i.e., set notation. Here we are defining a set R and we include all i in R such that there exists j such that f(j) = i. Here, j is from natural number set. f(5) = f(10) = f(15) = f(20) = ... = say x. Also, f(2) = f(3) = f(4) = f(6) = ... say y. So, to maximize the number of elements in R, we can take $x \neq y$ making 2 possible elements in R.