Consider two firms that produce complementary goods. For example, each firm could be producing hot dogs and hot dog buns or printers and ink cartridges. Therefore, each firm's inverse demand function is decreasing in its own production and increasing in the production of the other firm. In particular, if q1 and q2 are the firms' production levels, then their per-period profits are given by v1(q1, q2) = (20 q1 + q2)q1 2q 2 1 and v2(q1, q2) = (20 + q1 q2)q2 2q 2 2 .