diff --git a/.github/workflows/blank.yml b/.github/workflows/blank.yml index a643a38..f1fbc60 100644 --- a/.github/workflows/blank.yml +++ b/.github/workflows/blank.yml @@ -41,6 +41,7 @@ jobs: Exams/HKALE/1999/HKALE-1999.tex Exams/HKCEE/2001/HKCEE-2001.tex Exams/HKDSE/2020/HKDSE-2020.tex + Exams/Rutgers-Analysis/midterm.cecilia.tex MIT/Solutions/MIT.tex Misc/Bernoulli/Bernoulli.tex Misc/Cosine/Cosine.tex @@ -76,6 +77,7 @@ jobs: HKALE-1999.pdf HKCEE-2001.pdf HKDSE-2020.pdf + midterm.cecilia.pdf MIT.pdf Bernoulli.pdf Cosine.pdf diff --git a/Exams/Rutgers-Analysis/midterm.cecilia.tex b/Exams/Rutgers-Analysis/midterm.cecilia.tex new file mode 100644 index 0000000..58c97fd --- /dev/null +++ b/Exams/Rutgers-Analysis/midterm.cecilia.tex @@ -0,0 +1,164 @@ +\documentclass{article} +\usepackage{amsmath} +\usepackage{amssymb} +\usepackage{nth} +\usepackage[utf8]{inputenc} +\usepackage{graphicx} +\usepackage[final]{pdfpages} +\usepackage{hyperref} + +\title{Midterm Exam, Introduction to Real Analysis I} +\author{Cecilia Chan} +\date{February 2024} + +\begin{document} +\maketitle +\section*{Course Information} +The course homepage is \href{https://math.rutgers.edu/academics/undergraduate/courses/955-01-640-311-introduction-to-real-analysis-i}{Introduction to Real Analysis I}. + +The midterm pdf link is \href{https://math.rutgers.edu/images/test-311-10-2016.pdf}{here}. +\section*{Problem 1 (ok)} +\subsection*{Part 1} +True, because otherwise the real number would be the disjoint union of two countable sets. +\subsection*{Part 2} +Yes, because it is the union of countably many countable sets. +\subsection*{Part 3} +It depends on the space. If the space is complete (e.g. $ \mathbb{R}^N $), then the sequence is convergent (that's the definition of a space being complete). +\subsection*{Part 4} +False, consider the sequence $ \{1, -1, 1, \ldots \} $, it is bounded but not convergent. +\subsection*{Part 5} +True, to satisfy the epsilon challenge, just pick the maximum $ N $ for both subsequences. +\subsection*{Part 6} +True, because if that's not true, $ t - \epsilon $ is a smaller upper bound. +\subsection*{Part 7} +True, to satisfy the epsilon challenge, just pick the $ N $ corresponding to $ \frac{1}{L} \epsilon $ for $ f(x) $ where $ -L < g(x) < L$. +\subsection*{Part 8} +True, because otherwise at least one of the greatest lower bounds is actually not greatest. +\subsection*{Part 9} +True, the least upper bound exists and it is the limit. +\subsection*{Part 10} +False, 0 is also an accumulation point. + +\section*{Problem 2} +Let $ N = \lceil \frac{2}{\epsilon} \rceil $, then for $ n > N $, we have +\begin{eqnarray*} + n & > & N \\ + n & > & \frac{2}{\epsilon} \\ + \epsilon & > & \frac{2}{n} \\ + \frac{2}{n} & < & \epsilon \\ + \frac{2}{n} - 3 + 3 & < & \epsilon \\ + \frac{2}{n} - \frac{3n}{n} + 3 & < & \epsilon \\ + -(\frac{3n - 2}{n} - 3) & < & \epsilon +\end{eqnarray*} + +We also have $ \frac{3n - 2}{n} = 3 - \frac{2}{n} < 3 $, section + +\begin{eqnarray*} + \frac{3n - 2}{n} &=& 3 - \frac{2}{n} < 3 \\ + \frac{3n - 2}{3} - 3 &<& 0 \\ + &<& \epsilon +\end{eqnarray*} + +Together with have $ |\frac{3n - 2}{3} - 3| < \epsilon $ when $ n > \lceil \frac{2}{\epsilon} \rceil $, so we conclude + +\begin{eqnarray} + \lim_{n \to \infty} \frac{3n - 2}{n} = 3 +\end{eqnarray} + +\section*{Problem 3} +Let $ \delta = \min (\sqrt{\frac{\epsilon}{2}}, \frac{\epsilon}{8}) $. + +On one hand, we have: + +\begin{align*} + |x - 1| &< \delta \\ + |x - 1| &< \sqrt{\frac{\epsilon}{2}} \\ + |(x-1)^2| &< \frac{\epsilon}{2} &\text{square is okay because LHS $\ge$ 0} \\ +\end{align*} + +On the other hand, we have: + +\begin{align*} + |x - 1| &< \delta \\ + |x - 1| &< \frac{\epsilon}{8} \\ + |4(x-1)| &< \frac{\epsilon}{2} \\ +\end{align*} + +Therefore, by the triangle inequality, we have: + +\begin{align*} + & |x^2 + 2x + 1 - 4 | \\ + =& |x^2 - 2x + 1 + 4x - 4| \\ + =& |(x-1)^2 + 4(x-1)| \\ + <& |(x-1)^2| + |4(x-1)| \\ + <& \frac{\epsilon}{2} + \frac{\epsilon}{2} \\ + =& \epsilon +\end{align*} + +Therefore we proved that $ \lim\limits_{x \to 1} x^2 + 2x + 1 = 4 $ by the definition of the limit, it is possible to find a $ \delta $ such that $ |x - 1| < \delta $ implies $ |x^2 + 2x + 1 - 4| < \epsilon $. + +\section*{Problem 4} + +We claim that the limit is $ L = \frac{1 + \sqrt{17}}{2} $ + +Note that when $ L $ is designed to be the larger root of $ x^2 - x - 4 $, so when $ x > L $, $ x^2 - x - 4 > 0 $, and so we have: + +\begin{align*} + x^2 - x - 4 &> 0 & \text{ By the geometry of the graph} \\ + x^2 &> x + 4 \\ + x &> \sqrt{4 + x} & \text{ Because $ x > 0 $ and square root is increasing } \\ +\end{align*} + +Since $ a_1 = 4 > L $, the iteration will be decreasing. It is obviously bounded below by 0, so a limit exists. + +Next, we have: + +\begin{align*} + & \lim_{n \to \infty} a_n \\ + =& \lim_{n \to \infty} \sqrt{4 + a_{n-1}} \\ + =& \sqrt{\lim_{n \to \infty} 4 + a_{n-1}} & \text{ by the continuity of the square root function} \\ + =& \sqrt{4 + \lim_{n \to \infty} a_{n-1}} \\ + =& \sqrt{4 + \lim_{n \to \infty} a_{n}} \\ +\end{align*} + +So it must be a root of $ x = \sqrt{4 + x} $, and apparently it must be greater than 0, so it must be $ L $. + +\section*{Problem 5 (ok)} +\subsection*{Part a} +The use of L'Hopital's rule is justified by the $ \frac{\infty}{\infty} $ form of the limit. We have: +\begin{eqnarray*} + & & \lim_{n \to \infty}\frac{3n^3 - n + 100}{n^3 - n + 2} \\ + &=& \lim_{n \to \infty}\frac{9n^2 - 1}{3n^2 - 1} \\ + &=& \lim_{n \to \infty}\frac{18n}{6n} \\ + &=& 3 +\end{eqnarray*} + +\subsection*{Part b} +The use of L'Hopital's rule is justified by the $ \frac{0}{0} $ form of the limit. We have: +\begin{eqnarray*} + & & \lim_{x \to 0}\frac{\sqrt{x + 4} - 2}{x} \\ + &=& \lim_{x \to 0}\frac{\frac{1}{2}(x + 4)^{-\frac{1}{2}}}{1} \\ + &=& \frac{1}{4} +\end{eqnarray*} + +\section*{Problem 6 (ok)} +$ \lim_{x_0 \to 0} f(x) = 2 $ implies for any $ \epsilon > 0 $, there exists $ \delta > 0 $ such that $ 0 < |x - x_0| < \delta $ implies $ |f(x) - 2| < \epsilon $. In particular, for $ \epsilon = 1 $, there exists $ \delta > 0 $ such that $ 0 < |x - x_0| < \delta $ implies $ |f(x) - 2| < 1 $. + +Note that $ 0 < | x - x_0| < \delta $ is the same as $ x \in (x_0 - \delta, x_0 + \delta) $, and $ |f(x) - 2| < 1 $ is the same as $ f(x) \in (1, 3) $, in particular, $ f(x) > 1 $. + +The fact that $ x_0 $ is an accumulation point implies some $ x $ will exist in the given set. It is not strictly necessary because otherwise the statement would simply be vacuously true. + +\section*{Problem 7} +The Bolzano-Weierstrass theorem states that every bounded sequence has a convergent subsequence; in other words, if a sequence $ \{ a_n \} $ is bounded, then it has a convergent subsequence $ \{ a_{n_k} \} $. + +We can prove that by showing a monotone subsequence exists, because then the sequence of monotone and bounded, so it must converge. + +Assuming the sequence has a monotonic increasing subsequence, then we can take that subsequence. + +Otherwise it must have a maximum, so we can take that point. The rest of the sequence is not does not have a monotonic increasing subsequence either, so we can define the subsequence inductively by keep taking the maximum of the rest. + +By definition, this subsequence is monotonically decreasing. + +So we know there exists a monotonic subsequence, and it is also bounded, so it must converges. + +\end{document}