No, it's not the start of a joke!
I'm just wondering why are exact ODEs called exact?
All I can think of so far is that they don't end up with an integration constant on one of the variables.
But as the solution is e.g. f(x,y) = c it's still going to represent a family of solutions.
Any ideas? It's probably really obvious It's been annoying me for a few days now.
Thanks and merry Christmas and new year to you all!