I was solving a Physics problem, and for it to be consistent there should exist a function f(t) in real numbers and a time T, such that:
$$\int_{0}^{T} f(t) dt=0 $$
$$\int_{0}^{T} \int_{0}^{t} f(t') dt' dt=0$$
$$\int_{0}^{T} f(t) (\int_{0}^{t} f(t') dt') dt>0$$
i.e. the integral is zero, the...