I feel like individual productivity is a pretty much solved problem and comes down to picking your tools. Whether it's one out of a million 3rd party Todo/Note taking apps, or just whatever is available in your OS, is a matter of taste. If you want to be productive you will find your way and most importantly, just don't get too obsessed about not being at maximum productivity. Group productivity and collaboration, ho…
I feel like individual productivity is a pretty much solved problem and comes down to sitting on your ass and doing the actual work (yes, I sometimes have trouble doing that, but in the end 'just sit down and start doing it' is the best advice to listen to).
If by "solved" you mean we have a theory, then perhaps.
But truly solved would include how to get yourself to do it.