This paper presents a model in which public R&D stock is included as a quasi-fixed input in a variable cost function. Its price affects the long run desired level, while its shadow price indicates whether under (over) investment occurs in the short run.
Link
http://econpapers.repec.org/scripts/redir.pl?u=http%3A%2F%2Fdea.univpm.it%2Fquaderni%2Fpdf%2F238.pdf;h=repec:anc:wpaper:238