Let $\left(X_n\right)$ be a sequence of normed spaces and let
$$
X_0=\left\{x \in \prod_{n \in \mathbb{N}} X_n:\left\|x_n\right\| 0=0 \text { 人 }\right\}
$$
be their $c_0$-sum (with the norm $\|x\|=\sup \left\{\left\|x_n\right\|: n \in \mathbb{N}\right\}$ induced from the $l_{\infty}$-sum). Prove that $X_0$ is separable if and only if so is each of the spaces $X_n$.
5.19. Prove that the space $C^{(p)}[0,1]$ presents the sum of a finite-dimensional subspace and a space isomorphic to $C[0,1]$.