This 3 part problem deals with functions from the set P of cell phones in use in the US to the set of natural Numbers N.
a) Write one interesting function f:P N that is injective. Define the function by giving a rule for it, i.e., (f(x)=...)
b) write a function f:P N that is not injective. Again, give a rule
c) Explain why there is no function from P to N that is surjective