Gordon, Robert J. Is U.S. Economic Growth Over? Faltering Innovation Confronts the Six Headwinds. NBER Working Paper No. 18315. 2012. doi:10.3386/w18315.
Strumsky, D.; Lobo, J.; Tainter, J. A. Complexity and the productivity of innovation. Systems Research and Behavioral Science. 2010, 27 (5): 496. doi:10.1002/sres.1057.