This is trivial.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)=...)
To each person with a cell phone in the US attach a number n in N, so taht we can speak of person 1, person 2, etc.
Define f: P --> N by f(x) = the owner of x
b) write a function f:P N that is not injective. Again, give a rule
Now come up with a fucntion that maps each phone to the company that sold it...
c) Explain why there is no function from P to N that is surjective