Let be a continuous function. Is there necessarily an interval , with such that , such that the restriction of to is a rational function?
My guess is that the answer is negative. I tried to prove it as follows: I took a countable and dense set of irrational numbers and defined, for each , . This will work, in the sense that is continuous and that the restriction of to any interval is never a rational function. The problem is that, of course, in general, .