... …rm 1 s problem are then given byp 1 q 1 i+d(q 1 1)dq 1 1 1 1 1q 1 = 0 (13 .14 )p2 1 1 2q 1 = 0 (13 .15 )which, taking into account (13 .12 ), imply 1 1 1 2=p 1 p2+ ... 2d2q2; q 1 1dq 1 1dq 1 1d= 0 (13 .10 )Using the de…nition of the externality we can write (13 .10 ) as 1 + 222q2; q 1 1e 1 21 dq 1 1d= 0 (13 .11 )which, in view of (13 .9), ... thismaxfq 1 gnXi =1 piq 1 i+ (q 1 1) 1 1 q 1 (13 .13 )– there is explicit recognition in (13 .13 ) that the size of the sidepayment willdepend upon q 1 1, which is under …rm 1 s...