My question is this:

How do you find the relative complement between two functions?

f(x)=x+2 {3,4,5,6,7...}

f1(x)=3x {3,6,9,12...}

{x+2\3x}

Should give you the sequence: {4,5,7,8...}

Is it possible to find a new function that will predict this sequence?