Appendix
Proof of Lemma 1
proof. Part (i). Recall that the logconcavity of f implies that vˆi−1−F(vˆi)f(vˆi) is strictly increasing in vˆi. By the definition of v˜ and μ˜ , we have vˆi−1−F(vˆi)f(vˆi)≤0 if μ∈[μ˜,1], with the strict inequality if μ≠μ˜. Since xivˆi[1−F(vˆi)]≥0 for any μ, we reach the conclusion that ∂Πi∂μi<0 if μ∈(μ˜,1].
Part (ii). A necessary condition for an equilibrium μi∗ is ∂Πi∂μi(μi∗)=0. Following part (i) we must have μi∗∈[0,μ˜], if equilibrium exists.
Part (iii). We first show that ∂2Πi∂μi2<0 for μi∈[0,μ˜). By eq. (5),
∂2Πi∂μi2∝∂[vˆi−1−F(vˆi)f(vˆi)]∂μimi(μi,μj)+[vˆi−1−F(vˆi)f(vˆi)]∂mi(μi,μj)∂μi−xi∂{vˆi[1−F(vˆi)]}∂μi.
Now inspect each term in the above expression. Recall that, for μi∈[0,μ˜),vˆi−1−F(vˆi)f(vˆi)>0 and is strictly decreasing in μi. The term mi(μi,μj) is also positive and strictly decreasing in μi. By previous analysis, vˆi[1−F(vˆi)]=R(μi) is strictly increasing in μi for μi∈[0,μ˜). Therefore, ∂2Πi∂μi2<0 for μi∈[0,μ˜).
Next we show that ∂2Πi∂μi∂μj>0. Again by eq. (5),
∂2Πi∂μi∂μj∝[vˆi−1−F(vˆi)f(vˆi)]xj,
which is strictly greater than 0 for μi∈[0,μ˜).
Claim 1.Let W(v)≡vv−1−F(v)f(v) and Z(v)≡v[1−F(v)]v−1−F(v)f(v). Both W(v) and Z(v) are strictly decreasing invforv∈[v˜,v‾),and hence are strictly increasing inμforμ∈[0,μ˜).
proof. Taking derivatives, we get
dW(v)dv∝−1−F(v)f(v)+(1−F(v)f(v))′<0,
where the inequality uses the fact that (1−F(v)f(v))′<0 by the logconcavity of f(⋅). Similarly,
dZ(v)dv∝−f(v)[v−1−F(v)f(v)]2−v[1−F(v)][v−1−F(v)f(v)]′<0,
where the inequality uses the fact that [v−1−F(v)f(v)] is strictly increasing in v.
Proof of Lemma 2
proof. Recall that, by part (iii) of Lemma 1, μi and μj are strategic complements. And the domain of [0,μ˜] is compact. By Theorem 4.2(ii) of Vives (1990), the set of Nash equilibria is non-empty. To show the uniqueness of Nash equilibrium, we add eqs (6) and (7), and get
(12)xAvˆA∗[1−F(vˆA∗)]vˆA∗−1−F(vˆA∗)f(vˆA∗)+xBvˆB∗[1−F(vˆB∗)]vˆB∗−1−F(vˆB∗)f(vˆB∗)=1.
Using the definition of Z(v), the above equation can be written compactly as xAZ(vˆA∗)+xBZ(vˆB∗)=1. Now suppose there are two Nash equilibria: (μA1∗,μB1∗) and (μA2∗,μB2∗). By Theorem 4.2(ii) Vives (1990), the two Nash equilibria can be ordered. Without loss of generality, suppose (μA1∗,μB1∗)\lt(μA2∗,μB2∗). But by the monotonicity of Z(v), according to Claim 1, we have
xAZ(vˆA1∗)+xBZ(vˆB1∗)<xAZ(vˆA2∗)+xBZ(vˆB2∗),
which contradicts eq. (12). Therefore, there is a unique equilibrium.
Proof of Proposition 1
proof. Part (i). Suppose, to the contrary, μB∗≤μA∗. Since xA>xB, we have xAμA∗>xBμB∗. Taking the difference between eqs (6) and (7), we have
(13)xBμB∗−xAμA∗=12[xAZ(vˆA∗)−xBZ(vˆB∗)]=12[xAμA∗W(vˆA∗)−xBμB∗W(vˆB∗)].
The RHS of the above equation, (13), is strictly greater than 0, which follows from the presumption that μB∗≤μA∗, and the facts that Z(v)>0 and is increasing in μ (by Claim 1) for μ∈[0,μ˜), and xA>xB. This contradicts the result that the LHS of eq. (13) is strictly less than 0. Thus we must have μB∗>μA∗. It immediately follows that vˆA∗>vˆB∗.
To show xAμA∗>xBμB∗, suppose, to the contrary, xAμA∗≤xBμB∗. This means that the LHS of eq. (13) is greater than or equal to 0. Now consider the RHS of eq. (13). By the fact that μ˜>μB∗>μA∗ and by Claim 1, we have W(vˆB∗)\gtW(vˆA∗)>0. Combined with the presumption that xAμA∗≤xBμB∗, we draw the conclusion that the RHS of eq. (13) is strictly less than 0, a contradiction. Therefore, we must have xAμA∗>xBμB∗. It follows immediately that mA(μA∗,μB∗)<mB(μA∗,μB∗).
The difference in equilibrium profits can be written as
ΠB∗−ΠA∗∝αBmB∗vˆB∗[1−F(vˆB∗)]−αAmA∗vˆA∗[1−F(vˆA∗)].
Since αBmB∗>αAmA∗,vˆB∗[1−F(vˆB∗)]−vˆA∗[1−F(vˆA∗)]≥0 implies that ΠB∗−ΠA∗>0. Recall that v[1−F(v)] is decreasing in v for v∈[v˜,v‾]. Combining with the result in part (i) that vˆA∗>vˆB∗, we have vˆB∗[1−F(vˆB∗)]−vˆA∗[1−F(vˆA∗)]≥0. This proves that ΠA∗<ΠB∗.
Part (ii). The sign of pB∗−pA∗ can be expressed as
pB∗−pA∗∝αBqvˆB∗αBq+(1−αB)(1−q)−αAqvˆA∗αAq+(1−αA)(1−q).
By the above expression, pB∗−pA∗>0 is equivalent to
1+1−αBαB1−qq1+1−αAαA1−qq<vˆB∗vˆA∗.
Recall that μvˆ is strictly decreasing in vˆ for vˆ∈[v˜,v‾), which means that μB∗vˆB∗>μA∗vˆA∗ , or μA∗μB∗<vˆB∗vˆA∗. Thus, to prove pA∗<pB∗, it is enough to show that 1+1−αBαB1−qq1+1−αAαA1−qq≤μA∗μB∗. The fact that xAμA∗>xBμB∗ implies that μA∗μB∗>xBxA. Thus it is sufficient that 1+1−αBαB1−qq1+1−αAαA1−qq≤yByA.
Part (iii). When both xA and xB goes to 0, we have mi∗→12 and μi∗→μ˜, for i=A,B. Therefore,
αAq+(1−αA)(1−q)μA∗mA∗−αBq+(1−αB)(1−q)μB∗mB∗∝αAq+(1−αA)(1−q)−αBq+(1−αB)(1−q)>0,
where the last inequality follows the fact q\lt1/2.
Proof of Proposition 2
proof. Part (i). A consumer’s total net nuisance cost incurred on platform i is yiμi∗. The fact that yAμA∗>yBμB∗ follows directly from the result that xAμA∗>xBμB∗ (part (i) of Proposition 1).
Part (ii). Consider a type v firm advertising on both platforms: v≥vˆA∗>vˆB∗. Its profit per consumer and total profit on platform i are αiq(v−vˆi∗) and αiq(v−vˆi∗)mi∗, respectively. Since αA<αB,vˆA∗>vˆB∗, and mA∗<mB∗, we conclude that αAq(v−vˆA∗)\ltαBq(v−vˆB∗) and αAq(v−vˆA∗)mA∗<αBq(v−vˆB∗)mB∗.
Proof of Proposition 3
proof. Part (i). Since an increase in αB implies a decrease in xB, it is sufficient to show that ∂μA∗∂xB>0 and ∂μB∗∂xB<0. Define Wi≡W(vˆi∗). Differentiating eqs (6) and (7) with respect to xB, we get
(14)μB∗+xB∂μB∗∂xB=xA∂μA∗∂xB1+WA+μA∗dWAdμA∗,
(15)xA∂μA∗∂xB=μB∗+xB∂μB∗∂xB[1+WB]+xBμB∗dWBdμB∗∂μB∗∂xB.
From the above two equations, we can solve for ∂μB∗∂xB as follows:
(16)∂μB∗∂xB=−1+WA+μA∗dWAdμA∗1+WB−1μB∗1+WA+μA∗dWAdμA∗1+WB+μB∗dWBdμB∗−1xB<0,
where the inequality follows dWAdμA∗>0 and dWBdμB∗>0. From eqs (14) and (16), the sign of ∂μA∗∂xB can be expressed as
(17)∂μA∗∂xB∝μB∗+xB∂μB∗∂xB∝1+WA+μA∗dWAdμA∗μB∗dWBdμB∗>0.
Part (ii). It is sufficient to show that ∂(xAμA∗−xBμB∗)∂xB<0. More explicitly,
∂(xAμA∗−xBμB∗)∂xB=xA∂μA∗∂xB−μB∗−xB∂μB∗∂xB<0,
where the inequality follows eq. (14), which implies that μB∗+xB∂μB∗∂xB>xA∂μA∗∂xB.
Part (iii). Recall that vˆμ>0 and it is increasing in μ for μ∈[0,μ˜]. Thus by part (i), we have vˆA∗′μA∗′<vˆA∗μA∗ and vˆB∗′μB∗′>vˆB∗μB∗. By eq. (4), Πi∗∝αimi∗vˆi∗μi∗. Since αA remains the same, mA∗′<mA∗ by part (ii), and vˆA∗′μA∗′<vˆA∗μA∗, it must be the case that ΠA∗′<ΠA∗. Similarly, since αB′>αB,mB∗′>mB∗ by part (ii), and vˆB∗′μB∗′>vˆB∗μB∗, we must have ΠB∗′>ΠB∗.
Part (iv). To show pA∗′>pA∗, it is sufficient to show that ∂pA∗∂xB<0. By eq. (1), ∂pA∗∂xB∝dvˆA∗dμA∗∂μA∗∂xB<0, where the inequality follows ∂μA∗∂xB>0 in part (i).
The sign of pB∗′−pB∗ can be expressed as
pB∗′−pB∗∝−αBqvˆB∗αBq+(1−αB)(1−q)+αB′qvˆB∗′αB′q+(1−αB′)(1−q).
By the above expression, pB∗′−pB∗>0 is equivalent to
1+1−αB′αB′1−qq1+1−αBαB1−qq<vˆB∗′vˆB∗.
Since vˆμ>0 and it is increasing in μ for μ∈[0,μ˜], we have vˆB∗′μB∗′−vˆB∗μB∗>0, or μB∗μB∗′<vˆB∗′vˆB∗. Thus to show that pB∗−pB∗′<0, it is enough to show that 1+1−αB′αB′1−qq1+1−αBαB1−qq≤μB∗μB∗′. Note that xB′μB∗′<xBμB∗. To see this, by eqs (16) and (17), we have
∂(xBμB∗)∂xB=μB∗+xB∂μB∗∂xB>0.
This implies that ∂(xBμB∗)∂αB<0 and xB′μB∗′, since μB∗′>μB∗ by part (i) of Proposition 3. The fact that xB′μB∗′<xBμB∗ implies yB′yB<μB∗μB∗′. Therefore, to show pB∗−pB∗′<0 it is sufficient that 1+1−αB′αB′1−qq1+1−αBαB1−qq≤yB′yB.
Part (v). Since αA remains the same, mA∗′<mA∗, and μA∗′<μA∗, the total number of ads on platform A decreases. Regarding platform B, when t→∞, we have mB∗→12,mB∗′→12,μB∗→μ˜, and μB∗′→μ˜. Therefore,
αB′q+(1−αB′)(1−q)μB∗′mB∗′−αBq+(1−αB)(1−q)μB∗mB∗∝αB′q+(1−αB′)(1−q)−αBq+(1−αB)(1−q)<0,
since q\lt1/2. This means that for t big enough, the total number of ads on platform B decreases.
Now we show that the total combined relevant ads increase if the distribution of v is either uniform or exponential. Define ui≡vˆi−1−F(vˆi)f(vˆi) and ni≡vˆiμi. Note that ∂ni∂μi=ui>0, and ∂ui∂μi<0. Now the FOCs of eqs (6) and (7) can be written as:
uA∗mA∗=xAnA∗;uB∗mB∗=xBnB∗.
Taking derivatives w.r.t. αB, we get
∂μB∗∂αB=uB∗∂mB∗∂αB+knB∗xBuB∗−∂uB∗∂μBmB∗,∂mA∗∂αB=xAuA∗−∂uA∗∂μAmA∗uA∗∂μA∗∂αB,
where k=12tλq+γ1−2q>0. The total combined relevant ads are given by αAqμA∗mA∗+αBqμB∗mB∗. Taking derivatives w.r.t. αB and using the above equations, we get
∂(αAqμA∗mA∗+αBqμB∗mB∗)∂αB∝αAmA∗∂μA∗∂αB+αBmB∗∂μB∗∂αB+μB∗mB∗+∂mA∗∂αB(αAμA∗−αBμB∗)\gtαAmA∗+(αAμA∗−αBμB∗)−uB∗αBmB∗xBuB∗−∂uB∗∂μBmB∗xAuA∗−∂uA∗∂μAmA∗uA∗∂μA∗∂αB.
Since ∂μA∗∂αB<0, it is enough to show that the term in the braces is negative, which is equivalent to
αAmA∗(xBuB∗−∂uB∗∂μBmB∗)uA∗−αBmB∗(xAuA∗−∂uA∗∂μAmA∗)uB∗+(αAμA∗−αBμB∗)(xBuB∗−∂uB∗∂μBmB∗)(xAuA∗−∂uA∗∂μAmA∗)<0.
Using the facts that, αA<αB,xA>xB,mA∗<mB∗, and ∂ui∂μi<0, the following condition is sufficient
(18)−αA∂uB∗∂μBuA∗+αB∂uA∗∂μAuB∗+(αAμA∗−αBμB∗)∂uA∗∂μA∂uB∗∂μB<0.
Suppose v is uniformly distributed on [0,1], then ui∗=1−2μi∗, and ∂ui∗∂μi=−2. The inequality eq. (18) becomes
2αA(1−2μA∗)−2αB(1−2μB∗)+4(αAμA∗−αBμB∗)=2(αA−αB)<0,
which obviously holds.
Now suppose v has an exponential distribution with density f(v)=ξe−ξx,ξ>0. In this case, ui∗=1ξ(−lnμi∗−1) and ∂ui∗∂μi=−1ξμi∗. The inequality eq. (18) becomes
−αAμA∗(1+lnμA∗)+αBμB∗(1+lnμB∗)+(αAμA∗−αBμB∗)=αBμB∗lnμB∗−αAμA∗lnμA∗<0.
The above inequality holds because −(μB∗lnμB∗)\gt−(μA∗lnμA∗) by the facts that −μi∗lnμi∗=ξni∗,∂ni∗∂μi>0, and μB∗>μA∗.
Proof of Proposition 4
proof. Part (i). Following Proposition 3, we have μA∗′<μA∗. Since αA, and hence yA, does not change, it implies that yAμA∗′<yAμA∗. The fact that mA∗′<mA∗ means that yB′μB∗′−yAμA∗′<yBμB∗−yAμA∗. The above two inequalities imply that yB′μB∗′−yBμB∗<yAμA∗′−yAμA∗<0.
Part (ii). Firms with v∈(vˆB∗′,vˆB∗) are clearly better off since they were not participating on any platform initially. Firms with v∈[vˆB∗,vˆA∗) are also better off. To see this, note that before and after the change they only participate on platform B. A type v firm’s profit on platform B equals to αBqmB∗(v−vˆB∗). Since αB′>αB,mB∗′>mB∗,vˆB∗′<vˆB∗, we have αB′qmB∗′(v−vˆB∗′)\gtαBqmB∗(v−vˆB∗).
For firms participating on both platforms after the change (v≥vˆA∗′), a type v firm’s total profit on two platforms is given by αAqmA∗(v−vˆA∗)+αBqmB∗(v−vˆB∗). Taking derivatives w.r.t. αB, and using similar logic as in the proof of part (iii), in the case of uniform distribution we get
∂[αAqmA∗(v−vˆA∗)+αBqmB∗(v−vˆB∗)]∂αB>0⇐−αA∂uB∗∂μBuA∗+αB∂uA∗∂μAuB∗+[αA(v−vˆA∗)−αB(v−vˆB∗)]∂uA∗∂μA∂uB∗∂μB≤0⇔[2+4(v−1)](αA−αB)≤0,
which holds, since v≥vˆA∗′≥1/2.
In the case of exponential distribution,
∂[αAqmA∗(v−vˆA∗)+αBqmB∗(v−vˆB∗)]∂αB>0⇐0≥αAmA∗uA∗f(vˆA∗)(xBuB∗−∂uB∗∂μBmB∗)−αBmB∗uB∗f(vˆB∗)(xAuA∗−∂uA∗∂μAmA∗)+[αA(v−vˆA∗)−αB(v−vˆB∗)]∂uA∗∂μA∂uB∗∂μB
The term αAmA∗xBf(vˆA∗)−αBmB∗xAf(vˆB∗)∝αAnA∗f(vˆA∗)uA∗−αBnB∗f(vˆB∗)uB∗=αA1−F(vˆA∗)f(vˆA∗)W(vˆA∗)−αB1−F(vˆB∗)f(vˆB∗)W(vˆB∗)<0, since the hazard rate is constant and W(⋅) is decreasing. Now the original inequality boils down to
−αA1−F(vˆA∗)f(vˆA∗)(1+lnμA∗)+αB1−F(vˆB∗)f(vˆB∗)(1+lnμB∗)+αA(v−vˆA∗)−αB(v−vˆB∗)≤0⇐−αA(1+lnμA∗)+αB(1+lnμB∗)−αB(lnμB∗−lnμA∗)≤0⇔(αB−αA)(1+lnμA∗)≤0,
which obviously holds.
Proof of Proposition 5
proof. Given that αA=αB=α, the equilibrium allocation must be symmetric, and we denote the equilibrium variables as μ∗ and vˆ∗. By eqs (6) and (7), the equilibrium condition boils down to
yμ∗W∗=t,
where y=γ(1−q)(1−α)−(λ−γ)αq, and W∗=vˆ∗vˆ∗−1−F(vˆ∗)f(vˆ∗). Taking derivative with respect to α and rearranging, we get
∂μ∗∂α=−∂y∂αμ∗W∗yW∗+yμ∗dW∗dμ.
Recall that a consumer’s total net nuisance cost incurred (on either platform) is yμ∗. The change in the total net nuisance cost induced by a change in α is given by
∂yμ∗∂α=∂y∂αμ∗+y∂μ∗∂α=∂y∂αμ∗+y−∂y∂αμ∗W∗yW∗+yμ∗dW∗dμ=∂y∂αμ∗2dW∗dμW∗+μ∗dW∗dμ<0,
where the inequality uses the facts that W∗>0,dWdμ>0 (Claim 1), and ∂y∂α<0.
Proof of Proposition 6
proof. Part (i). We first show that the first term in eq. (11) is positive. In particular
12[2tkμ∗−y(α)dμ∗dα]=122tkdZ∗dμ∗(μ∗dZ∗dμ∗−Z∗)∝μ∗dZ∗dμ∗−Z∗,
since dZ∗dμ∗>0. But
μ∗dZ∗dμ∗−Z∗∝μ∗2f2(vˆ∗)+μ∗vˆ∗f3(vˆ∗)[f2(vˆ∗)+f′(vˆ∗)(1−F(vˆ∗))]>0,
where the last inequality uses the fact that the terms in the brackets is positive due to the logconcavity of f(⋅).
The second term in eq. (11) is clearly positive, since
∫vˆ∗v‾vf(v)dv−μ∗vˆ∗=∫vˆ∗v‾(v−vˆ∗)f(v)dv>0.
Now we add the first term and the third term of eq. (11) together. Given qv‾≤1/2 and t≥1/2, to show the sum of the two terms is positive, it is sufficient that the following expression is positive:
(μ∗dZ∗dμ∗−Z∗)(2+dZ∗dμ∗)+(Z∗2−μ∗2dZ∗dμ∗)(1+dZ∗dμ∗)+Z∗(Z∗−μ∗)>0.
Since Z∗≥μ∗ and μ∗dZ∗dμ∗−Z∗>0, the following condition is sufficient:
(1+dZ∗dμ∗)[μ∗(1−μ∗)dZ∗dμ∗−Z∗+Z∗2]≥0.
But again by Z∗≥μ∗ and μ∗dZ∗dμ∗−Z∗>0,
μ∗(1−μ∗)dZ∗dμ∗−Z∗+Z∗2>(1−μ∗)Z∗−Z∗+Z∗2=Z∗(Z∗−μ∗)≥0.
Therefore, when qv‾≤1/2 and t≥1/2,Ho(α)−H∗(α) is strictly positive. This further implies that, by the convexity of C(⋅),α∗<αo.
Part (ii). Since v is uniformly distributed, μ˜=1/2, and
Z∗=μ∗(1−μ∗)1−2μ∗;dZ∗dμ∗=1+2μ∗(1−μ∗)(1−2μ∗)2.
Now eq. (11) becomes
(19)Ho(α)−H∗(α)\gtαqvˆ∗[k4x2dZ∗dμ∗−kμ∗Z∗+μ∗[dZ∗dμ∗+(dZ∗dμ∗)2]2dZ∗dμ∗+(dZ∗dμ∗)2]∝1−7μ∗+14μ∗2−10μ∗3.
It can be verified that expression eq. (19) 1−7μ+14μ2−10μ3 is decreasing in μ for μ∈[0,1/2]. Therefore, if μ∗≤μˆ, where μˆ∈(0,1/2) is the solution to 1−7μ+14μ2−10μ3=0, then Ho(α)−H∗(α)>0. Now define xˆ as 12xˆ=Z(μˆ)=μˆ(1−μˆ)1−2μˆ. We have μ∗≤μˆ if x≥xˆ. Since x(α) reaches its minimum at α‾ , x(α)≥xˆ if x(α‾)≥xˆ. To summarize, if x(α‾)≥xˆ, then μ∗≤μˆ, and Ho(α)−H∗(α)>0, which implies α∗<αo.
Part (iii). Given that v is uniformly distributed, qv‾<1/2. Thus, by part (i), for α∗>αo to occur it must be the case that t\lt1/2. By part (ii), for α∗>αo to occur it must be the case that x(α‾)\ltxˆ.
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