Thread: Recursive Deinition of order pairs problem

Give a recursive definition of each of these sets of ordered pairs of
positive integers. [Hint: Plot the points in the set in the plane and look for lines
containing points in the set.]
a) S={(a,b) | a∈Z+ ,b∈Z+ , and a+b is odd}
b) S={(a,b) | a∈Z+ ,b∈Z+ , and a∣b}

???