This paper investigates the effect of spillovers in a model of endogenous technical change resulting from learning or network effects on the existence of a lower bound to market concentration.
Link
http://econpapers.repec.org/scripts/redir.pl?u=http%3A%2F%2Fjeps.repec.org%2Fpapers%2F05-006.pdf;h=repec:jep:wpaper:05006