Hello,

We have some Polynomial P(x): between the set of all smooth maps on the torus.

Now my book says following:

Since the smooth functions on the torus are characterised by having rapidly decreasing Fourier coefficients, and we have the following equation: there is a s.t. for almost all we have

for all .

so it follows immediately that is an isomorphism for almost every \alpha.

The equation above are no problems to me. I know them. But i really don't see, why this is an immediate consequence from the assumptions above??

Do you have an idea, why the F-Transformation and the fact, that it is decreasing rapidly can help me to show that P is an isom.??

if someone wants to see the book, Partial differential equations ... - Google Bücher

it is Prop.10.1 on page 248

Regards!