# Dichotomy Paper

I have a couple questions about the Dichotomy paper of Miller:

1) In Theorem 9 (the Perfect Set Theorem) there is a step that says to “recursively construct a continuous $\psi:2^{\omega}\to 2^{\omega}$ such that $(\varphi\circ\psi)[N_s]\cap(\varphi\circ\psi)[N_t]=\emptyset$ whenever $t,s\in 2^n$ for some $n$. He says that you can do it because $varphi|U$ is non-constant for nonempty open sets.

I’ve tried doing something like the schemes where we assign a clopen set to each node of $2^{<\omega}$ and assign to an $x\in 2^{\omega}$ the intersection of the clopen sets for its inital segments. But I haven't really been able to make it work.

2) In the proof of the silver Dichotomy, we get a perfect clique and the homomorphism ends these to perfectly many equivalence classes. But I am unsure of why there must be a perfect clique.

3) Finally, for Lusin-Novikov, I'm not entirely sure why $G_0$ has to map into one vertical section. I know that each connected component has to go into one section, since we are applying a homomorphism. But the graph isn't connected (right?).

Help with these would be appreciated. Overall the paper looks interesting. I haven't been able to get into the rest of it as much I would have liked. But I think working through these examples will help when it comes to understanding the rest of the paper.

# Image of Fourier Transform

In analysis today, we were talking about the fourier transform and how as a map from $L_2$ to $L_2$ it is onto, but as a map from $L_1$ to $C_0$ it isn’t onto and the range isn’t even closed. Then Dr. Jury said that the range is really hard to describe.

So after class I asked him if “hard to describe” meant “analytic but non-Borel” since the range is analytic by definition. He didn’t know off-hand. I did a quick look online and didn’t find other than people saying that the range is hard to characterize.

So, is there any result saying it is $\Sigma^1_1$ complete or at least non-Borel? I would be interested to know, and to see the proof.