https://en.wikipedia.org/wiki/Arithmetical_hierarchy https://en.wikipedia.org/wiki/Analytical_hierarchy https://en.wikipedia.org/wiki/Borel_hierarchy https://en.wikipedia.org/wiki/Projective_hierarchy They have weird Sigma's and Pi's, and are apparently about how hard it is to describe a subset of the natural numbers or the real line. What are these hierarchies and how are they used? submitted by /u/thekeyofPhysCrowSta [link] [comments]