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If [math]x [/math] is a number (e.g. a modular number), then we say [math]y [/math] is a multiplicative inverse of [math]x [/math] if [math]xy = 1 [/math] (or [math]xy = \href{/cs2800/wiki/index.php/Modular_number}{⟦1⟧} [/math] in the case of modular numbers).
Definition: Unit
If [math]x [/math] is a number that has a multiplicative inverse, then [math]x [/math] is called a unit.