Solution
= Solution
{c}
Take \b[$Y=UX_1$], where $U$ is uniform on $(0,1)$ and independent of $X_1$. Conditioning on $U$ gives
$$
\mathbb Ee^{tUX_1}=\int_0^1\mathbb E(e^{tuX_1})du=\int_0^1M_1(ut)du.
$$
For $t$ near zero this is finite because the <moment-generating function> of $X_1$ is finite on a neighborhood of zero; <Tonelli's theorem> justifies the conditioning integral for the nonnegative exponential.