There are many different, but equivalent, axiomatizations of Peano arithmetic. While some axiomatizations, such as the one just described, use a signature that only has symbols for 0 and the successor, addition, and multiplications operations, other axiomatizations use the language of ordered semirings, including an additional order relation symbol. One such axiomatization begins with the following axioms that describe a discrete ordered semiring.[8]
- . , i.e., addition is associative.
- . , i.e., addition is commutative.
- . , i.e., multiplication is associative.
- . , i.e., multiplication is commutative.
- . , i.e., the distributive law.
- . , i.e., zero is the identity element for addition.
- . , i.e., one is the identity element for multiplication.
- . , i.e., the '<' operator is transitive.
- . , i.e., the '<' operator is irreflexive.
- . .
- . .
- . .
- . . .
- . .
- . .
The theory defined by these axioms is known as PA–; PA is obtained by adding the first-order induction schema.