What inclusion exists between the sets $E$ and $E^{\prime}$, where $E$ is the set of real numbers $x$ for which the application $t \mapsto f(t)e^{-\lambda(t)x}$ is integrable on $\mathbb{R}^+$, and $E^{\prime}$ is the set of real numbers $x$ for which the integral $\int_0^{+\infty} f(t)e^{-\lambda(t)x}\,dt$ converges?