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043736b26e Continued Fractions edits 2026-04-13 16:18:23 -07:00
2 changed files with 12 additions and 12 deletions

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@@ -78,7 +78,7 @@ An \textit{infinite continued fraction} is an expression of the form
a_0 + \cfrac{1}{a_1+\cfrac{1}{a_2 + \cfrac{1}{a_3 + \cfrac{1}{a_4 + ...}}}}
\]
where $a_0, a_1, a_2, ...$ are in $\mathbb{Z}^+_0$.
To prove that this expression actually makes sense and equals a finite number
Showing that this expression converges to a finite number
is beyond the scope of this worksheet, so we assume it for now.
This is denoted $[a_0, a_1, a_2, ...]$.
@@ -133,7 +133,7 @@ A few examples are below. We denote the repeating sequence with a line.
\problem{}
\begin{itemize}
\item Show that $\sqrt{2} = [1, \overline{2}]$.
\item Show that $\sqrt{5} = [1, \overline{4}]$.
\item Show that $\sqrt{5} = [2, \overline{4}]$.
\end{itemize}
\hint{use the same strategy as \ref{irrational} but without a calculator.}
@@ -159,7 +159,7 @@ Express the following continued fractions in the form $\frac{a+\sqrt{b}}{c}$ whe
\problem{Challenge II}
Let $\alpha = [~a_0,~ ...,~ a_r,~ \overline{a_{r+1},~ ...,~ a_{r+p}}~]$ be any periodic continued fraction. \par
Prove that $\alpha$ is of the form $\frac{a+\sqrt{b}}{c}$ for some integers $a,b,c$ where $b$ is not a perfect square.
Show that $\alpha$ is of the form $\frac{a+\sqrt{b}}{c}$ for some integers $a,b,c$ where $b$ is not a perfect square.
@@ -168,7 +168,7 @@ Prove that $\alpha$ is of the form $\frac{a+\sqrt{b}}{c}$ for some integers $a,b
\problem{Challenge III}
Prove that any number of the form $\frac{a+\sqrt{b}}{c}$ where $a,b,c$ are integers
Show that any number of the form $\frac{a+\sqrt{b}}{c}$ where $a,b,c$ are integers
and $b$ is not a perfect square can be written as a periodic continued fraction.

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@@ -65,7 +65,7 @@ Verify the recursive formula for $1\leq j\leq 3$ for the convergents $C_j$ of: \
\problem{Challenge IV}<rec>
Prove that $p_n = a_np_{n-1} + p_{n-2}$ and $q_n = a_nq_{n-1} + q_{n-2}$ by induction.
Show that $p_n = a_np_{n-1} + p_{n-2}$ and $q_n = a_nq_{n-1} + q_{n-2}$ by induction.
\begin{itemize}
\item As the base case, verify the recursive formulas for $n=1$ and $n=2$.
\item Assume the recursive formulas hold for $n\leq m$ and show the formulas hold for $m+1$.
@@ -97,7 +97,7 @@ we will show that $p_n q_{n-1} - p_{n-1}q_n = (-1)^{n-1}$.
\problem{Challenge VI}
\problem{Challenge V}
Similarly derive the formula $p_nq_{n-2}-p_{n-2}q_n = (-1)^{n-2}a_n$.
@@ -141,7 +141,7 @@ We will show that $|\alpha-C_n|<\frac{1}{q_n^2}$.
We are now ready to prove a fundamental result in the theory of rational approximation.
\problem{Dirichlet's approximation theorem}
Let $\alpha$ be any irrational number.
Prove that there are infinitely many rational numbers $\frac{p}{q}$ such that $|\alpha - \frac{p}{q}| < \frac{1}{q^2}$.
Show that there are infinitely many rational numbers $\frac{p}{q}$ such that $|\alpha - \frac{p}{q}| < \frac{1}{q^2}$.
@@ -154,8 +154,8 @@ Prove that there are infinitely many rational numbers $\frac{p}{q}$ such that $|
\problem{Challenge VII}
Prove that if $\alpha$ is \emph{rational}, then there are only \emph{finitely} many rational numbers $\frac{p}{q}$
\problem{Challenge VI}
Show that if $\alpha$ is \emph{rational}, then there are only \emph{finitely} many rational numbers $\frac{p}{q}$
satisfying $|\alpha - \frac{p}{q} | < \frac{1}{q^2}$.
@@ -195,8 +195,8 @@ Let $\frac{a}{b}$ and $\frac{c}{d}$ be consecutive elements of the Farey sequenc
\problem{Challenge VIII}<farey>
Prove that $bc-ad=1$ for $\frac{a}{b}$ and $\frac{c}{d}$ consecutive rational numbers in Farey sequence of order $n$.
\problem{Challenge VII}<farey>
Show that $bc-ad=1$ for $\frac{a}{b}$ and $\frac{c}{d}$ consecutive rational numbers in Farey sequence of order $n$.
\begin{itemize}[itemsep=2mm]
\item In the plane, draw the triangle with vertices (0,0), $(b,a)$, $(d,c)$.
@@ -237,7 +237,7 @@ $|\alpha - \frac ab| \geq |\alpha - \frac{p_n}{q_n}|$
\problem{Challenge X}
\problem{Challenge VIII}
Prove the following strengthening of Dirichlet's approximation theorem.
If $\alpha$ is irrational, then there are infinitely many rational numbers
$\frac{p}{q}$ satisfying $|\alpha - \frac pq| < \frac{1}{2q^2}$.