Nitpick, under standard definitions, 2 is a unit, over the field of rationals (or reals, or many other systems). EDIT: for a less trivial example, 2 is a unit

Nitpick, under standard definitions, 2 is a unit, over the field of rationals (or reals, or many other systems). EDIT: for a less trivial example, 2 is a unit in the integers mod 9, but 3 isn't.