I need help with this problem.
Give an example of a sequence of integrable functions fn:[a,b]-->R convergent (pointwise) to a non-integrable function f:[a,b]-->R.
I don't think that from the assumption that f be non-negative, non-integrable it follows that is integrable.
For the Riemann integral it is easy to give an example of a non-negative and bounded function that is non-integrable (e.g. the characteristic function of ), and in the case of the Lebesgue integral we must consider the possibility that f might not even be measurable.
So, I think, one would have to be a bit more specific about the cause of the non-integrability of f.