Multiplicative Ordering on Integers
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Theorem
Let $x, y, z \in \Z$ such that $z > 0$.
Then:
- $x < y \iff z x < z y$
- $x \le y \iff z x \le z y$
Proof
Let $z > 0$.
Let $M_z: \Z \to \Z$ be the mapping defined as:
- $\forall x \in \Z: \map {M_z} x = z x$
It is sufficient to show that $M_z$ is an order embedding from $\struct {\Z, +, \le}$ to itself.
By Monomorphism from Total Ordering, it is sufficient to show that:
- $x < y \implies z x < z y$
Let $x < y$.
Then:
- $0 < y - x$
So $z \in \N$.
Hence by Natural Numbers are Non-Negative Integers:
- $y - x \in \N$
Thus by Ordering on Natural Numbers is Compatible with Multiplication:
- $z \paren {y - x} \in \N$
Therefore:
- $0 < z \paren {y - x} = z y - z x$
That is:
- $z x < z y$
$\blacksquare$
Sources
- 1951: Nathan Jacobson: Lectures in Abstract Algebra: Volume $\text { I }$: Basic Concepts ... (previous) ... (next): Introduction $\S 5$: The system of integers
- 1965: Seth Warner: Modern Algebra ... (previous) ... (next): Chapter $\text {IV}$: Rings and Fields: $20$. The Integers: Theorem $20.10: 1^\circ, 2^\circ$
- 1982: P.M. Cohn: Algebra Volume 1 (2nd ed.) ... (previous) ... (next): Chapter $2$: Integers and natural numbers: $\S 2.1$: The integers: $\mathbf Z. \, 8$
- 1994: H.E. Rose: A Course in Number Theory (2nd ed.) ... (previous) ... (next): $1$ Divisibility: $1.1$ The Euclidean algorithm and unique factorization