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Commit 5841619b authored by Fabian Nuraddin Alexander Gabel's avatar Fabian Nuraddin Alexander Gabel :speech_balloon:
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......@@ -11,7 +11,8 @@
\end{Definition}
We make essential use of Dedekind's theorem to prove the following result:
\begin{Theorem}[Convergence of bounded and monotonic sequences]\label{thm:monbndseq}
\begin{Theorem}[Convergence of bounded and monotonic sequences]
\label{thm:monbndseq}
Let $(a_n)_{n\in\mathbb{N}}$ be a~real sequence that has one of the following properties:
\begin{itemize}
\item[--] $(a_n)_{n\in\mathbb{N}}$ is monotonically increasing and bounded from above;
......@@ -47,7 +48,7 @@ Thus, $(a_n)$ is bounded from below. For showing monotonicity, we consider
\[a_{n+1}-a_n=\frac{a_n+\frac2{a_n}}{2}-a_n=\frac{1}{2a_n}(2-a_n^2).\]
In particular, if $n\geq2$, we have that $a_n>0$ and $2-a_n^2\leq0$. Thus, $a_{n+1}-a_n\leq0$ for $n\geq2$. An~application of Theorem~\ref{thm:bndmonseq} (resp.\ the slight generalisation in Remark %\ref{rem:monseqgen}
from above)
now leads to the existence of some $a\in\mathbb{R}$ with $a=\lim_{n o\infty}a_n$.\\[2ex]
now leads to the existence of some $a\in\mathbb{R}$ with $a=\lim_{n \to\infty}a_n$.\\[2ex]
To compute the limit, we make use of the relation $\lim_{n \to\infty}a_n=\lim_{n\to\infty}a_{n+1}$ (follows directly from the Definition of limits) and the formulae for limits. This yields
\[a=\lim_{n \to\infty}a_n=\lim_{n\to\infty}a_{n+1}=\lim_{n\to\infty}\frac{a_n+\frac2{a_n}}{2}=\frac{a+\frac2{a}}{2}.\]
This relation leads to the equation $2-a^2=0$, i.e., we either have $a=\sqrt{2}$ or $a=-\sqrt{2}$. However, the latter solution cannot be a limit since all sequence elements are positive. Therefore, we have
......
......@@ -310,7 +310,7 @@
},
"114": {
"id": "114",
"label": "Convergence of Bounded Monotonic Sequences",
"label": "Bounded Monotonic\nSequences",
"meta": " ",
"content": "If a sequence of real numbers is bounded and monotonic, then it is convergent.",
"notes": "114-snippet.html",
......@@ -1889,6 +1889,31 @@
"target": "104",
"label": "The proof and a lot of applications of the sandwich theorem are only possible because the limit theorems allow for elementary calculations with limits."
},
"101-114": {
"source": "101",
"target": "114",
"label": "If a sequence is bounded and monotonic, then it converges."
},
"102-114": {
"source": "102",
"target": "114",
"label": "Boundedness alone does not give convergence yet, one also needs monotonicity."
},
"106-114": {
"source": "106",
"target": "114",
"label": "The convergence of bounded sequences follows from Dedekind's theorem which guarantees the existence of suprema of bounded sets."
},
"105-114": {
"source": "106",
"target": "114",
"label": "The limit of a monotonic increasing and bounded sequence is precisely the supremum of its function values. An analogous connection holds for the infimum."
},
"013-102": {
"source": "013",
"target": "102",
"label": "The image of a sequence, i.e. the set of all values that the sequence attains, is a bounded set."
},
"203-207": {
"source": "203",
"target": "207",
......
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