here “W” is the wright omega function (or Lamber function), “a” is a constant varying from 0.5 to 0.65 and P is a constant.
When putting a =0.5, a closed from solution is found easily, but when trying to generate solution for each value of “a”, Wolfram Mathematica still running without converging.
I doubt there is any general form. A lesser but related problem would be to handle the equation r+r^a+b==0 and solve for r. Even for concrete values of b I do not know what an analytic solution might be.