Let $\mathcal{S}:= \mathcal{S}(\mathbb{R}^n)$ be the Schwartz space of smooth functions with rapid decay. The question is pretty simply stated in the title. Pseudo-differential act continuously on the space $\mathcal{S}$, it is therefore natural to wonder whether they are the only ones:



> 
> Is there a continuous operator $T:\mathcal{S} \to \mathcal{S}$ which is not a pseudo-differential operator? i.e. not of the form:
> $$T(f)(x) = \int\_{\mathbb{R}^n}a(x,\xi)\hat f(\xi)e^{ix\xi}d\xi$$
> 
> 
> For some appropriate symbol $a(x,\xi)$ (of class $S^m$ for some $m$)
> 
> 
> 


EDIT: For a more interesting follow up version of this question (which is not trivially false) see [here](https://mathoverflow.net/q/285203/22810).



Is every continuous endomorphism of the Schwartz space a pseudo-differential operator?
No, the most obvious example is the reflection operator: $Rf(x) = f(-x)$ this is not pseudolocal (in fact the $\xi$-compotent of the wavefront set gets a sign flip). Also the Fourier transform.


More generally, every compactly supported Fourier integral operator with non-trivial Lagrangian (not the co-normal to the diagonal) is not a pseudodifferential operator, but preserves the Schwartz-space.


