On the concept of analytic hardness
On the concept of analytic hardness
Let $H\subseteq Z\subseteq 2^{\omega }$. Using only classical descriptive set theory we prove that if Borel functions from $2^{\omega }$ to $Z$ give as preimages of $H$ all analytic subsets of $2^{\omega }$, then so do continuous injections. This strengthens a theorem Kechris proved by means of effective descriptive set …