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Add theorem that no anomalous pairs means commutative to blueprint
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ericluap committed Jul 30, 2024
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51 changes: 50 additions & 1 deletion blueprint/src/content.tex
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Expand Up @@ -185,4 +185,53 @@ \section{Content}

\begin{theorem}
An ordered semigroup without anomalous pairs is an Archimedean commutative semigroup.
\end{theorem}
\end{theorem}
\begin{proof}
Let $a$ and $b$ be elements of an ordered semigroup $S$.
If either $a$ or $b$ is zero, then they commute.

We begin with the case that $a$ and $b$ are positive.
If $a + b < b + a$, then for all $n\in \mathbb{N}^+$, we have that
\begin{align}
(n+1)(a+b) &= a + n(b+a) + b \\
&> n(b+a) + b \\
&> n(b+a) \\
&> n(a+b)
\end{align}
And so $a+b$ and $b+a$ form an anomalous pair.

The same for the case that $b + a < a + b$.
Therefore, we must have that $a+b = b+a$.

The case where $a$ and $b$ are negative follows similarly.

We now look at the case where $a$ is negative and $b$ is positive.
If the element $0$ exists and $a+b = 0$, then $a + b + a = a$ and so $b+a = 0$.
Therefore, $a$ and $b$ commute.

If $a + b$ is positive, then
\begin{align}
(a+b) + (a+b) &> a + b \\
(b + a) + b &> b \\
b + a &> 0
\end{align}

We already showed that positive elements commute and so
\[ (b+a) + b = b + (b + a)\]

Now we look at the case where $a+b < b+a$.
Then we have that
\begin{align}
2(a + b) &= a + ((b+a) + b) \\
&= a + (b + (b + a)) \\
&= (a + b) + (b + a) \\
&> (a + b) + (a + b) \\
&= 2(a + b)
\end{align}
Which is a contradiction.
We get a similar contradiction in the case that $b+a < a+b$.
Therefore, $a+b = b+a$.

The case where $a+b$ is negative follows similarly.
The case where $b$ is negative and $a$ is positive is symmetric.
\end{proof}
2 changes: 1 addition & 1 deletion blueprint/web/dep_graph_document.html
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Expand Up @@ -547,7 +547,7 @@ <h1 id="doc_title">Dependencies</h1>
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.on("end", interactive);

latexLabelEscaper = function(label) {
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37 changes: 37 additions & 0 deletions blueprint/web/index.html
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Expand Up @@ -491,6 +491,9 @@ <h1 id="a0000000003">0.2 Content</h1>

<a class="icon proof" href="index.html#a0000000027">#</a>

<a class="icon proof" href="index.html#a0000000028"><svg class="icon icon-cogs "><use xlink:href="symbol-defs.svg#icon-cogs"></use></svg>
</a>



</div>
Expand All @@ -500,6 +503,40 @@ <h1 id="a0000000003">0.2 Content</h1>

</div>
</div>
<div class="proof_wrapper" id="a0000000028">
<div class="proof_heading">
<span class="proof_caption">
Proof
</span>
<span class="expand-proof"></span>
</div>
<div class="proof_content">
<p>Let \(a\) and \(b\) be elements of an ordered semigroup \(S\). If either \(a\) or \(b\) is zero, then they commute. </p>
<p>We begin with the case that \(a\) and \(b\) are positive. If \(a + b {\lt} b + a\), then for all \(n\in \mathbb {N}^+\), we have that </p>
<div class="displaymath" id="a0000000029">
\begin{align} (n+1)(a+b) & = a + n(b+a) + b \\ & {\gt} n(b+a) + b \\ & {\gt} n(b+a) \\ & {\gt} n(a+b) \end{align}
</div>
<p> And so \(a+b\) and \(b+a\) form an anomalous pair. </p>
<p>The same for the case that \(b + a {\lt} a + b\). Therefore, we must have that \(a+b = b+a\). </p>
<p>The case where \(a\) and \(b\) are negative follows similarly. </p>
<p>We now look at the case where \(a\) is negative and \(b\) is positive. If the element \(0\) exists and \(a+b = 0\), then \(a + b + a = a\) and so \(b+a = 0\). Therefore, \(a\) and \(b\) commute. </p>
<p>If \(a + b\) is positive, then </p>
<div class="displaymath" id="a0000000030">
\begin{align} (a+b) + (a+b) & {\gt} a + b \\ (b + a) + b & {\gt} b \\ b + a & {\gt} 0 \end{align}
</div>
<p>We already showed that positive elements commute and so </p>
<div class="displaymath" id="a0000000031">
\[ (b+a) + b = b + (b + a) \]
</div>
<p>Now we look at the case where \(a+b {\lt} b+a\). Then we have that </p>
<div class="displaymath" id="a0000000032">
\begin{align} 2(a + b) & = a + ((b+a) + b) \\ & = a + (b + (b + a)) \\ & = (a + b) + (b + a) \\ & {\gt} (a + b) + (a + b) \\ & = 2(a + b) \end{align}
</div>
<p> Which is a contradiction. We get a similar contradiction in the case that \(b+a {\lt} a+b\). Therefore, \(a+b = b+a\). </p>
<p>The case where \(a+b\) is negative follows similarly. The case where \(b\) is negative and \(a\) is positive is symmetric. </p>

</div>
</div>

</div> <!--main-text -->
</div> <!-- content-wrapper -->
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