Question

Give an example to show that a natural number is not closed under subtraction

Answer 1

Answer:

so the set of natural number is not closed under subtraction. You can prove that for any natural numbers a,b there is at most one c such that b+c=a. That means that we can define z=a−b to mean z+b=a. This means that if z=a−b and w=a−b then z=w - that is, subtraction is well-defined when it is defined.