Question

I have a context free grammar related question which is kind of related to math (inequalities). I'm stuck on one problem of my homework and I'm getting the impression that my professor made a mistake. The question regards a context free grammar for the following language:

{a^ib^j| 0 <= i < j < 2i}

Since i and j both represent string concatenation they must all be contained within the natural numbers.

Therefore that inequality makes no sense for i = 0 and i = 1. if i = 0, 2i also = 0 so j has to be 0 < j < 0 which makes no sense.
same with i = 1, 1 < j < 2.