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from Peano's axioms (see natural number) and prove some simple properties. The set of natural numbers will be denoted by N; zero is taken to be a natural number.
The operation of addition, commonly written as infix operator +, is a function of N x N -> N
a + b = c
a is called the augend, b is called the addend, while c is called the sum.
By convention, a+ is referred as the successor of a as defined in the Peano postulates.
The first is referred as AP1, the second as AP2.
We prove by mathematical induction on b.
Base: (a.0) = [by AP1] a = [by AP1] (a+0) for all a
Induction hypothese: (a.b)=(a+b) for all a
We prove by mathematical induction on c.
Base: (a+b)+0 = [by AP1] a+b = [by AP1] a+(b+0) for all a,b
Induction hypothesis: (a+b)+c = a+(b+c) for all a,b
We prove by mathematical induction on b.
Base: a+0=a=0+a and a+1=a+=1+a for all a
Proof of base is by mathematical induction on a.
Induction hypothesis: a+b=b+a for all a