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a<br />
,a 20<br />
1 2<br />
M<br />
a 1 M<br />
a 2<br />
P P<br />
a1 a2<br />
10<br />
0<br />
5 10 15 20<br />
t<br />
Figure 10: Advertising =11<br />
v 1<br />
,v 2<br />
1.5<br />
1.0<br />
0.5<br />
P P<br />
v1 v2<br />
M<br />
v 2<br />
M<br />
v 1<br />
0.0<br />
5 10 15 20<br />
t<br />
Figure 11: Quality =11<br />
pattern is explained by the fact that the public platform tends to increase<br />
the socially beneficial advertising.<br />
5.3 Welfare comparisons<br />
In this subsection we compare the social welfare obtained in the three regimes.<br />
First, we compare social welfare in the mixed and the zero duopoly. This<br />
comparison is illustrated in Figure 13, where in region and<br />
in region . According to Figure 13, when advertising is<br />
very harmful the zero duopoly is always better than the conventional mixed<br />
duopoly, from the social welfare point of view. Otherwise, the conventional<br />
mixed duopoly can be socially preferred, as stated in the following proposition.<br />
Proposition 1 In the comparisons between the mixed and zero duopoly, we<br />
obtain that there exists a critical value of k defined as ∗ (), thatisdecreasing<br />
16<br />
18