  We study a dynamic oligopoly with differentiated goods by differential game
approach under general demand and cost functions. We show that the steady state
value of the R&D investment by each firm is decreasing with respect to the
number of firms, and the steady state value of the industry R&D investment is
increasing with respect to the number of firms. Also we show that if there is
no spillover, whether the R&D investment of each firm given the cost level in
the memoryless closed-loop case is larger or smaller than that in the open-loop
case depends on whether the strategic variables are strategic substitutes or
strategic complements. Further we show that the memoryless closed-loop solution
and the feedback solution (by the Hamilton-Jacobi-Bellman equation) are
equivalent.
