Robert J. Barro, Xavier Sala-i-Martin, Economic growth 2nd ed. (2004), Ch. 7 : Technological Change: Schumpeterian Models of Quality Ladders
Xavier Sala i Martín: Frases en inglés
http://www.columbia.edu/~xs23/keynes/keynes2.htm And this may help explain why.
Xavier Sala-i-Martin, Xavier Sala-i-Martin's Home Page http://www.columbia.edu/~xs23/keynes/keynes1.htm.