[Math] The Schwartz space is not normable

metric-spacesschwartz-distributions

The Schwartz space of rapidly decreasing function (as well as their derivatives) on $\mathbb R^n$ is a Fréchet space, whose (metric complete) topology is given by the usual countable family of semi-norms $(p_k)_{k\in \mathbb N}$
$$
p_k(\phi)=\max_{\vert \alpha\vert, \vert \beta\vert\le k}\Vert x^\alpha\partial_x^\beta
\phi\Vert_{L^\infty(\mathbb R^n)}.
$$
Is there a simple proof of the fact that this topological space is not normable?

Best Answer

More elementary than Ascoli:

If it was normable, it would mean that there exists a norm $n$ which is continuous, hence for some $k$, $n(\phi) \leqslant C\ p_k(\phi)$ and which defines the topology, i.e. such that for all $k$, $p_k(\phi) \leqslant C_k\ n(\phi)$.

This would imply that all the norms $p_k$ are equivalent for $k$ large enough, which is not the case: for example $\exp(-|x|^2) \sin(N x_1) /N^k$ tends to $0$ for the first $k$ norms but not for the following.

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