The zero element 0 defined by (A2) and the negative −a defined by (A3) are not claimed to be unique by the axioms. We will see later on that there is only one zero element in a ring, and that each element has only one negative.
The zero element 0 defined by (A2) and the negative −a defined by (A3) are not claimed to be unique by the axioms. We will see later on that there is only one zero element in a ring, and that each element has only one negative.