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\begin{document}
\section{\protect\vspace{1pt}Ma 116 Lecture 2/7/00}
\section{Sequences}
\vspace{0in}Please note that in addition to the material below this lecture
incorporated material from the Visual Calculus web site. The material on
sequences is at \hyperref{Visual Sequences}{}{}{%
http://archives.math.utk.edu/visual.calculus/6/}. (To use this link hold
down the Ctrl key and click.)
\subsection{ Definition}
\vspace{0in}
\begin{definition}
\vspace{1pt}A \emph{sequence} is simply an \emph{ordered list of numbers},
and the numbers in the list are called the \emph{terms} of the sequence. \
There may be a finite or an infinite number of terms. \ If we don't say
whether it is a finite or infinite sequence, we normally mean an infinite
sequence.
\end{definition}
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\begin{example}
Each of the following lists is a sequence:
\end{example}
\begin{center}
\begin{tabular}{llll}
1. & $1,2,3,4,5,\cdots $ & The counting numbers. & An infinite sequence. \\
2. & $10,9,8,7,6,5,4,3,2,1,0$ & Blast Off! & A finite sequence. \\
3. & $3,1,4,1,5,9,2,6,5,3,\cdots $ & The digits of $\pi $. & An infinite
sequence. \\
4. & $1,\dfrac{1}{2},\dfrac{1}{4},\dfrac{1}{8},\dfrac{1}{16},\dfrac{1}{32}%
,\cdots $ & Keep halving. & An infinite sequence.%
\end{tabular}
\end{center}
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\vspace{1pt}
\vspace{1pt}In general, we denote the terms of a sequence as a letter with
an integer subscript:
\qquad $a_{1},a_{2},a_{3},a_{4},\cdots $
and we frequently give a formula for the $n^{\text{th}}$ term. \ For
example, the terms of the sequence, \ $b_{n}=\dfrac{1}{2n+1}$ \ for $%
n=1,2,3,\cdots $\ are
\qquad $b_{1}=\dfrac{1}{3},\;b_{2}=\dfrac{1}{5},\;b_{3}=\dfrac{1}{7},\;b_{4}=%
\dfrac{1}{9},\;\cdots $
Normally the terms are counted starting from $n=1$, as above, but they can
start from any number:
\vspace{1pt}Remark: Another definition of a sequence is
\begin{definition}
A \emph{sequence} is a set of numbers which has a one-to-one correspondence
with the natural numbers.
\end{definition}
\paragraph{\protect\vspace{1pt}}
\begin{exercise}
Write out the first $5$ terms of the sequence $a_{n}=\dfrac{n}{n-1}$ for $%
n=2,3,4,\cdots \;$\dotfill
\CustomNote[\hyperref{\TCIIcon{BITMAPSETAnswer}{0.1609in}{0.1487in}{0in}}{}{%
}{}]{Margin Hint}{$a_{n}=\dfrac{n}{n-1}$ for $n=2,3,4,\cdots $%
\par
$a_{2}=\dfrac{2}{1},\;a_{3}=\dfrac{3}{2},\;a_{4}=\dfrac{4}{3},\;a_{5}=\dfrac{%
5}{4},\;a_{6}=\dfrac{6}{5}\;\cdots $%
\par
\vspace{1pt}
\par
Why did we start from $n=2$?}
\end{exercise}
\vspace{1pt}
To indicate an entire sequence, we enclose the general term in braces: \ $%
\left\{ a_{n}\right\} $ \ If necessary, we can also indicate the range of
the index: \ $\left\{ a_{n}\right\} \rule[-0.05in]{0in}{0.18in}%
_{n=1}^{\infty }$ \ For example, the sequence in the preceding example would
be denoted $\left\{ \dfrac{n}{n-1}\right\} _{n=2}^{\infty }$.
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\begin{center}
\vspace{1pt}
\end{center}
\paragraph{\textbf{Examples and Notations:}}
$\{a_{n}\}_{n=m}^{\infty }$ denotes the sequence starting at $a_{m}$. Thus
\begin{equation*}
{\Large \{}\frac{1}{2^{n}}{\Large \}}_{n=2}^{\infty }{\Large =}\left\{ \frac{%
1}{4}{\Large ,}\frac{1}{8}{\Large ,}\frac{1}{16}{\Large ,...}\right\}
\end{equation*}
\begin{equation*}
{\Large \{}\frac{(-1)^{n}}{n!}{\Large \}}_{n=3}^{\infty }{\Large =}\left\{
\frac{-1}{3!}{\Large ,}\frac{1}{4!}{\Large ,}\frac{-1}{5!}{\Large ,...}%
\right\}
\end{equation*}
Not all sequences can be defined in the above manner. \ For example, let $%
a_{n}$ be the digit in the $n^{th}$ decimal place of the number $\pi $. \
Then $\{a_{n}\}_{n=1}^{\infty }$ is a sequence whose first few terms are $%
\{1,4,1,5,9,...\}$.
\textbf{Definition:} \ A \emph{sequence} is a set of numbers which has a
one-to-one correspondence with the natural numbers.
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\subsection{ Limits}
\vspace{0in}
The limit of the sequence $a_{n}$, denoted $\lim\limits_{n\rightarrow \infty
}a_{n}$, \ tells what number the terms are approaching as we go further and
further out in the sequence (as $n$ gets arbitrarily large.)
\begin{tabular}{l}
For example, the plot of the \\
sequence $a_{n}=\dfrac{2n+1}{n}$\ is\quad $\Longrightarrow $ \\
As $n$ gets larger, the terms $a_{n}$ \\
are approaching $2$. \ So we \\
write \\
$\quad \lim\limits_{n\rightarrow \infty }a_{n}=\lim\limits_{n\rightarrow
\infty }\dfrac{2n+1}{n}=2$.%
\end{tabular}
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When we are given a sequence, the typical questions we ask are: ``Does the
sequence have a limit?'' \ ``If so, what is the limit of the sequence?'' \ \
For now and for most purposes, the following will suffice:
\vspace{1pt}
\begin{definition}
The sequence \emph{has a limit} if the terms $a_{n}$ get closer and closer
to a \textsl{finite} number $L$ as $n$ gets arbitrarily large. \ In that
case, we say $L$ is the limit and we write $\lim\limits_{n\rightarrow \infty
}a_{n}=L$.
If a sequence has a limit $L$, we say it is \emph{convergent} and that it
\emph{converges to} $L$.
If a sequence does not have a limit, we say it is \emph{divergent} or that
it \emph{diverges}.
\end{definition}
\vspace{1pt}
If the sequence does not have a limit, it may still have an infinite limit.
\
\vspace{1pt}
\begin{definition}
If the terms $a_{n}$ get\emph{\ arbitrarily large and positive} as $n$ gets
arbitrarily large, then we say the limit is \emph{positive infinity} and
write $\lim\limits_{n\rightarrow \infty }a_{n}=\infty $. \ We also say the
sequence \emph{diverges to} $\infty $.\qquad \fbox{\underline{%
\CustomNote[Example]{Margin Hint}{%
The sequence $a_{n}=1+\sqrt{n}$ diverges to $\infty $ because $1+\sqrt{n}$
gets arbitrarily large and positive as $n$ gets large. \ Its plot is:
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If the terms $a_{n}$ get \emph{arbitrarily large and negative} as $n$ gets
arbitrarily large, then we say the limit is \emph{negative infinity} and
write $\lim\limits_{n\rightarrow \infty }a_{n}=-\infty $. \ We also say the
sequence \emph{diverges to} $-\infty $.\qquad \fbox{\underline{%
\CustomNote[Example]{Margin Hint}{%
The sequence $a_{n}=1-\sqrt{n}$ diverges to $-\infty $ because $1-\sqrt{n}$
gets arbitrarily large and negative as $n$ gets large. \ Its plot is:
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\end{definition}
\vspace{1pt}
To say that the limit is positive or negative infinity does \textsl{not} say
that the limit exists! \ It merely says the \textsl{way} in which it does
not exist, i.e. the \textsl{way} in which it \textsl{diverges}.
\vspace{1pt}
\textsl{Note:} \ It is certainly possible to diverge without diverging to $%
\infty $ or $-\infty $.\qquad \fbox{\underline{%
\CustomNote[Example]{Margin Hint}{%
The sequence $a_{n}=\left( -1\right) ^{n}+\dfrac{1}{n}$ has the plot
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\par
Since the terms oscillate between numbers which get closer to $1$ and
numbers which get closer to $-1$, the sequence does not converge; it
diverges. \ However, it does not diverge to $\infty $ or $-\infty $. \ We
say it is \emph{oscillatory divergent}.}}}
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\vspace{1pt}
In general, the process of finding the limit of a sequence is essentially
the same as finding the limit of a continuous function or finding its
horizontal asymptote. \ Examples appear on subsequent pages.
\subsection{\protect\vspace{0in}Precise Definition of Limits}
\vspace{0in}
We previously said that $\lim\limits_{n\rightarrow \infty }a_{n}=L$ means
that the terms $a_{n}$ get arbitrarily close to a\ finite number $L$ as $n$
gets arbitrarily large. \ We now make this more precise by explaining the
use of the word ``arbitrarily.'' \ The idea we want is: if we specify how
close to $L$ the terms $a_{n}$ should be (called an \emph{output tolerance})
then for the part of the sequence beyond some point (called a \emph{tail of
the sequence}), the terms are all within that tolerance. \ The output
tolerance is denoted by epsilon $\left( \varepsilon \right) $ and the tail
of the sequence begins with some term $a_{N}$ whose index is $n=N$.
\vspace{1pt}
\begin{definition}
We say the \emph{limit} of the sequence $\left\{ a_{n}\right\} $ \emph{exists%
} and is \emph{equal to} a \textsl{finite} number $L$ and we write\qquad $%
\lim\limits_{n\rightarrow \infty }a_{n}=L$\qquad to mean:
\qquad \qquad For all $\varepsilon >0$, there is an integer $N$ such that
\emph{if} $n\geq N$ \emph{then} $\left| a_{n}-L\right| <\varepsilon $%
.\medskip
If a sequence has a limit $L$, we say it is \emph{convergent} and that it
\emph{converges to} $L$.
If a sequence does not have a limit, we say it is \emph{divergent} or that
it \emph{diverges}.
\end{definition}
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\vspace{1pt}
We also need a precise definition of an infinite limit. \ The idea here is
that the terms $a_{n}$ get larger than any specified number $M$\ and after
some point the terms stay larger than $M$.
\vspace{1pt}
\begin{definition}
We say the limit of the sequence $\left\{ a_{n}\right\} $ is \emph{positive
infinity} and we write\qquad $\lim\limits_{n\rightarrow \infty }a_{n}=\infty
$\qquad to mean:
\qquad \qquad For all $M>0$, there is an integer $N$ such that \emph{if} $%
n\geq N$ \emph{then} $a_{n}>M$.\medskip
We say the limit of the sequence $\left\{ a_{n}\right\} $ is \emph{negative
infinity} and we write\qquad $\lim\limits_{n\rightarrow \infty
}a_{n}=-\infty $\qquad to mean:
\qquad \qquad For all $M<0$, there is an integer $N$ such that \emph{if} $%
n\geq N$ \emph{then} $a_{n}