434 lines
23 KiB
TeX
434 lines
23 KiB
TeX
\documentclass[12pt,letterpaper]{article}
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\usepackage[left=2cm, right=2.5cm, top=2.5cm, bottom=2.5cm]{geometry}
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\usepackage{enumitem}
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\newcommand{\st}{\ \text{s.t.}\ }
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\newtheorem{definition}{Definition}[section]
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\newtheorem{theorem}{Theorem}[section]
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\newtheorem*{theorem*}{Theorem}
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\newtheorem{corollary}{Corollary}[section]
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\newtheorem{lemma}[theorem]{Lemma}
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\setlist[enumerate]{font=\bfseries}
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\renewcommand{\qedsymbol}{$\blacksquare$}
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\author{Alexander J. Tusa}
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\title{Real Analysis Homework 5}
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\begin{document}
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\maketitle
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\begin{enumerate}
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\item For the following sequences, i) write out the first 5 terms, ii) Use the Monotone Sequence Property to show that the sequences converges.
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\begin{enumerate}
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\item \textbf{Section 3.3}
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\begin{enumerate}
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\item[2)] Let $x_1 > 1$ and $x_{n+1} := 2-1/x_n$ for $n \in \N$. Show that $(x_n)$ is bounded and monotone. Find the limit.
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\\\\ The first five terms of this sequence are $x_1 \geq 2,x_2 \geq \frac{3}{2}, x_3 \geq \frac{4}{3}, x_4 \geq \frac{5}{4}, x_5 \geq \frac{6}{5}, \dots \approx x_1 \ge 2, x_2 \geq 1.5, x_3 \geq 1.3333, x_4 \geq 1.25, x_5 \geq 1.2, \dots$. This sequence appears to be decreasing.
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\\\\Recall the Monotone Sequence Property:
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\begin{theorem*}{Monotone Sequence Property}
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A monotone sequence of real numbers is convergent if and only if it is bounded. Further,
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\begin{enumerate}
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\item If $X=(x_n)$ is a bounded increasing sequence, then
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\[\lim (x_n) = \sup \{x_n:n \in \N\}\]
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\item If $Y=(y_n)$ is a bounded decreasing sequence, then
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\[\lim (y_n) = \inf \{y_n : n \in \N \}\]
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\end{enumerate}
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\end{theorem*}
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To show that this sequence converges, we must first find the possible limit points (fixed points) of this sequence. So,
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\begin{align*}
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x&=2-\frac{1}{x} \\
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x^2 &= 2x -1 \\
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x^2 - 2x + 1 &= 0 \\
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(x-1)^2 &= 0
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\end{align*}
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Thus, $x=1$ is a possible limit of this sequence.
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\\\\Now, we will prove that $(x_n)$ is bounded by $1$, and since we hypothesized that $(x_n)$ is decreasing, we say that $(x_n)$ is bounded below by 1.
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\begin{proof}
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We want to show that the sequence $(x_n)$ is bounded below by 1; that is, we want to show that $1 \leq x_n,\ \forall\ n \in \N$. We prove it by method of mathematical induction.
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\\\\\textbf{Basis Step:} Let $n=1$. Then
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\begin{align*}
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x_n &\geq x_{n+1}, &\text{by the definition of decreasing,} \\
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x_1 &\geq x_{1+1} \\
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x_1 &\geq x_2
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\end{align*}
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Since $x_1>1 \Rightarrow \frac{1}{x_1} < 1$, we have
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\[x_2 = 2-\frac{1}{x_1} > 1\]
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\[\Rightarrow 1 < x_2 < 2.\]
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Since $x_1>1$ and because $1 < x_2 < 2$, we have that $x_1 \geq x_2$.
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\\\\\textbf{Inductive Step:} Assume $1 < x_n < 2,\ \forall\ n \in \N$.
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\\\\\textbf{Show:} Now we want to show that $x_n \leq x_{n+1}$.
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\\So,
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\[1 < x_n <2\]
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\[1 > \frac{1}{x_n} > \frac{1}{2}\]
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\[-1 < -\frac{1}{x_n} < -\frac{1}{2}\]
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\[1 < 2-\frac{1}{x_n} < 2-\frac{1}{2} < 2\]
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\[1 < x_{n+1} < 2\]
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Thus we have that $(x_n)$ is bounded between 1 and 2.
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\end{proof}
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Now we need to show that $(x_n)$ is monotone decreasing; that is, we must show that $x_1 \geq x_2 \geq \dots \geq x_n$.
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\begin{proof}
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We want to show that $x_1 \geq x_2 \geq \dots \geq x_n,\ \forall\ n \in \N$. We prove it by method of mathematical induction.
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\\\\\textbf{Basis Step:} Let $n=1$. Then since $x_1 >1$ is given, we have that $\frac{1}{x_1} < 1$. This yields $x_2=2-\frac{1}{x_1}>1$, as was determined for the boundedness proof, and thus we have that $1 < x_2 < 2$. This means that $1 > \frac{1}{x_2} > \frac{1}{2}$, and since $\frac{1}{2} \leq \frac{1}{x_n}$, we have $x_2 \geq x_1$.
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\\\\\textbf{Inductive Step:} Assume $x_n \geq x_{n+1}\ \forall\ n \in \N$.
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\\\\\textbf{Show:} We now want to show that $x_{n+2} \leq x_{n+1}$.
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\\So,
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\[x_{n+2}=2-\frac{1}{x_{x+1}}\]
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Recall the inductive hypothesis, in that $x_n \geq x_{n+1} \Rightarrow \frac{1}{x_n} \leq \frac{1}{x_{n+1}}$. Thus,
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\[-\frac{1}{x_n} \geq -\frac{1}{x_{n+1}}\]
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\[\Rightarrow 2-\frac{1}{x_n} \leq 2 -\frac{1}{x_{n+1}}\]
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\[x_{n+1} \leq x_{n+2}\]
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$\therefore$ we have that $x_1 \geq x_2 \geq \dots \geq x_n,\ \forall\ n \in \N$.
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\end{proof}
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Thus $(x_n)$ is monotone decreasing.
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\\\\By the \textit{Monotone Sequence Property}, since we have shown that $(x_n)$ is both bounded (and thus converges), and that $(x_n)$ is monotone decreasing, we have that
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\begin{align*}
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\lim (x_n) &= \inf \{x_n: n \in \N\} \\
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&=\inf (1,2) \\
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&= 1
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\end{align*}
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Hence the sequence converges to the previously found possible limit of 1. \\
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\item[3)] Let $x_1 > 1$ and $x_{n+1} := 1 + \sqrt{x_n - 1}$ for $n \in \N$. Show that $(x_n)$ is decreasing and bounded below by $2$. Find the limit.
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\\\\The first 5 terms of this sequence are $x_1 \geq 2, x_2 \geq 2, x_3 \geq 2, x_4 \geq 2, x_5 \geq 2, \dots$. Notice the following, however:
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\begin{align*}
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x_{n+1} \leq x_n &\iff 1+\sqrt{x_n - 1} \leq x_n \\
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&\iff \sqrt{x_n -1} \leq x_n -1
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\end{align*}
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which we know is always true since the square root function is a decreasing function.
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\\\\Now we must find the possible limit points (fixed points) of this sequence. So,
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\begin{align*}
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x &= 1 + \sqrt{x-1} \\
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x-1 &= \sqrt{x-1} \\
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x-1 &= (x-1)^2 \\
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x-1 &= x^2 -2x +1 \\
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(x-1)-(x^2-2x+1) &= 0 \\
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-x^2+3x-2 &=0 \\
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-(x^2-3x+2) &= 0 \\
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-(x-1)(x-2) &= 0 \\
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(x-1)(x-2) &= 0
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\end{align*}
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Thus $x=1$, or $x=2$. These are the possible limits of $(x_n)$. Since we hypothesized that $(x_n)$ is decreasing, then we say that $(x_n)$ is bounded below by 2, since we are given that $x_1 > 1$.
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\\\\Now we will prove that $(x_n)$ is bounded below by 2.\\
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\begin{proof}
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We want to show that $(x_n)$ is bounded below by 1; that is, we want to show that $1 \leq x_n,\ \forall\ n \in \N$. We prove it by method of mathematical induction.
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\\\\\textbf{Basis Step:} Let $n=1$. Then we are given that $x_1 \geq 2$.
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\\\\\textbf{Inductive Step:} Assume that $x_n \geq 2,\ \forall\ n \in \N$.
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\\\\\textbf{Show:} We now want to show that $x_{n+1} \geq 2,\ \forall\ n \in \N$.
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\\\\So,
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\begin{align*}
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x_{n+1} &= 1+\sqrt{x_n -1} \\
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&\geq 1+\sqrt{2 -1} \\
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&=1 + 1 \\
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&= 2
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\end{align*}
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Thus, $x_n \geq 2,\ \forall\ n \in \N$. By the definition of boundedness, we have that $(x_n)$ is bounded below by 2.
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\end{proof}
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Since we have also shown earlier that $(x_n)$ is monotone decreasing, we have that by the monotone sequence property, since $(x_n)$ is bounded, $(x_n)$ converges, and since $(x_n)$ is monotone decreasing, we have:
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\begin{align*}
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\lim (x_n) &= \inf \{x_n:n \in \N\} \\
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&=2
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\end{align*}
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\item[7)] Let $x_1 := a>0$ and $x_{n+1} := x_n+1/x_n$ for $n \in \N$. Determine whether $(x_n)$ converges or diverges.
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\\\\The first 5 terms of this sequence are $x_1 \geq 1, x_2 \geq 2, x_3 \geq \frac{5}{2}, x_4 \geq \frac{29}{10}, x_5 \geq \frac{941}{290}, \dots \approx x_1 \geq 1, x_2 \geq 2, x_3 \geq 2.5, x_4 \geq 2.9, x_5 \geq 3.244828, \dots$. This sequence appears to be increasing. We show this to be true as follows:
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\begin{align*}
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x_{n+1} \geq x_n &\iff x_n + \frac{1}{x_n} \geq x_n \\
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&\iff x_n^2 + 1 \geq x_n^2 \\
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&\iff 1 \geq 0
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\end{align*}
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which is true.
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However, notice that one of the terms of the sequence is $x_n$. We know that $x_n$ is an unbounded sequence. Thus, we can infer that $(x_n)$ is unbounded above. We show this as follows:
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\begin{align*}
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x_{n+1}^2 &= \left(x_n + \frac{1}{x_n} \right)^2 \\
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&= x_n^2+2+\frac{1}{x_n^2} \\
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&> x_n^2 +2
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\end{align*}
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Since:
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\[x_{n+1}^2 > x_n^2+2 > x_{n-1}^2 +4 > \dots > x_1^2+2 \cdot n = a^2+2 \cdot n\]
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\[\Downarrow\]
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\[x_n > \sqrt{a^2 + 2 \cdot (n-1)}\]
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Since the right hand side of this inequality is unbounded, the left hand side is also unbounded.
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\\\\Thus we have that this sequence $(x_n)$ is unbounded above.
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\\\\Since this sequence is increasing and unbounded above, we have that the sequence is divergent.\\
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\item[8)] Let $(a_n)$ be an increasing sequence, $(b_n)$ be a decreasing sequence, and assume that $a_n \leq b_n$ for all $n \in \N$. Show that $\lim (a_n) \leq \lim (b_n)$, and thereby deduce the Nested Intervals Property 2.5.2 from the Monotone Convergence Theorem 3.3.2.
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\\\\Since $(a_n)$ is an increasing sequence, we know that $(a_1 \leq a_2 \leq \dots \leq a_n)$, and since $(b_n)$ is a decreasing sequence, we know that $(b_1 \geq b_2 \geq \dots \geq b_n)$. Also, since we have that $a_n \leq b_n,\ \forall\ n \in \N$, we know that $(a_n)$ is bounded above by $(b_1)$. Thus, by the \textit{Monotone Convergence Theorem}, we know that
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\[\lim (a_n) = \sup \{a_n: n \in \N\}\]
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Also, since $(b_n)$ is a decreasing sequence such that it is bounded below by $(a_1)$, by the \textit{Monotone Convergence Theorem}, we have
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\[\lim (b_n) = \inf \{b_n: n \in \N\}\]
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Recall Theorem 3.2.5:
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\begin{theorem*}
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If $X=(x_n)$ and $Y=(y_n)$ are convergent sequences of real numbers and if $x_n \leq y_n\ \forall\ n \in \N$, then $\lim (x_n) \leq \lim (y_n)$.
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\end{theorem*}
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Also, recall the \textit{Nested Intervals Property}:
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\begin{theorem*}
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If $I_n=[a_n,b_n],\ n \in \N$, is a nested sequence of closed bounded intervals, then there exists a number $\xi \in \R \st \xi \in I_n\ \forall\ n \in \N$.
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\end{theorem*}
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Note that we have a nested sequence of closed, bounded intervals: $[a_n, b_n],\ n \in \N$. Since we showed that $\lim (a_n) \leq \lim (b_n)$, (and we are given that $(a_n)$ is increasing and $(b_n)$ is decreasing), we know that there exists $\xi$ such that
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\[\lim (a_n) \leq \xi \leq \lim (b_n)\]
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which means that $\xi \in [a_n, b_n],\ \forall\ n \in \N$.
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\end{enumerate}
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\item $a_1 = 1,\ a_{n+1}=\frac{a_n^2+5}{2a_n}$
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\\\\The first 5 terms of this sequence are $1, 3, \frac{7}{3}, \frac{47}{21}, \frac{2207}{987}, \dots \approx 1, 3, 2.3333, 2.2381, 2.2361, \dots$. This is a decreasing sequence.
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\\\\First, we must find the possible limits (fixed points) of the sequence. So,
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\begin{align*}
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a&=\frac{a^2+5}{2a} \\
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2a^2 &= a^2+5 \\
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a^2 &= 5 \\
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a &= \pm \sqrt{5}
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\end{align*}
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Since we're given that $a_1=1$, we know that the most likely lower bound will be $\sqrt{5}$.
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\\\\Now we want to show that $(a_n)$ is bounded below by $\sqrt{5}$.
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\begin{proof}
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We want to show that $a_n \geq \sqrt{5},\ \forall\ n \in \N$. We prove it by method of mathematical induction.
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\\\\\textbf{Basis Step:} Since $1 \geq \sqrt{5}$, we have that $a_1 \geq \sqrt{5}$
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\\\\\textbf{Inductive Step:} Assume that $a_n \geq \sqrt{5}\ \forall\ n \in \N$.
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\\\\\textbf{Show:} We want to show that $a_{n+1} \geq \sqrt{5}\ \forall\ n \in \N$. So,
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\[a_{n+1} = \frac{a_n^2 + 5}{2a_n}\]
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\begin{align*}
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(a_n-\sqrt{5})^2 &\geq 0 \\
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a_n^2 -2\sqrt{5}a_n +5 &\geq 0 \\
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a_n^2 +5 &\geq 2\sqrt{5}a_n \\
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\Downarrow \\
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\frac{a_n^2+5}{2a_n} &\geq \frac{2\sqrt{5}a_n}{2a_n} \\
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\frac{a_n^2+5}{2a_n} &\geq \sqrt{5} \\
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a_{n+1} \geq \sqrt{5}
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\end{align*}
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Thus we have that $(a_n)$ is bounded below by $\sqrt{5}$.
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\end{proof}
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Now we must show that $(a_n)$ is monotone decreasing.\\
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\begin{proof}
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We want to show that $(a_n)$ is monotone decreasing; that is, we want to show that $(a_2 \geq a_3 \geq \dots \geq a_n),\ \forall\ n \geq 2$. We prove it by method of mathematical induction.
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\\\\\textbf{Basis Step:} Since $3 \geq \frac{7}{3}$, we have that $a_2 \geq a_3$.
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\\\\\textbf{Inductive Step:} Assume that $a_n \geq a_{n+1},\ \forall\ n \geq 2$.
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\\\\\textbf{Show:} We want to show that $a_{n+2} \leq a_{n+1},\ \forall\ n \geq 2$.
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\\So,
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\[a_{n+2} = \frac{a_{n+1}^2+5}{2a_{n+1}} \leq \frac{a_n^2+5}{2a_n}\]
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Since we have:
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\begin{align*}
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a_{n+1} &\geq \sqrt{5}, &\text{by the previous proof of boundedness} \\
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a_{n+1}^2 &\geq 5
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\end{align*}
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We can equivalently write the inequality as
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\[\frac{a_{n+1}^2+5}{2a_{n+1}} \leq \frac{a_{n+1}^2+a_{n+1}^2}{2a_{n+1}}=a_{n+1}\]
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Thus we have that $(a_n)$ is monotone decreasing.
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\end{proof}
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Since $(a_n)$ is both monotone decreasing and bounded, we have
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\begin{align*}
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\lim (a_n) &= \inf \{a_n:n \in \N\} \\
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&= \sqrt{5}
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\end{align*}
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\item $a_1 = 5,\ a_{n+1}=\sqrt{4+a_n}$
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\\\\The first 5 terms of this sequence are 5, 3, $\sqrt{7}$, $ \frac{\sqrt{14}}{2} + \frac{\sqrt{2}}{2} $, $ \frac{\sqrt{2 \cdot (\sqrt{14}+\sqrt{2}+8)}}{2} $, $\dots$, $\approx$ 5, 3, 2.64575131106, 2.57793547457, 2.5647486182, $\dots$. This sequence is decreasing.
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\\\\First, we must find the possible limits (fixed points) of the sequence. So,
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\begin{align*}
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a&=\sqrt{4+a} \\
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\sqrt{4+a} &= a \\
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4+a &= a^2 \\
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-a^2+a+4 &= 0 \\
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a^2-a-4 &= 0 \\
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a^2-a&=4 \\
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a^2-a+\frac{1}{4}&=4+\frac{1}{4} \\
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a^2-a+\frac{1}{4}&=\frac{17}{4} \\
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(a-\frac{1}{2})^2&=\frac{17}{4} \\
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a-\frac{1}{2}&=\pm \frac{\sqrt{17}}{2}
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\end{align*}
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So we have that $a=\frac{1}{2} + \frac{\sqrt{17}}{2}$, or $a=\frac{1}{2}-\frac{\sqrt{17}}{2}$. We must now check these solutions for correctness; so,
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\begin{align*}
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a \Rightarrow \frac{1}{2}-\frac{\sqrt{17}}{2} &=\frac{1}{2} \left(1-\sqrt{17}\right) \\
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&\approx -1.56155 \\\\
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\sqrt{a+4} &= \sqrt{\left(\frac{1}{2}-\frac{\sqrt{17}}{2}\right)+4} \\
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&=\frac{\sqrt{9-\sqrt{17}}}{\sqrt{2}} \\
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&\approx 1.56155
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\end{align*}
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Thus, this solution is incorrect. Now we must validate that $a=\frac{1}{2}+\frac{\sqrt{17}}{2}$ is correct. So,
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\begin{align*}
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a \Rightarrow \frac{1}{2}+\frac{\sqrt{17}}{2} &= \frac{1}{2}\left(1+\sqrt{17}\right) \\
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&\approx 2.56155 \\\\
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\sqrt{a+4} &= \sqrt{\left(\frac{\sqrt{17}}{2}+\frac{1}{2}\right)+4} \\
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&= \frac{\sqrt{9+\sqrt{17}}}{\sqrt{2}} \\
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&\approx 2.56155
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\end{align*}
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Thus $a=\frac{1}{2}+\frac{\sqrt{17}}{2}$ is a correct solution.
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\\\\Now we want to show that $(a_n)$ is bounded below by $\frac{1}{2}+\sqrt{17}$.
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\begin{proof}
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We want to show that $a_n \geq \frac{1}{2}+\frac{\sqrt{17}}{2},\ \forall\ n \in \N$, by the definition of a lower bound. We prove this by method of mathematical induction.
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\\\\\textbf{Basis Step:} Since $5 \geq \frac{1}{2}+\frac{\sqrt{17}}{2}$, we have that $a_1 \geq \frac{1}{2}+\frac{\sqrt{17}}{2}$.
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\\\\\textbf{Inductive Step:} Assume $a_n \geq \frac{1}{2}+\frac{\sqrt{17}}{2},\ \forall\ n \in \N$.
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\\\\\textbf{Show:} We now want to show that $ a_{n+1} \geq \frac{1}{2}+\frac{\sqrt{17}}{2}\ \forall\ n \in \N$. So,
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\begin{align*}
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a_{n+1} &= \sqrt{4+a_n}, &\text{by the definition of the sequence} \\
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&\geq \sqrt{4+\left(\frac{1}{2}+\frac{\sqrt{17}}{2}\right)}, &\text{by the inductive hypothesis} \\
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&\geq \sqrt{\frac{8}{2}+\frac{1}{2}+\frac{\sqrt{17}}{2}} \\
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&\geq \sqrt{\frac{9+\sqrt{17}}{2}} \\
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&\geq \sqrt{\frac{1}{2}\left(9+\sqrt{17}\right)} \\
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&\geq \sqrt{\frac{1}{4}+\frac{\sqrt{17}}{2}+\frac{17}{4}}, &\text{by expressing } \frac{9+\sqrt{17}}{2} \text{ as a square} \\
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&\geq \sqrt{\frac{1+2\sqrt{17}+17}{4}} \\
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&\geq \sqrt{\frac{1+2\sqrt{17}+(\sqrt{17})^2}{4}} \\
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&\geq \sqrt{\frac{(\sqrt{17}+1)^2}{4}} \\
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&\geq \sqrt{\frac{1}{4}\left(1+\sqrt{17}\right)^2} \\
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&\geq \frac{\sqrt{(1+\sqrt{17})^2}}{\sqrt{4}} \\
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&\geq \frac{\sqrt{17}+1}{2} \\
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&\geq \frac{1}{2} + \frac{\sqrt{17}}{2}
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\end{align*}
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Thus we have that $ a_{n+1} \geq \frac{1}{2} + \frac{\sqrt{17}}{2}\ \forall\ n \in \N$.
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\end{proof}
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Now, we want to show that $ (a_n) $ is monotone decreasing; that is, we want to show that $(a_1 \geq a_2 \geq \dots \geq a_n)$.
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\begin{proof}
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We want to show that $(a_1 \geq a_2 \geq \dots \geq a_n),\ \forall\ n \in \N$. We prove this by method of mathematical induction.
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\\\\\textbf{Basis Step:} Since $5 \geq 3$, we have that $a_1 \geq a_2$.
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\\\\\textbf{Inductive Step:} Assume $a_n \geq a_{n+1}\ \forall\ n \in \N$.
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\\\\\textbf{Show:} We want to show that $a_{n+1} \geq a_{n+2}\ \forall\ n \in \N$. So,
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\begin{align*}
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a_{n+2} &= \sqrt{4+a_{n+1}} &\text{by the definition of the sequence} \\
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&\leq \sqrt{4+a_n} &\text{by the inductive hypothesis} \\
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&=a_{n+1}
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\end{align*}
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Thus we have that $a_{n+1} \geq a_{n+2}\ \forall\ n \in \N$.
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\end{proof}
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Since $(a_n)$ is both bounded and monotone decreasing, by the \textit{Monotone Convergence Theorem}, we have that $(a_n)$ converges. Also by the \textit{Monotone Sequence Property}, we have that $(a_n)$ converges to the following:
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\begin{align*}
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\lim (a_n) &= \inf \{a_n: n \in \N\} \\
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&= \frac{1}{2}+\frac{\sqrt{17}}{2} \approx 2.56155281281
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\end{align*}
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\end{enumerate}
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|
|
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\item
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|
\begin{enumerate}
|
|
\item Show $a_n=\frac{3 \cdot 5 \cdot 7 \cdot \dots (2n-1)}{2 \cdot 4 \cdot 6 \dots (2n)}$ converges to $A$ where $0 \leq A < 1/2$.
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\\\\\textbf{TODO}
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|
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\item Show $b_n = \frac{2 \cdot 4 \cdot 6 \dots (2n)}{3 \cdot 5 \cdot 7 \dots (2n+1)}$ converges to $B$ where $0 \leq B < 2/3$.
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\\\\\textbf{TODO}
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|
\end{enumerate}
|
|
|
|
\item \textbf{Section 3.4}
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\begin{enumerate}
|
|
\item[1)] Give an example of an unbounded sequence that has a convergent subsequence.
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|
\\\\Let $a_n:=(-1)^n$. This sequence diverges since it oscillates between 1 and $-1$, however if we define $b_n:=(a_{2n})$; that is, let $(b_n)$ be the sequence of all terms of $(a_n)$ such that $n$ is even. Thus, $(b_n)$ is a subsequence of $(a_n)$, but $(b_n)$ converges since $a_2=1, a_4=1, \dots, a_{2n}=1$. Thus while $(a_n)$ is a divergent sequence, the subsequence $(b_n)$ of $(a_n)$ converges for all $n \in \N$.\\
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|
|
|
\item[3)] Let $(f_n)$ be the Fibonacci sequence of Example 3.1.2(d), and let $x_n := f_{n+1}/f_n$. Given that $\lim (x_n) =L$ exists, determine the value of $L$.
|
|
\\\\We can rewrite $(x_n)$ as follows:
|
|
\begin{align*}
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|
x_n &= \frac{f_{n+1}}{f_n} \\
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|
&=\frac{f_n+f_{n-1}}{f_n} \\
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|
&= 1+\frac{f_{n-1}}{f_n} \\
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|
&= 1+\frac{\frac{1}{f_n}}{f_{n-1}} \\
|
|
&= 1+\frac{1}{x_{n-1}}
|
|
\end{align*}
|
|
Since we're given that $L=\lim (x_n)$ exists and since we just showed that it's equal to $\lim (x_{n-1})$, we get the following:
|
|
\begin{align*}
|
|
x_n &= \left. 1+\frac{1}{x_{n-1}}\ \ \ \ \ \right| \lim \\
|
|
\lim (x_n) &= 1+\frac{1}{\lim(x_{n-1})} \\
|
|
L &= \left. 1+\frac{1}{L}\ \ \ \ \ \right| \cdot L \\
|
|
L^2 &= L + 1 \\
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|
L^2-L-1 &= 0 \\
|
|
L_{1,2} &= \frac{1 \pm \sqrt{1-4 \cdot 1 \cdot (-1)}}{2} \\
|
|
L_1 &= \frac{1-\sqrt{5}}{2} <0 \\
|
|
L_2 &= \frac{1+\sqrt{5}}{2}>0
|
|
\end{align*}
|
|
Now, since $f_n >0 \Rightarrow x_n >0 \Rightarrow L>0$, we can infer that the proper limit is
|
|
\[L=\frac{1+\sqrt{5}}{2}\]
|
|
\\
|
|
|
|
\item[4a)] Show that the sequence $(1-(-1)^n+1/n)$ converges.
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|
\\\\ Let $(x_n):=(1-(-1)^n+1/n)$. Let $(z_n)=(x_{2n})$, and $(w_n)=(x_{2n-1})$ be subsequence of $(x_n)$. Then $(z_n)$ is the subsequence of all terms of $(x_n)$ such that $n$ is even, and $(w_n)$ is the subsequence of all terms of $(x_n)$ such that $n$ is odd.
|
|
\\\\These subsequences yield the following:
|
|
\[z_n = x_{2n} = 1-(-1)^{2n}+\frac{1}{2n}=1-1+\frac{1}{2n}=\frac{1}{2n}\]
|
|
\[w_n=x_{2n-1}=1-(-1)^{2n-1}+\frac{1}{2n-1}=1+1+\frac{1}{2n-1}=2+\frac{1}{2n-1}\]
|
|
Now, if we take the limit of each sequence as $n \rightarrow \infty$ yields
|
|
\[\lim_{n \to \infty} (z_n) = 0 \neq 2 = \lim_{n \to \infty} (w_n)\]
|
|
Recall Theorem 3.4.5 \textit{Divergence Criteria}:
|
|
\begin{theorem*}
|
|
If a sequence $X=(x_n)$ of real numbers has either of the following properties, then $X$ is divergent.
|
|
\begin{enumerate}
|
|
\item $X$ has two convergent subsequences $X'=(x_{n_k})$ and $X''=(x_{r_k})$ whose limits are not equal.
|
|
|
|
\item $X$ is unbounded
|
|
\end{enumerate}
|
|
\end{theorem*}
|
|
Thus by the \textit{Divergence Criteria}, we have that since $(z_n)$ and $(w_n)$ satisfy the first property of the \textit{Divergence Criteria}, we can conclude that the sequence $(x_n)$ is divergent. \\
|
|
|
|
\item[16)] Give an example to show that Theorem 3.4.9 fails if the hypothesis that $X$ is a bounded sequences is dropped.
|
|
\\\\Recall \textit{Theorem 3.4.9}:
|
|
\begin{theorem*}
|
|
If $(x_n)$ is a bounded sequence of real numbers, then the following statements for a real number $x^*$ are equivalent:
|
|
\begin{enumerate}
|
|
\item $x^*= \lim \sup (x_n)$.
|
|
|
|
\item If $\varepsilon > 0$, there are at most a finite number of $n \in \N$ such that $x^*+\varepsilon<x_n$, but an infinite number of $n \in \N$ such that $x^*-\varepsilon<x_n$.
|
|
|
|
\item If $u_m=\sup \{x_n : n \geq m\}$, then $x^*=\inf \{u_m:m \in \N\}=\lim (u_m)$.
|
|
|
|
\item If $S$ is the set of subsequential limits of $(x_n)$, then $x^*= \sup (S)$.\\
|
|
\end{enumerate}
|
|
\end{theorem*}
|
|
Consider the sequence $((-1)^nn)$. We note that any subsequence of this sequence is unbounded and thus this sequence has no convergent subsequence. Due to this, all of the conditions of \textit{Theorem 3.4.9} are satisfied vacuously, save the condition concerning boundedness. However, this sequence doesn't converge, but both oscillates and diverges towards $\infty$ and $-\infty$. Thus if the boundedness criterion of the theorem is dropped, this theorem fails.
|
|
|
|
\item[18)] Show that if $(x_n)$ is a bounded sequence, then $(x_n)$ converges if and only if $\lim \sup (x_n) = \lim \inf (x_n)$.
|
|
\begin{proof}
|
|
Let $(x_n)$ be a bounded sequence. We want to show that $(x_n)$ converges if and only if $\lim \sup (x_n) = \lim \inf (x_n)$.
|
|
\\\\\textbf{TODO}
|
|
\end{proof}
|
|
|
|
\item[19)] Show that if $(x_n)$ and $(y_n)$ are bounded sequences, then
|
|
\[\lim \sup (x_n + y_n) \leq \lim \sup (x_n) + \lim \sup (y_n).\]
|
|
Give an example in which the two sides are not equal.
|
|
\\\\\textbf{TODO}
|
|
\end{enumerate}
|
|
\item
|
|
\begin{enumerate}
|
|
\item Show that $x_n=e^{\sin (5n)}$ has a convergent subsequence.
|
|
|
|
\item Give an example of a bounded sequence with three subsequences converging to three different numbers.
|
|
|
|
\item Give an example of a sequence $x_n$ with $\lim \sup x_n = 5$ and $\lim \sup x_n = -3$.
|
|
|
|
\item Let $\lim \sup x_n = 2$. True or False: if $n$ is sufficiently large, then $x_n > 1.99$.
|
|
|
|
\item Compute the infimum, supremum, limit infimum, and limit supremum for $a_n = 3 - (-1)^n - (-1)^n/n$.
|
|
\end{enumerate}
|
|
|
|
\item Prove or justify, if true. Provide a counterexample, if false.
|
|
\begin{enumerate}
|
|
\item If $a_n$ and $b_n$ are strictly increasing, then $a_n + b_n$ is strictly increasing.
|
|
|
|
\item If $a_n$ and $b_n$ are strictly increasing, then $a_n \cdot b_n$ is strictly increasing.
|
|
|
|
\item If $a_n$ and $b_n$ are monotonic, then $a_n + b_n$ is monotonic.
|
|
|
|
\item If $a_n$ and $b_n$ are monotonic, then $a_n \cdot b_n$ is monotonic.
|
|
|
|
\item If a monotone sequence is bounded, then it is convergent.
|
|
|
|
\item If a bounded sequence is monotone, then it is convergent.
|
|
|
|
\item If a convergent sequence is monotone, then it is bounded.
|
|
|
|
\item If a convergent sequence is bounded, then it is monotone.
|
|
\end{enumerate}
|
|
\end{enumerate}
|
|
\end{document} |