Multiplicative Inverse in Ring of Integers Modulo m/Proof 1

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Theorem

Let $\struct {\Z_m, +_m, \times_m}$ be the ring of integers modulo $m$.


Then $\eqclass k m \in \Z_m$ has an inverse in $\struct {\Z_m, \times_m}$ if and only if $k \perp m$.


Proof

First, suppose $k \perp m$.

That is:

$\gcd \set {k, m} = 1$

Then, by Bézout's Identity:

$\exists u, v \in \Z: u k + v m = 1$


Thus:

$\eqclass {u k + v m} m = \eqclass {u k} m = \eqclass u m \eqclass k m = \eqclass 1 m$

Thus $\eqclass u m$ is an inverse of $\eqclass k m$.


Suppose that:

$\exists u \in \Z: \eqclass u m \eqclass k m = \eqclass {u k} m = 1$.

Then:

$u k \equiv 1 \pmod m$

and:

$\exists v \in \Z: u k + v m = 1$


Thus from Bézout's Identity:

$k \perp m$

$\blacksquare$


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