I can think of a few, but "linear/nonlinear" doesn't seem like the right way to look at it. From what I've read, productivity has more to do with statistical distributions:
http://xw2k.nist.gov/dads/HTML/lotkaslaw.html
Very curiously, these distributions are self-similar in that even when you restrict the sample to top-tier academics for instance, they still hold.
Very curiously, these distributions are self-similar in that even when you restrict the sample to top-tier academics for instance, they still hold.