I've noticed a couple of conference titles which reference something called 
"topology in dimension 3.5," such as [this one](http://math.rice.edu/NewsEvents/Conferences/TopologyConference2016/index.html) and [this one](https://sites.google.com/sjsu.edu/thompscharbyfest/home). This subject seems quite mysterious to me — it looks like it's part of geometric topology, but I'm guessing it's not about 3.5-dimensional manifolds, so what kinds of things are studied in 3.5-dimensional topology?



What is "topology in dimension 3.5"?
cw answer: As mentioned in the comments, this just refers to the relations between 3-dimensional and 4-dimensional topology.


