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Don't use a PID+UFD for 5G
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vEnhance committed Jul 23, 2024
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Expand Up @@ -847,10 +847,10 @@ \section{\problemhead}
\end{problem}

\begin{dproblem}
How many prime ideals of $\ZZ[\sqrt 2]$ are \emph{not} maximal ideals?
How many prime ideals of $\ZZ[\sqrt{2017}]$ are \emph{not} maximal ideals?
\label{prob:dedekind_sample}
\begin{hint}
Show that the quotient $\ZZ[\sqrt2]/I$ has finitely many elements
Show that the quotient $\ZZ[\sqrt{2017}]/I$ has finitely many elements
for any nonzero prime ideal $I$.
Therefore, the quotient is an integral domain, it is also a field,
and thus $I$ was a maximal ideal.
Expand All @@ -860,13 +860,13 @@ \section{\problemhead}
We contend every other prime ideal is maximal.

Indeed, let $I$ be any ideal (not necessarily prime),
and let $a + b \sqrt 2$ be a nonzero element of it.
Then $I$ also contains $(a^2-2b^2)$.
and let $a + b \sqrt{2017}$ be a nonzero element of it.
Then $I$ also contains $(a^2-2017b^2)$.
That means when taking modulo $I$ we may take modulo the integer
$n \coloneqq |a^2-2b^2| \neq 0$.
$n \coloneqq |a^2-2017b^2| \neq 0$.

So every element in $R$ is equivalent modulo $I$
to an element of the form $x + y \sqrt 2$,
to an element of the form $x + y \sqrt{2017}$,
where $x,y \in \{0, 1, \dots, n-1\}$.
In other words, the quotient $R/I$ has at most finitely many elements.

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