1. Global Research and Development Expenditures: Fact Sheet, Report R44283, update from Sept 19, 2019. https://fas.org/sgp/crs/misc/R44283.pdf
2. A century of physics;R Sinatra;Nature Physics,2015
3. Collison P, Nielsen M. Science is getting less bang for its buck. The Atlantic. 2018; Nov 16.
4. Lord Kelvin. Maybe in an address before the British Association for the Advancement of Science. 1900.