obtain. The crucial mathematical operation which yields these approximations is that of passage to the limit, the limit being where the notion of infinity comes in. The limits of interest may be zero, finite, or infinite. Zero or finite limits usually provide the approximations that are sought: Things difficult to calculate in a realistic, finite, context are replaced by their limits as an approximation.

The first and most frequently encountered mathematical construct which may possess a limit is that of a sequence. A sequence is a countably infinite collection of things, such as numbers, vectors, matrices, or more general mathematical objects, and thus by its mere definition cannot be represented in the actual physical world. But some sequences are nevertheless very familiar. Consider the most famous sequence of all: the sequence
$${1,2,3, \ldots}$$
of the natural numbers. This is a simple-minded example perhaps, but one that exhibits some of the important properties which sequences may possess.

## 经济代写|计量经济学代写Econometrics代考|Rates of Convergence

We covered a lot of ground in the last section, so much so that we have by now, even if very briefly, touched on all the important purely mathematical topics to be discussed in this chapter. What remains is to flesh out the treatment of some matters and to begin to apply our theory to statistics and econometrics. The subject of this section is rates of convergence. In treating it we will introduce some very important notation, called the $O$, o notation, which is read as “big- $O$, little-o notation.” Here $O$ and $o$ stand for order and are often referred to as order symbols. Roughly speaking, when we say that some quantity is, say, $O(x)$, we mean that is of the same order, asymptotically, as the quantity $x$, while when we say that it is $o(x)$, we mean that it is of lower order than the quantity $x$. Just what this means will be made precise below.

In the last section, we discussed the random variable $b_n$ at some length and saw from (4.05) that its variance converged to zero, because it was proportional to $n^{-1}$. This implies that the sequence converges in probability to zero, and it can be seen that the higher moments of $b_n$, the third, fourth, and so on, must also tend to zero as $n \rightarrow \infty$. A somewhat tricky calculation, which interested readers are invited to try for themselves, reveals that the fourth moment of $b_n$ is
$$E\left(b_n^4\right)=\frac{3}{16} n^{-2}-\frac{1}{8} n^{-3},$$
that is, the sum of two terms, one proportional to $n^{-2}$ and the other to $n^{-3}$. The third moment of $b_n$, like the first, is zero, simply because the random variable is symmetric about zero, a fact which implies that all its odd-numbered moments vanish. Thus the second, third, and fourth moments of $b_n$ all converge to zero, but at different rates. Again, the two terms in the fourth moment (4.11) converge at different rates, and it is the term which is proportional to $n^{-2}$ that has the greatest importance asymptotically.

1,2,3,…

## 经济代写|计量经济学代写Econometrics代考|Rates of Convergence

$$E\left(b_n^4\right)=\frac{3}{16} n^{-2}-\frac{1}{8} n^{-3},$$

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## 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。