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