I'm trying to show why it isn't possible to define an order of magnitude on $\mathbb Z_n$ (modular arithmetic) that satisfies the ordering properties of $\mathbb Z$.
By letting addition to be $\oplus$ and multiplication $\otimes$, I know the following about $\mathbb Z_n$:
Closed under $\oplus$ and $\otimes$.
$\oplus$ and $\otimes$ are commutative.
$\oplus$ and $\otimes$ are associative.
$0$ is $\oplus$ identity and $1$ is $\otimes$ identity.
Cancellation: $(-a) \oplus a = 0$.
$$a \otimes (b \oplus c) = a \otimes b \oplus a \otimes c,$$
$$(b \oplus c) \otimes a = b \otimes a \oplus c \otimes a .$$
How would I prove that at least one of the ordering properties of $\mathbb Z$ does not hold for $\mathbb Z_n$?
I will be definitely using the ordering properties of $\mathbb Z$:
exactly one of $a < b$ or $a = b$ or $a > b$ holds;
if $a < b$ and $b < c$, then $a < c$;
if $a < b$ then $a + c < b + c$;
if $a < b$ and $c > 0$ then $a \cdot c < b \cdot c$.
In a way I can see that one of these properties would collapse in $\mathbb Z_n$, but I cannot prove it. By contradiction perhaps? (start with $a < b$ and find end up with $a > b$?)
Any help much appreciated.