Define the following predicate P(x,y,z) for naturals x and y and positive naturals z.
If x < z, P(x,y,z) is true iffiff x = y.
If y > = z, then P(x,y,z) is false.
If y < z and x >= z, then P(x,y,z) is true iff P(x - z,y,z) is true.
Prove that if y < z, then P(x,y,z) is true iff (∃r:x=rz+y) where r is n∈N