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brief note on Whitney extension theorem
Is there a form of the Whitney extension theorem in the generality of Fréchet manifolds?
I did look into this, I’d have to go back and check. It depends on what sort of closed sets you allow the initial functions on. If you have a mapping space it might be easier.
Thanks, David. Yes, I’d just need it for mapping spaces (or spaces of sections). Also, I don’t need any constraints on the derivatives, just the statement that a smooth function has an extension. Thinking about it though, I realize that I don’t need it for compact subspaces, but for closed subspaces. Hm, maybe I am not asking an optimal question here.
What I am really wondering is what to make of definition 19 in Collini 16. Here the ambient Fréchet space is that of smooth functions $C^\infty(X)$ on some smooth manifold, inside we consider a subspace $S \subset C^\infty(X)$ of those that satisfy some differential equation (a Klein-Gordon equation with interaction inhomogenity).
Collini wants to talk about smooth functions on the Fréchet submanifold $S$. In definition 19 he defines these to be those functions on $S$ for which there exists an extension to a smooth function on a neighbourhood of $S$ in $C^\infty(X)$. (He also has constraints on wave front sets of their functional derivatives, but I think this is an issue to be dealt with separately, so let’s ignore this).
All I am wondering is whether this defintion 19 (without the wave front condition) secretly reduces simply to smooth functions on $S$.
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