Bertrand Model of Price Competition Advanced Microeconomic Theory 1
Bertrand Model of Price Competition
Advanced Microeconomic Theory 1
Bertrand Model of Price Competition
โข Consider:โ An industry with two firms, 1 and 2, selling a
homogeneous product
โ Firms face market demand ๐ฅ(๐), where ๐ฅ(๐) is continuous and strictly decreasing in ๐
โ There exists a high enough price (choke price) าง๐ < โ such that ๐ฅ(๐) = 0 for all ๐ > าง๐
โ Both firms are symmetric in their constant marginal cost ๐ > 0, where ๐ฅ ๐ โ (0, โ)
โ Every firm ๐ simultaneously sets a price ๐๐
Advanced Microeconomic Theory 2
Bertrand Model of Price Competition
โข Firm ๐โs demand is
๐ฅ๐(๐๐ , ๐๐) =
๐ฅ(๐๐) if ๐๐ < ๐๐
1
2๐ฅ(๐๐) if ๐๐ = ๐๐
0 if ๐๐ > ๐๐
โข Intuition: Firm ๐ captures โ all market if its price is the lowest, ๐๐ < ๐๐
โ no market if its price is the highest, ๐๐ > ๐๐
โ shares the market with firm ๐ if the price of both firms coincide, ๐๐ = ๐๐
Advanced Microeconomic Theory 3
Bertrand Model of Price Competition
โข Given prices ๐๐ and ๐๐, firm ๐โs profits are
therefore(๐๐ โ ๐) โ ๐ฅ๐ (๐๐ , ๐๐)
โข We are now ready to find equilibrium prices in the Bertrand duopoly model.
โ There is a unique NE (๐๐โ, ๐๐
โ) in the Bertrand
duopoly model. In this equilibrium, both firms set prices equal to marginal cost, ๐๐
โ = ๐๐โ = ๐.
Advanced Microeconomic Theory 4
Bertrand Model of Price Competition
โข Letโs us describe the best response function of firm ๐.
โข If ๐๐ < ๐, firm ๐ sets its price at ๐๐ = ๐.โ Firm ๐ does not undercut firm ๐ since that would entail
negative profits.
โข If ๐ < ๐๐ < ๐๐, firm ๐ slightly undercuts firm ๐, i.e., ๐๐ = ๐๐ โ ๐.โ This allows firm ๐ to capture all sales and still make a
positive margin on each unit.
โข If ๐๐ > ๐๐, where ๐๐ is a monopoly price, firm ๐ does not need to charge more than ๐๐, i.e., ๐๐ = ๐๐.โ ๐๐ allows firm ๐ to capture all sales and maximize profits
as the only firm selling a positive output.
Advanced Microeconomic Theory 5
pj
pk
pm
pm
c
c
pj (pk)
45ยฐ-line (pj = pk)
Bertrand Model of Price Competition
โข Firm ๐โs best response has:โ a flat segment for all
๐๐ < ๐, where ๐๐(๐๐) = ๐
โ a positive slope for all ๐ < ๐๐ < ๐๐, where firm ๐ charges a price slightly below firm ๐
โ a flat segment for all ๐๐ > ๐๐, where ๐๐(๐๐) = ๐๐
Advanced Microeconomic Theory 6
pj
pk
c
c
pj (pk)
pk (pj)
pm
pm
45ยฐ-line (pj = pk)
Bertrand Model of Price Competition
โข A symmetric argument applies to the construction of the best response function of firm ๐.
โข A mutual best response for both firms is
(๐1โ, ๐2
โ) = (๐, ๐)where the two best response functions cross each other.
โข This is the NE of the Bertrand modelโ Firms make no economic
profits.
Advanced Microeconomic Theory 7
Bertrand Model of Price Competition
โข With only two firms competing in prices we obtain the perfectly competitive outcome, where firms set prices equal to marginal cost.
โข Price competition makes each firm ๐ face an infinitely elastic demand curve at its rivalโs price, ๐๐.
โ Any increase (decrease) from ๐๐ infinitely reduces (increases, respectively) firm ๐โs demand.
Advanced Microeconomic Theory 8
Bertrand Model of Price Competition
โข How much does Bertrand equilibrium hinge into our assumptions? โ Quite a lot
โข The competitive pressure in the Bertrand model with homogenous products is ameliorated if we instead consider:โ Price competition (but allowing for heterogeneous
products)
โ Quantity competition (still with homogenous products)
โ Capacity constraints
Advanced Microeconomic Theory 9
Bertrand Model of Price Competition
โข Remark:โ How our results would be affected if firms face
different production costs, i.e., 0 < ๐1 < ๐2?
โ The most efficient firm sets a price equal to the marginal cost of the least efficient firm, ๐1 = ๐2.
โ Other firms will set a random price in the uniform interval
[๐1, ๐1 + ๐]
where ๐ > 0 is some small random increment with probability distribution ๐ ๐, ๐ > 0 for all ๐.
Advanced Microeconomic Theory 10
Cournot Model of Quantity Competition
Advanced Microeconomic Theory 11
Cournot Model of Quantity Competition
โข Let us now consider that firms compete in quantities.
โข Assume that:โ Firms bring their output ๐1 and ๐2 to a market, the
market clears, and the price is determined from the inverse demand function ๐(๐), where ๐ = ๐1 + ๐2.
โ ๐(๐) satisfies ๐โ(๐) < 0 at all output levels ๐ โฅ 0,
โ Both firms face a common marginal cost ๐ > 0
โ ๐(0) > ๐ in order to guarantee that the inverse demand curve crosses the constant marginal cost curve at an interior point.
Advanced Microeconomic Theory 12
Cournot Model of Quantity Competition
โข Let us first identify every firmโs best response function
โข Firm 1โs PMP, for a given output level of its rival, เดค๐2,
max๐1โฅ0
๐ ๐1 + เดค๐2
Price
๐1 โ ๐๐1
โข When solving this PMP, firm 1 treats firm 2โs production, เดค๐2, as a parameter, since firm 1 cannot vary its level.
Advanced Microeconomic Theory 13
Cournot Model of Quantity Competition
โข FOCs:๐โฒ(๐1 + เดค๐2)๐1 + ๐(๐1 + เดค๐2) โ ๐ โค 0
with equality if ๐1 > 0โข Solving this expression for ๐1, we obtain firm 1โs
best response function (BRF), ๐1(เดค๐2).โข A similar argument applies to firm 2โs PMP and its
best response function ๐2(เดค๐1). โข Therefore, a pair of output levels (๐1
โ, ๐2โ) is NE of
the Cournot model if and only if ๐1
โ โ ๐1(เดค๐2) for firm 1โs output๐2
โ โ ๐2(เดค๐1) for firm 2โs outputAdvanced Microeconomic Theory 14
Cournot Model of Quantity Competition
โข To show that ๐1โ, ๐2
โ > 0, let us work by contradiction, assuming ๐1
โ = 0. โ Firm 2 becomes a monopolist since it is the only firm
producing a positive output.
โข Using the FOC for firm 1, we obtain๐โฒ(0 + ๐2
โ)0 + ๐(0 + ๐2โ) โค ๐
or ๐(๐2โ) โค ๐
โข And using the FOC for firm 2, we have๐โฒ(๐2
โ + 0)๐2โ + ๐(๐2
โ + 0) โค ๐
or ๐โฒ(๐2โ)๐2
โ + ๐(๐2โ) โค ๐
โข This implies firm 2โs MR under monopoly is lower than its MC. Thus, ๐2
โ = 0.Advanced Microeconomic Theory 15
Cournot Model of Quantity Competition
โข Hence, if ๐1โ = 0, firm 2โs output would also be
zero, ๐2โ = 0.
โข But this implies that ๐(0) < ๐ since no firm produces a positive output, thus violating our initial assumption ๐(0) > ๐. โ Contradiction!
โข As a result, we must have that both ๐1โ > 0 and
๐2โ > 0.
โข Note: This result does not necessarily hold when both firms are asymmetric in their production cost.
Advanced Microeconomic Theory 16
Cournot Model of Quantity Competition
โข Example (symmetric costs):
โ Consider an inverse demand curve ๐(๐) = ๐ โ๐๐, and two firms competing ร la Cournot both facing a marginal cost ๐ > 0.
โ Firm 1โs PMP is๐ โ ๐(๐1 + เดค๐2) ๐1 โ ๐๐1
โ FOC wrt ๐1:๐ โ 2๐๐1 โ ๐ เดค๐2 โ ๐ โค 0
with equality if ๐1 > 0
Advanced Microeconomic Theory 17
Cournot Model of Quantity Competition
โข Example (continue):
โ Solving for ๐1, we obtain firm 1โs BRF
๐1(เดค๐2) =๐โ๐
2๐โ
เดค๐2
2
โ Analogously, firm 2โs BRF
๐2(เดค๐1) =๐โ๐
2๐โ
เดค๐1
2
Advanced Microeconomic Theory 18
Cournot Model of Quantity Competition
Advanced Microeconomic Theory 19
โข Firm 1โs BRF:โ When ๐2 = 0, then
๐1 =๐โ๐
2๐, which
coincides with its output under monopoly.
โ As ๐2 increases, ๐1decreases (i.e., firm 1โs and 2โs output are strategic substitutes)
โ When ๐2 =๐โ๐
๐, then
๐1 = 0.
Cournot Model of Quantity Competition
Advanced Microeconomic Theory 20
โข A similar argument applies for firm 2โs BRF.
โข Superimposing both firmsโ BRFs, we obtain the Cournot equilibrium output pair (๐1
โ, ๐2โ).
Cournot Model of Quantity Competition
Advanced Microeconomic Theory 21
q1
q2
a โ c
q1(q2)
2b
q2(q1)
a โ cb
a โ cb
a โ c2b
a โ c3b
a โ c3b
(q1,q2 ) * *
45ยฐ-line (q1 = q2)q1 + q2 = qc =a โ c
b
q1 + q2 = qm =a โ c2b
Perfect competition
Monopoly
45ยฐ
Cournot Model of Quantity Competition
โข Cournot equilibrium output pair (๐1โ, ๐2
โ) occurs at the intersection of the two BRFs, i.e.,
(๐1โ, ๐2
โ) =๐โ๐
3๐,
๐โ๐
3๐
โข Aggregate output becomes
๐โ = ๐1โ + ๐2
โ =๐โ๐
3๐+
๐โ๐
3๐=
2(๐โ๐)
3๐
which is larger than under monopoly, ๐๐ =๐โ๐
2๐,
but smaller than under perfect competition, ๐๐ =๐โ๐
๐.
Advanced Microeconomic Theory 22
Cournot Model of Quantity Competition
โข The equilibrium price becomes
๐ ๐โ = ๐ โ ๐๐โ = ๐ โ ๐2 ๐โ๐
3๐=
๐+2๐
3
which is lower than under monopoly, ๐๐ =๐+๐
2, but
higher than under perfect competition, ๐๐ = ๐.
โข Finally, the equilibrium profits of every firm ๐
๐๐โ = ๐ ๐โ ๐๐
โ โ ๐๐๐โ =
๐+2๐
3
๐โ๐
3๐โ ๐
๐โ๐
3๐=
๐โ๐ 2
4๐
which are lower than under monopoly, ๐๐ =๐โ๐ 2
4๐,
but higher than under perfect competition, ๐๐ = 0.
Advanced Microeconomic Theory 23
Cournot Model of Quantity Competition
โข Quantity competition (Cournot model) yields less competitive outcomes than price competition (Bertrand model), whereby firmsโ behavior mimics that in perfectly competitive marketsโ Thatโs because, the demand that every firm faces in
the Cournot game is not infinitely elastic. โ A reduction in output does not produce an infinite
increase in market price, but instead an increase of โ ๐โฒ(๐1 + ๐2).
โ Hence, if firms produce the same output as under marginal cost pricing, i.e., half of
๐โ๐
2, each firm would
have incentives to deviate from such a high output level by marginally reducing its output.
Advanced Microeconomic Theory 24
Cournot Model of Quantity Competition
โข Equilibrium output under Cournot does not coincide with the monopoly output either.
โ Thatโs because, every firm ๐, individually increasing its output level ๐๐, takes into account how the reduction in market price affects its own profits, but ignores the profit loss (i.e., a negative external effect) that its rival suffers from such a lower price.
โ Since every firm does not take into account this external effect, aggregate output is too large, relative to the output that would maximize firmsโ joint profits.
Advanced Microeconomic Theory 25
Cournot Model of Quantity Competition
โข Example (Cournot vs. Cartel):
โ Let us demonstrate that firmsโ Cournot output is larger than that under the cartel.
โ PMP of the cartel ismax๐1,๐2
(๐ โ ๐(๐1+๐2))๐1 โ ๐๐1
+ (๐ โ ๐(๐1+๐2))๐2 โ ๐๐2
โ Since ๐ = ๐1 + ๐2, the PMP can be written asmax๐1,๐2
๐ โ ๐(๐1+๐2) (๐1+๐2) โ ๐(๐1+๐2)
= max๐
๐ โ ๐๐ ๐ โ ๐๐ = ๐๐ โ ๐๐2 โ ๐๐
Advanced Microeconomic Theory 26
Cournot Model of Quantity Competition
โข Example (continued):
โ FOC wrt ๐
๐ โ 2๐๐ โ ๐ โค 0
โ Solving for ๐, we obtain the aggregate output
๐โ =๐โ๐
2๐
which is positive since ๐ > ๐, i.e., ๐(0) = ๐ > ๐.
โ Since firms are symmetric in costs, each produces
๐๐ =๐
2=
๐โ๐
4๐
Advanced Microeconomic Theory 27
Cournot Model of Quantity Competition
โข Example (continued):
โ The equilibrium price is
๐ = ๐ โ ๐๐ = ๐ โ ๐๐โ๐
2๐=
๐+๐
2
โ Finally, the equilibrium profits are
๐๐ = ๐ โ ๐๐ โ ๐๐๐
=๐+๐
2โ
๐โ๐
4๐โ ๐
๐โ๐
4๐=
๐โ๐ 2
8๐
which is larger than firms would obtain under
Cournot competition, ๐โ๐ 2
9๐.
Advanced Microeconomic Theory 28
Cournot Model of Quantity Competition: Cournot Pricing Rule
โข Firmsโ market power can be expressed using a variation of the Lerner index.โ Consider firm ๐โs profit maximization problem
๐๐ = ๐(๐)๐๐ โ ๐๐(๐๐)
โ FOC for every firm ๐
๐โฒ ๐ ๐๐ + ๐ ๐ โ ๐๐ = 0
or ๐(๐) โ ๐๐ = โ๐โฒ ๐ ๐๐
โ Multiplying both sides by ๐ and dividing them by ๐(๐)yield
๐๐ ๐ โ ๐๐
๐(๐)=
โ๐โฒ ๐ ๐๐
๐(๐)๐
Advanced Microeconomic Theory 29
Cournot Model of Quantity Competition: Cournot Pricing Rule
โ Recalling 1
๐= โ๐โฒ ๐ โ
๐
๐ ๐, we have
๐๐ ๐ โ๐๐
๐(๐)=
1
๐๐๐
or ๐ ๐ โ๐๐
๐(๐)=
1
๐
๐๐
๐
โ Defining ๐ผ๐ โก๐๐
๐as firm ๐โs market share, we obtain
๐ ๐ โ ๐๐
๐(๐)=
๐ผ๐
๐
which is referred to as the Cournot pricing rule.
Advanced Microeconomic Theory 30
Cournot Model of Quantity Competition: Cournot Pricing Rule
โ Note:
When ๐ผ๐ = 1, implying that firm ๐ is a monopoly, the
IEPR becomes a special case of the Cournot price rule.
The larger the market share ๐ผ๐ of a given firm, the
larger the price markup of firm ๐.
The more inelastic demand ๐ is, the larger the price markup of firm ๐.
Advanced Microeconomic Theory 31
Cournot Model of Quantity Competition: Cournot Pricing Rule
โข Example (Merger effects on Cournot Prices):โ Consider an industry with ๐ firms and a constant-
elasticity demand function ๐(๐) = ๐๐โ1, where ๐ > 0 and ๐ = 1.
โ Before merger, we have๐๐ต โ ๐
๐๐ต=
1
๐โน ๐๐ต =
๐๐
๐ โ 1
โ After the merger of ๐ < ๐ firms ๐ โ ๐ + 1 firms remain in the industry, and thus
๐๐ด โ ๐
๐๐ด=
1
๐ โ ๐ + 1โน ๐๐ด =
๐ โ ๐ + 1 ๐
๐ โ ๐Advanced Microeconomic Theory 32
Cournot Model of Quantity Competition: Cournot Pricing Rule
โข Example (continued):โ The percentage change in prices is
%ฮ๐ =๐๐ด โ ๐๐ต
๐๐ต=
๐ โ ๐ + 1 ๐๐ โ ๐
โ๐๐
๐ โ 1๐๐
๐ โ 1
=๐ โ 1
๐(๐ โ ๐)> 0
โ Hence, prices increase after the merger.
โ Also, %ฮ๐ increases as the number of merging firms ๐ increases
๐%ฮ๐
๐๐=
๐ โ 1
๐ ๐ โ ๐ 2> 0
Advanced Microeconomic Theory 33
%ฮp
k20 40 60 80 100
0.10
0.20%ฮp(k)
Cournot Model of Quantity Competition: Cournot Pricing Rule
โข Example (continued):
โ The percentage increase in price after the merger, %ฮ๐, as a function of the number of merging firms, ๐.
โ For simplicity, ๐ =100.
Advanced Microeconomic Theory 34
Cournot Model of Quantity Competition: Asymmetric Costs
โข Assume that firm 1 and 2โs constant marginal costs of production differ, i.e., ๐1 > ๐2, so firm 2 is more efficient than firm 1. Assume also that the inverse demand function is ๐ ๐ = ๐ โ ๐๐, and ๐ = ๐1 + ๐2.
โข Firm ๐โs PMP is
max๐๐
๐ โ ๐(๐๐ + ๐๐) ๐๐ โ ๐๐๐๐
โข FOC:๐ โ 2๐๐๐ โ ๐๐๐ โ ๐๐ = 0
Advanced Microeconomic Theory 35
Cournot Model of Quantity Competition: Asymmetric Costs
โข Solving for ๐๐ (assuming an interior solution) yields firm ๐โs BRF
๐๐(๐๐) =๐ โ ๐๐
2๐โ
๐๐
2โข Firm 1โs optimal output level can be found by plugging
firm 2โs BRF into firm 1โs
๐1โ =
๐ โ ๐1
2๐โ
1
2
๐ โ ๐2
2๐โ
๐1โ
2โบ ๐1
โ =๐ โ 2๐1 + ๐2
3๐
โข Similarly, firm 2โs optimal output level is
๐2โ =
๐ โ ๐2
2๐โ
๐1โ
2=
๐ + ๐1 โ 2๐2
3๐
Advanced Microeconomic Theory 36
Cournot Model of Quantity Competition: Asymmetric Costs
โข The output levels (๐1โ, ๐2
โ) also vary when (๐1, ๐2)changes
๐๐1โ
๐๐1= โ
2
3๐< 0 and
๐๐1โ
๐๐2=
1
3๐> 0
๐๐2โ
๐๐1=
1
3๐> 0 and
๐๐2โ
๐๐2= โ
2
3๐< 0
โข Intuition: Each firmโs output decreases in its own costs, but increases in its rivalโs costs.
Advanced Microeconomic Theory 37
q1
q2a โ c2
2b
a โ c1
2b
a โ c1
b
a โ c2
b
(q1,q2 ) * *
q1(q2)
q2(q1)
Cournot Model of Quantity Competition: Asymmetric Costs
โข BRFs for firms 1 and 2
when ๐1 >๐+๐2
2(i.e.,
only firm 2 produces).
โข BRFs cross at the vertical axis where ๐1
โ = 0 and ๐2
โ > 0 (i.e., a corner solution)
Advanced Microeconomic Theory 38
Cournot Model of Quantity Competition:๐ฝ > 2 firms
โข Consider ๐ฝ > 2 firms, all facing the same constant marginal cost ๐ > 0. The linear inverse demand curve is ๐ ๐ = ๐ โ ๐๐, where ๐ =ฯ๐ฝ ๐๐.
โข Firm ๐โs PMP is
max๐๐
๐ โ ๐ ๐๐ +
๐โ ๐
๐๐ ๐๐ โ ๐๐๐
โข FOC:
๐ โ 2๐๐๐โ โ ๐
๐โ ๐
๐๐โ โ ๐ โค 0
Advanced Microeconomic Theory 39
Cournot Model of Quantity Competition:๐ฝ > 2 firms
โข Solving for ๐๐โ, we obtain firm ๐โs BRF
๐๐โ =
๐ โ ๐
2๐โ
1
2
๐โ ๐
๐๐โ
โข Since all firms are symmetric, their BRFs are also symmetric, implying ๐1
โ = ๐2โ = โฏ = ๐๐ฝ
โ. This
implies ฯ๐โ ๐ ๐๐โ = ๐ฝ๐๐
โ โ ๐๐โ = ๐ฝ โ 1 ๐๐
โ.
โข Hence, the BRF becomes
๐๐โ =
๐ โ ๐
2๐โ
1
2๐ฝ โ 1 ๐๐
โ
Advanced Microeconomic Theory 40
Cournot Model of Quantity Competition:๐ฝ > 2 firms
โข Solving for ๐๐โ
๐๐โ =
๐ โ ๐
๐ฝ + 1 ๐which is also the equilibrium output for other ๐ฝ โ 1firms.
โข Therefore, aggregate output is
๐โ = ๐ฝ๐๐โ =
๐ฝ
๐ฝ + 1
๐ โ ๐
๐and the corresponding equilibrium price is
๐โ = ๐ โ ๐๐โ =๐ + ๐ฝ๐
๐ฝ + 1Advanced Microeconomic Theory 41
Cournot Model of Quantity Competition:๐ฝ > 2 firms
โข Firm ๐โs equilibrium profits are ๐๐
โ = ๐ โ ๐๐โ ๐๐โ โ ๐๐๐
โ
= ๐ โ ๐๐ฝ
๐ฝ + 1
๐ โ ๐
๐
๐ โ ๐
๐ฝ + 1 ๐โ ๐
๐ โ ๐
๐ฝ + 1 ๐
=๐ โ ๐
๐ฝ + 1 ๐
2
= ๐๐โ 2
Advanced Microeconomic Theory 42
Cournot Model of Quantity Competition:๐ฝ > 2 firms
โข We can show that
lim๐ฝโ2
๐๐โ =
๐ โ ๐
2 + 1 ๐=
๐ โ ๐
3๐
lim๐ฝโ2
๐โ =2(๐ โ ๐)
2 + 1 ๐=
2(๐ โ ๐)
3๐
lim๐ฝโ2
๐โ =๐ + 2๐
2 + 1=
๐ + 2๐
3
which exactly coincide with our results in the Cournot duopoly model.
Advanced Microeconomic Theory 43
Cournot Model of Quantity Competition:๐ฝ > 2 firms
โข We can show that
lim๐ฝโ1
๐๐โ =
๐ โ ๐
1 + 1 ๐=
๐ โ ๐
2๐
lim๐ฝโ1
๐โ =1(๐ โ ๐)
1 + 1 ๐=
๐ โ ๐
2๐
lim๐ฝโ1
๐โ =๐ + 1๐
1 + 1=
๐ + ๐
2
which exactly coincide with our findings in the monopoly.
Advanced Microeconomic Theory 44
Cournot Model of Quantity Competition:๐ฝ > 2 firms
โข We can show that lim๐ฝโโ
๐๐โ = 0
lim๐ฝโโ
๐โ =๐ โ ๐
๐lim๐ฝโโ
๐โ = ๐
which coincides with the solution in a perfectly competitive market.
Advanced Microeconomic Theory 45
Product Differentiation
Advanced Microeconomic Theory 46
Product Differentiation
โข So far we assumed that firms sell homogenous (undifferentiated) products.
โข What if the goods firms sell are differentiated?
โ For simplicity, we will assume that product attributes are exogenous (not chosen by the firm)
Advanced Microeconomic Theory 47
Product Differentiation: Bertrand Model
โข Consider the case where every firm ๐, for ๐ ={1,2}, faces demand curve
๐๐(๐๐ , ๐๐) = ๐ โ ๐๐๐ + ๐๐๐
where ๐, ๐, ๐ > 0 and ๐ โ ๐.
โข Hence, an increase in ๐๐ increases firm ๐โs sales.
โข Firm ๐โs PMP:max๐๐โฅ0
(๐ โ ๐๐๐ + ๐๐๐)๐๐
โข FOC: โ2๐๐๐ + ๐๐๐ = 0
Advanced Microeconomic Theory 48
Product Differentiation: Bertrand Model
โข Solving for ๐๐, we find firm ๐โs BRF
๐๐(๐๐) =๐ + ๐๐๐
2๐โข Firm ๐ also has a symmetric BRF.
โข Note:
โ BRFs are now positively sloped
โ An increase in firm ๐โs price leads firm ๐ to increase his, and vice versa
โ In this case, firmsโ choices (i.e., prices) are strategic complements
Advanced Microeconomic Theory 49
Product Differentiation: Bertrand Model
Advanced Microeconomic Theory 50
p1
p2
a
2b
p2 *
p1(p2)
p2(p1)
p1 *
a
2b
Product Differentiation: Bertrand Model
โข Simultaneously solving the two BRS yields
๐๐โ =
๐
2๐ โ ๐with corresponding equilibrium sales of
๐๐โ(๐๐
โ, ๐๐โ) = ๐ โ ๐๐๐
โ + ๐๐๐โ =
๐๐
2๐ โ ๐and equilibrium profits of
๐๐โ = ๐๐
โ โ ๐๐โ ๐๐
โ, ๐๐โ =
๐
2๐ โ ๐
๐๐
2๐ โ ๐
=๐2๐
2๐ โ ๐ 2
Advanced Microeconomic Theory 51
Product Differentiation: Cournot Model
โข Consider two firms with the following linear inverse demand curves
๐1(๐1, ๐2) = ๐ผ โ ๐ฝ๐1 โ ๐พ๐2 for firm 1๐2(๐1, ๐2) = ๐ผ โ ๐พ๐1 โ ๐ฝ๐2 for firm 2
โข We assume that ๐ฝ > 0 and ๐ฝ > ๐พโ That is, the effect of increasing ๐1 on ๐1 is larger than
the effect of increasing ๐1 on ๐2
โ Intuitively, the price of a particular brand is more sensitive to changes in its own output than to changes in its rivalโs output
โ In other words, own-price effects dominate the cross-price effects.
Advanced Microeconomic Theory 52
Product Differentiation: Cournot Model
โข Firm ๐โs PMP is (assuming no costs)max๐๐โฅ0
(๐ผ โ ๐ฝ๐๐ โ ๐พ๐๐)๐๐
โข FOC:๐ผ โ 2๐ฝ๐๐ โ ๐พ๐๐ = 0
โข Solving for ๐๐ we find firm ๐โs BRF
๐๐(๐๐) =๐ผ
2๐ฝโ
๐พ
2๐ฝ๐๐
โข Firm ๐ also has a symmetric BRF
Advanced Microeconomic Theory 53
Product Differentiation: Cournot Model
Advanced Microeconomic Theory 54
q1
q2
(q1,q2 ) * *
q1(q2)
q2(q1)ฮฑ2ฮฒ
ฮฑฮณ
ฮฑ2ฮฒ
ฮฑฮณ
Product Differentiation: Cournot Model
โข Comparative statics of firm ๐โs BRF
โ As ๐ฝ โ ๐พ (products become more homogeneous), BRF becomes steeper. That is, the profit-maximizing choice of ๐๐ is more sensitive to changes in ๐๐ (tougher competition)
โ As ๐พ โ 0 (products become very differentiated), firm ๐โs BRF no longer depends on ๐๐ and becomes
flat (milder competition)
Advanced Microeconomic Theory 55
Product Differentiation: Cournot Model
โข Simultaneously solving the two BRF yields
๐๐โ =
๐ผ
2๐ฝ + ๐พfor all ๐ = {1,2}
with a corresponding equilibrium price of
๐๐โ = ๐ผ โ ๐ฝ๐๐
โ โ ๐พ๐๐โ =
๐ผ๐ฝ
2๐ฝ + ๐พ
and equilibrium profits of
๐๐โ = ๐๐
โ๐๐โ =
๐ผ๐ฝ
2๐ฝ + ๐พ
๐ผ
2๐ฝ + ๐พ=
๐ผ2๐ฝ
2๐ฝ + ๐พ 2
Advanced Microeconomic Theory 56
Product Differentiation: Cournot Model
โข Note:
โ As ๐พ increases (products become more homogeneous), individual and aggregate output decrease, and individual profits decrease as well.
โ If ๐พ โ ๐ฝ (indicating undifferentiated products), then
๐๐โ =
๐ผ
2๐ฝ+๐ฝ=
๐ผ
3๐ฝas in standard Cournot models of
homogeneous products.
โ If ๐พ โ 0 (extremely differentiated products), then
๐๐โ =
๐ผ
2๐ฝ+0=
๐ผ
2๐ฝas in monopoly.
Advanced Microeconomic Theory 57
Dynamic Competition
Advanced Microeconomic Theory 58
Dynamic Competition: Sequential Bertrand Model with Homogeneous Products
โข Assume that firm 1 chooses its price ๐1 first, whereas firm 2 observes that price and responds with its own price ๐2.
โข Since the game is a sequential-move game (rather than a simultaneous-move game), we should use backward induction.
Advanced Microeconomic Theory 59
Dynamic Competition: Sequential Bertrand Model with Homogeneous Products
โข Firm 2 (the follower) has a BRF given by
๐2(๐1) = แ๐1 โ ๐ if ๐1 > ๐๐ if ๐1 โค ๐
while firm 1โs (the leaderโs) BRF is ๐1 = ๐
โข Intuition: the follower undercuts the leaderโs price ๐1 by a small ๐ > 0 if ๐1 > ๐, or keeps it at ๐2 = ๐ if the leader sets ๐1 = ๐.
Advanced Microeconomic Theory 60
Dynamic Competition: Sequential Bertrand Model with Homogeneous Products
โข The leader expects that its price will be:โ undercut by the follower when ๐1 > ๐ (thus yielding
no sales)โ mimicked by the follower when ๐1 = ๐ (thus entailing
half of the market share)
โข Hence, the leader has (weak) incentives to set a price ๐1 = ๐.
โข As a consequence, the equilibrium price pair remains at (๐1
โ, ๐2โ) = (๐, ๐), as in the
simultaneous-move version of the Bertrand model.
Advanced Microeconomic Theory 61
Dynamic Competition: Sequential Bertrand Model with Heterogeneous Products
โข Assume that firms sell differentiated products, where firm ๐โs demand is
๐๐ = ๐ท๐(๐๐ , ๐๐)
โ Example: ๐๐(๐๐, ๐๐) = ๐ โ ๐๐๐ + ๐๐๐, where ๐, ๐, ๐ >0 and ๐ > ๐
โข In the second stage, firm 2 (the follower) solves following PMP
max๐2โฅ0
๐2 = ๐2๐2 โ ๐๐ถ(๐2)
= ๐2๐ท2(๐2, ๐1) โ ๐๐ถ(๐ท2(๐2, ๐1)๐2
)
Advanced Microeconomic Theory 62
Dynamic Competition: Sequential Bertrand Model with Heterogeneous Products
โข FOCs wrt ๐2 yield
๐ท2(๐2, ๐1) + ๐2
๐๐ท2(๐2, ๐1)
๐๐2
โ๐๐๐ถ ๐ท2(๐2, ๐1)
๐๐ท2(๐2, ๐1)
๐๐ท2(๐2, ๐1)
๐๐2
Using the chain rule
= 0
โข Solving for ๐2 produces the followerโs BRF for every price set by the leader, ๐1, i.e., ๐2(๐1).
Advanced Microeconomic Theory 63
Dynamic Competition: Sequential Bertrand Model with Heterogeneous Products
โข In the first stage, firm 1 (leader) anticipates that the follower will use BRF ๐2(๐1) to respond to each possible price ๐1, hence solves following PMP
max๐1โฅ0
๐1 = ๐1๐1 โ ๐๐ถ ๐1
= ๐1๐ท1 ๐1, ๐2 ๐1
๐ต๐ ๐น2
โ ๐๐ถ ๐ท1 ๐1, ๐2(๐1)
๐1
Advanced Microeconomic Theory 64
Dynamic Competition: Sequential Bertrand Model with Heterogeneous Products
โข FOCs wrt ๐1 yield
๐ท1(๐1, ๐2) + ๐1
๐๐ท1(๐1, ๐2)
๐๐1+
๐๐ท1(๐1, ๐2)
๐๐2(๐1)
๐๐2(๐1)
๐๐1
New Strategic Effect
โ๐๐๐ถ ๐ท1(๐1, ๐2)
๐๐ท1(๐1, ๐2)
๐๐ท1(๐1, ๐2)
๐๐1+
๐๐ท1(๐1, ๐2)
๐๐2(๐1)
๐๐2(๐1)
๐๐1
New Strategic Effect
= 0
โข Or more compactly as
๐ท1(๐1, ๐2) + ๐1 โ๐๐๐ถ ๐ท1(๐1, ๐2)
๐๐ท1(๐1, ๐2)
๐๐ท1(๐1, ๐2)
๐๐11 +
๐๐2(๐1)
๐๐1
๐๐๐ค
= 0
Advanced Microeconomic Theory 65
Dynamic Competition: Sequential Bertrand Model with Heterogeneous Products
โข In contrast to the Bertrand model with simultaneous price competition, an increase in firm 1โs price now produces an increase in firm 2โs price in the second stage.
โข Hence, the leader has more incentives to raise its price, ultimately softening the price competition.
โข While a softened competition benefits both the leader and the follower, the real beneficiary is the follower, as its profits increase more than the leaderโs.
Advanced Microeconomic Theory 66
Dynamic Competition: Sequential Bertrand Model with Heterogeneous Products
โข Example: โ Consider a linear demand ๐๐ = 1 โ 2๐๐ + ๐๐, with no
marginal costs, i.e., ๐ = 0.
โ Simultaneous Bertrand model: the PMP ismax๐๐โฅ0
๐๐ = ๐๐ โ (1 โ 2๐๐ + ๐๐) for any ๐ โ ๐
where FOC wrt ๐๐ produces firm ๐โs BRF
๐๐(๐๐) =1
4+
1
4๐๐
โ Simultaneously solving the two BRFs yields ๐๐โ =
1
3โ
0.33, entailing equilibrium profits of ๐๐โ =
2
9โ 0.222.
Advanced Microeconomic Theory 67
Dynamic Competition: Sequential Bertrand Model with Heterogeneous Products
โข Example (continued):
โ Sequential Bertrand model: in the second stage, firm 2โs (the followerโs) PMP is
max๐2โฅ0
๐2 = ๐2 โ 1 โ 2๐2 โ ๐1
where FOC wrt ๐2 produces firm 2โs BRF
๐2(๐1) =1
4+
1
4๐1
โ In the first stage, firm 1โs (the leaderโs) PMP is
max๐1โฅ0
๐1 = ๐1 โ 1 โ 2๐1 +1
4+
1
4๐1
๐ต๐ ๐น2
= ๐1 โ1
4(5 โ 7๐1)
Advanced Microeconomic Theory 68
Dynamic Competition: Sequential Bertrand Model with Heterogeneous Products
โข Example (continued):
โ FOC wrt ๐1, and solving for ๐1, produces firm 1โs
equilibrium price ๐1โ =
5
14= 0.36.
โ Substituting ๐1โ into the BRF of firm 2 yields
๐2โ 0.36 =
1
4+
1
40.36 = 0.34.
โ Equilibrium profits are hence
๐1โ = 0.36
1
45 โ 7 0.36 = 0.223 for firm 1
๐2โ = 0.34 1 โ 2 0.34 + 0.36 = 0.230 for firm 2
Advanced Microeconomic Theory 69
p1
p2
p1(p2)
p2(p1)
โ
ยผ
ยผ
Prices with sequential price competition
0.36
0.34โ
Prices with simultaneous price competition
Dynamic Competition: Sequential Bertrand Model with Heterogeneous Products
โข Example (continued):โ Both firmsโ prices and
profits are higher in the sequential than in the simultaneous game.
โ However, the follower earns more than the leader in the sequential game! (second moverโs advantage)
Advanced Microeconomic Theory 70
Dynamic Competition: Sequential Cournot Model with Homogenous Products
โข Stackelberg model: firm 1 (the leader) chooses output level ๐1, and firm 2 (the follower) observing the output decision of the leader, responds with its own output ๐2(๐1).
โข By backward induction, the followerโs BRF is ๐2(๐1) for any ๐1.
โข Since the leader anticipates ๐2(๐1) from the follower, the leaderโs PMP is
max๐1โฅ0
๐ ๐1 + ๐2(๐1)๐ต๐ ๐น2
๐1 โ ๐๐ถ1(๐1)
Advanced Microeconomic Theory 71
Dynamic Competition: Sequential Cournot Model with Homogenous Products
โข FOCs wrt ๐1 yields
๐ ๐1 + ๐2(๐1) + ๐โฒ ๐1 + ๐2(๐1) ๐1 +๐๐2(๐1)
๐๐1๐1
โ๐๐๐ถ1(๐1)
๐๐1= 0
or more compactly
๐ ๐ + ๐โฒ ๐ ๐1 + ๐โฒ ๐๐๐2(๐1)
๐๐1๐1
Strategic Effect
โ๐๐๐ถ1 ๐1
๐๐1= 0
โข This FOC coincides with that for standard Cournot model with simultaneous output decisions, except for the strategic effect.
Advanced Microeconomic Theory 72
Dynamic Competition: Sequential Cournot Model with Homogenous Products
โข The strategic effect is positive since ๐โฒ(๐) < 0and
๐๐2(๐1)
๐๐1< 0.
โข Firm 1 (the leader) has more incentive to raise ๐1relative to the Cournot model with simultaneous output decision.
โข Intuition (first-mover advantage): โ By overproducing, the leader forces the follower to
reduce its output ๐2 by the amount ๐๐2(๐1)
๐๐1.
โ This helps the leader sell its production at a higher price, as reflected by ๐โฒ(๐); ultimately earning a larger profit than in the standard Cournot model.
Advanced Microeconomic Theory 73
Dynamic Competition: Sequential Cournot Model with Homogenous Products
โข Example:โ Consider linear inverse demand ๐ = ๐ โ ๐, where
๐ = ๐1 + ๐2, and a constant marginal cost of ๐.
โ Firm 2โs (the followerโs) PMP is
max๐2
(๐ โ ๐1 โ ๐2)๐2 โ ๐๐2
โ FOC:
๐ โ ๐1 โ 2๐2 โ ๐ = 0
โ Solving for ๐2 yields the followerโs BRF
๐2 ๐1 =๐โ๐1โ๐
2
Advanced Microeconomic Theory 74
Dynamic Competition: Sequential Cournot Model with Homogenous Products
โข Example (continued):โ Plugging ๐2 ๐1 into the leaderโs PMP, we get
max๐1
๐ โ ๐1 โ๐โ๐1โ๐
2๐1 โ ๐๐1 =
1
2(๐ โ ๐1 โ ๐)
โ FOC:1
2๐ โ 2๐1 โ ๐ = 0
โ Solving for ๐1, we obtain the leaderโs equilibrium output level ๐1
โ =๐โ๐
2.
โ Substituting ๐1โ into the followerโs BRF yields the
followerโs equilibrium output ๐2โ =
๐โ๐
4.
Advanced Microeconomic Theory 75
Dynamic Competition: Sequential Cournot Model with Homogenous Products
Advanced Microeconomic Theory 76
q1
q2
a โ c
q1(q2)
2
q2(q1)
a โ c2
Cournot Quantities
Stackelberg Quantities
Dynamic Competition: Sequential Cournot Model with Homogenous Products
โข Example (continued):
โ The equilibrium price is
๐ = ๐ โ ๐1โ โ ๐2
โ =๐ + 3๐
4โ And the resulting equilibrium profits are
๐1โ =
๐+3๐
4
๐โ๐
2โ ๐
๐โ๐
2=
๐โ๐ 2
8
๐2โ =
๐+3๐
4
๐โ๐
4โ ๐
๐โ๐
4=
๐โ๐ 2
16
Advanced Microeconomic Theory 77
Price
a
a + c2
a โ c2b
Monopoly
Units
a + 2c3
a + 3c4
2(a โ c)3b
3(a โ c)4b
a โ cb
ab
pm =
pCournot =
pStackelberg =
pP.C. =pBertrand = c
Cournot
Stackelberg
Bertrand and Perfect Competition
Dynamic Competition: Sequential Cournot Model with Homogenous Products
โข Linear inverse demand๐ ๐ = ๐ โ ๐
โข Symmetric marginal costs ๐ > 0
Advanced Microeconomic Theory 78
Dynamic Competition: Sequential Cournot Model with Heterogeneous Products
โข Assume that firms sell differentiated products, with inverse demand curves for firms 1 and 2
๐1(๐1, ๐2) = ๐ผ โ ๐ฝ๐1 โ ๐พ๐2 for firm 1
๐2(๐1, ๐2) = ๐ผ โ ๐พ๐1 โ ๐ฝ๐2 for firm 2
โข Firm 2โs (the followerโs) PMP is
max๐2
(๐ผ โ ๐พ๐1 โ ๐ฝ๐2) โ ๐2
where, for simplicity, we assume no marginal costs.
โข FOC:๐ผ โ ๐พ๐1 โ 2๐ฝ๐2 = 0
Advanced Microeconomic Theory 79
Dynamic Competition: Sequential Cournot Model with Heterogeneous Products
โข Solving for ๐2 yields firm 2โs BRF
๐2(๐1) =๐ผโ๐พ๐1
2๐ฝ
โข Plugging ๐2 ๐1 into the leaderโs firm 1โs (the leaderโs) PMP, we get
max๐1
๐ผ โ ๐ฝ๐1 โ ๐พ๐ผโ๐พ๐1
2๐ฝ๐1 =
max๐1
๐ผ2๐ฝโ๐พ
2๐ฝโ
2๐ฝ2โ๐พ2
2๐ฝ๐1 ๐1
โข FOC:
๐ผ2๐ฝโ๐พ
2๐ฝโ
2๐ฝ2โ๐พ2
๐ฝ๐1 = 0
Advanced Microeconomic Theory 80
Dynamic Competition: Sequential Cournot Model with Heterogeneous Products
โข Solving for ๐1, we obtain the leaderโs equilibrium output level ๐1
โ =๐ผ(2๐ฝโ๐พ)
2(2๐ฝ2โ๐พ2)
โข Substituting ๐1โ into the followerโs BRF yields the
followerโs equilibrium output
๐2โ =
๐ผโ๐พ๐1โ
2๐ฝ=
๐ผ(4๐ฝ2โ2๐ฝ๐พโ๐พ2)
4๐ฝ(2๐ฝ2โ๐พ2)
โข Note:
โ ๐1โ > ๐2
โ
โ If ๐พ โ ๐ฝ (i.e., the products become more homogeneous), (๐1
โ, ๐2โ) convege to the standard Stackelberg values.
โ If ๐พ โ 0 (i.e., the products become very differentiated), (๐1
โ, ๐2โ) converge to the monopoly output ๐๐ =
๐ผ
2๐ฝ.
Advanced Microeconomic Theory 81
Capacity Constraints
Advanced Microeconomic Theory 82
Capacity Constraints
โข How come are equilibrium outcomes in the standard Bertrand and Cournot models so different?
โข Do firms really compete in prices without facing capacity constraints? โ Bertrand model assumes a firm can supply infinitely
large amount if its price is lower than its rivals.
โข Extension of the Bertrand model:โ First stage: firms set capacities, เดค๐1 and เดค๐2, with a cost
of capacity ๐ > 0โ Second stage: firms observe each otherโs capacities
and compete in prices, simultaneously setting ๐1 and ๐2
Advanced Microeconomic Theory 83
Capacity Constraints
โข What is the role of capacity constraint?โ When a firmโs price is lower than its capacity, not all
consumers can be served.โ Hence, sales must be rationed through efficient
rationing: the customers with the highest willingness to pay get the product first.
โข Intuitively, if ๐1 < ๐2 and the quantity demanded at ๐1 is so large that ๐(๐1) > เดค๐1, then the first เดค๐1units are served to the customers with the highest willingness to pay (i.e., the upper segment of the demand curve), while some customers are left in the form of residual demand to firm 2.
Advanced Microeconomic Theory 84
p
q
p2
p1
Q(p2) Q(p1)
Q(p)
q1, firm 1's capacity
q1 Unserved customers by firm 1
These units become residual demand for firm 2.
Q2(p2) โ q1
1st
2nd
3rd
4th
5th
6th
Capacity Constraints
Advanced Microeconomic Theory 85
โข At ๐1 the quantity demanded is ๐(๐1), but only เดค๐1 units can be served.
โข Hence, the residual demand is ๐(๐1) โเดค๐1.
โข Since firm 2 sets a price of ๐2, its demand will be ๐(๐2).
โข Thus, a portion of the residual demand , i.e., ๐(๐2) โ เดค๐1, is captured.
Capacity Constraints
โข Hence, firm 2โs residual demand can be expressed as
แ๐ ๐2 โ เดค๐1 if ๐ ๐2 โ เดค๐1 โฅ 0
0 otherwise
โข Should we restrict เดค๐1 and เดค๐2 somewhat?
โ Yes. A firm will never set a huge capacity if such capacity entails negative profits, independently of the decision of its competitor.
Advanced Microeconomic Theory 86
Capacity Constraints
โข How to express this rather obvious statement with a simple mathematical condition?โ The maximal revenue of a firm under monopoly is
max๐
(๐ โ ๐)๐, which is maximized at ๐ =๐
2, yielding
profits of ๐2
4.
โ Maximal revenues are larger than costs if ๐2
4โฅ ๐ เดค๐๐, or
solving for เดค๐๐,๐2
4๐โฅ เดค๐๐.
โ Intuitively, the capacity cannot be too high, as otherwise the firm would not obtain positive profits regardless of the opponentโs decision.
Advanced Microeconomic Theory 87
Capacity Constraints: Second Stage
โข By backward induction, we start with the second stage (pricing game), where firms simultaneously choose prices ๐1 and ๐2 as a function of the capacity choices เดค๐1 and เดค๐2.
โข We want to show that in this second stage, both firms set a common price
๐1 = ๐2 = ๐โ = ๐ โ เดค๐1 โ เดค๐2
where demand equals supply, i.e., total capacity,
๐โ = ๐ โ เดค๐, where เดค๐ โก เดค๐1 + เดค๐2
Advanced Microeconomic Theory 88
Capacity Constraints: Second Stage
โข In order to prove this result, we start by assuming that firm 1 sets ๐1 = ๐โ. We now need to show that firm 2 also sets ๐2 = ๐โ, i.e., it does not have incentives to deviate from ๐โ.
โข If firm 2 does not deviate, ๐1 = ๐2 = ๐โ, then it sells up to its capacity เดค๐2.
โข If firm 2 reduces its price below ๐โ, demand would exceed its capacity เดค๐2. As a result, firm 2 would sell the same units as before, เดค๐2, but at a lower price.
Advanced Microeconomic Theory 89
Capacity Constraints: Second Stage
โข If, instead, firm 2 charges a price above ๐โ, then ๐1 = ๐โ < ๐2 and its revenues become
๐2๐(๐2) = แ
๐2(๐ โ ๐2 โ เดค๐1) if ๐ โ ๐2 โ เดค๐1 โฅ 00 otherwise
โข Note: โ This is fundamentally different from the standard
Bertrand model without capacity constraints, where an increase in price by a firm reduces its sales to zero.
โ When capacity constraints are present, the firm can still capture a residual demand, ultimately raising its revenues after increasing its price.
Advanced Microeconomic Theory 90
Capacity Constraints: Second Stage
โข We now find the maximum of this revenue function. FOC wrt ๐2 yields:
๐ โ 2๐2 โ เดค๐1 = 0 โบ ๐2 =๐ โ เดค๐1
2โข The non-deviating price ๐โ = ๐ โ เดค๐1 โ เดค๐2 lies above
the maximum-revenue price ๐2 =๐โ เดค๐1
2when
๐ โ เดค๐1 โ เดค๐2 >๐ โ เดค๐1
2โบ ๐ > เดค๐1 + 2เดค๐2
โข Since ๐2
4๐โฅ เดค๐๐ (capacity constraint), we can obtain
๐2
4๐+ 2
๐2
4๐> เดค๐1 + 2เดค๐2 โ
3๐2
4๐> เดค๐1 + 2เดค๐2
Advanced Microeconomic Theory 91
Capacity Constraints: Second Stage
โข Therefore, ๐ > เดค๐1 + 2เดค๐2 holds if ๐ >3๐2
4๐which,
solving for ๐, is equivalent to 4๐
3> ๐.
Advanced Microeconomic Theory 92
Capacity Constraints: Second Stage
โข When 4๐
3> ๐ holds,
capacity constraint ๐2
4๐โฅ
เดค๐๐ transforms into 3๐2
4๐>
เดค๐1 + 2เดค๐2, implying ๐โ >
๐2 = ๐ โเดค๐1
2.
โข Thus, firm 2 does not have incentives to increase its price ๐2 from ๐โ, since that would lower its revenues.
Advanced Microeconomic Theory 93
Capacity Constraints: Second Stage
โข In short, firm 2 does not have incentives to deviate from the common price
๐โ = ๐ โ เดค๐1 โ เดค๐2
โข A similar argument applies to firm 1 (by symmetry).
โข Hence, we have found an equilibrium in the pricing stage.
Advanced Microeconomic Theory 94
Capacity Constraints: First Stage
โข In the first stage (capacity setting), firms simultaneously select their capacities เดค๐1 and เดค๐2.
โข Inserting stage 2 equilibrium prices, i.e.,
๐1 = ๐2 = ๐โ = ๐ โ เดค๐1 โ เดค๐2,
into firm ๐โs profit function yields๐๐(เดค๐1, เดค๐2) = (๐ โ เดค๐1 โ เดค๐2)
๐โ
เดค๐๐ โ ๐ เดค๐๐
โข FOC wrt capacity เดค๐๐ yields firm ๐โs BRF
เดค๐๐(เดค๐๐) =๐ โ ๐
2โ
1
2เดค๐๐
Advanced Microeconomic Theory 95
Capacity Constraints: First Stage
โข Solving the two BRFs simultaneously, we obtain a symmetric solution
เดค๐๐ = เดค๐๐ =๐ โ ๐
3
โข These are the same equilibrium predictions as those in the standard Cournot model.
โข Hence, capacities in this two-stage game coincide with output decisions in the standard Cournot model, while prices are set equal to total capacity.
Advanced Microeconomic Theory 96
Endogenous Entry
Advanced Microeconomic Theory 97
Endogenous Entry
โข So far the number of firms was exogenous
โข What if the number of firms operating in a market is endogenously determined?
โข That is, how many firms would enter an industry where
โ They know that competition will be a la Cournot
โ They must incur a fixed entry cost ๐น > 0.
Advanced Microeconomic Theory 98
Endogenous Entry
โข Consider inverse demand function ๐(๐), where ๐denotes aggregate output
โข Every firm ๐ faces the same total cost function, ๐(๐๐), of producing ๐๐ units
โข Hence, the Cournot equilibrium must be symmetricโ Every firm produces the same output level ๐(๐), which is a
function of the number of entrants.
โข Entry profits for firm ๐ are
๐๐ ๐ = ๐ ๐ โ ๐ ๐๐
๐(๐)
๐ ๐ โ ๐ ๐ ๐
Production Costs
โ เธ๐นFixed Entry Cost
Advanced Microeconomic Theory 99
Endogenous Entry
โข Three assumptions (valid under most demand and cost functions):
โ individual equilibrium output ๐(๐) is decreasing in ๐;
โ aggregate output ๐ โก ๐ โ ๐(๐) increases in ๐;
โ equilibrium price ๐(๐ โ ๐(๐)) remains above marginal costs regardless of the number of entrants ๐.
Advanced Microeconomic Theory 100
Endogenous Entry
โข Equilibrium number of firms:
โ The equilibrium occurs when no more firms have incentives to enter or exit the market, i.e., ๐๐(๐๐) = 0.
โ Note that individual profits decrease in ๐, i.e.,
๐โฒ ๐ = ๐ ๐๐ ๐ โ ๐โฒ ๐ ๐
+๐๐(๐)
๐๐
โ
+ ๐ ๐ ๐โฒ ๐๐ ๐
โ
๐[๐๐ ๐ ]
๐๐+
< 0
Advanced Microeconomic Theory 101
Endogenous Entry
โข Social optimum:
โ The social planner chooses the number of entrants ๐๐ that maximizes social welfare
max๐
๐ ๐ โก เถฑ0
๐๐(๐)
๐ ๐ ๐๐ โ ๐ โ ๐ ๐ ๐ โ ๐ โ ๐น
Advanced Microeconomic Theory 102
p
Q
p(n q(n))
p(Q)
n c (q(n))
n c (q)A
B
C
D
n q(n)
Endogenous Entry
โข 0
๐๐(๐)๐ ๐ ๐๐ =
๐ด + ๐ต + ๐ถ + ๐ท
โข ๐ โ ๐ ๐ ๐ =
๐ถ + ๐ท
โข Social welfare is thus ๐ด + ๐ต minus total entry costs ๐ โ ๐น
Advanced Microeconomic Theory 103
Endogenous Entry
โ FOC wrt ๐ yields
๐ ๐๐ ๐ ๐๐๐ ๐
๐๐+ ๐ ๐ โ ๐ ๐ ๐ โ ๐๐โฒ ๐ ๐
๐๐ ๐
๐๐โ ๐น = 0
or, re-arranging,
๐ ๐ + ๐ ๐ ๐๐ ๐ โ ๐โฒ ๐ ๐๐๐(๐)
๐๐= 0
โ Hence, marginal increase in ๐ entails two opposite effects on social welfare:
a) the profits of the new entrant increase social welfare (+, appropriability effect)
b) the entrant reduces the profits of all previous incumbents in the industry as the individual sales of each firm decreases upon entry (-, business stealing effect)
Advanced Microeconomic Theory 104
Endogenous Entry
โข The โbusiness stealingโ effect is represented by:
๐ ๐ ๐๐ ๐ โ ๐โฒ ๐ ๐๐๐(๐)
๐๐< 0
which is negative since ๐๐(๐)
๐๐< 0 and
๐ ๐ ๐๐ ๐ โ ๐โฒ ๐ ๐ > 0 by definition.
โข Therefore, an additional entry induces a
reduction in aggregate output by ๐๐๐(๐)
๐๐, which in
turn produces a negative effect on social welfare.
Advanced Microeconomic Theory 105
Endogenous Entry
โข Given the negative sign of the business stealing effect, we can conclude that
๐โฒ ๐ = ๐ ๐ + ๐ ๐ ๐๐ ๐ โ ๐โฒ ๐ ๐๐๐ ๐
๐๐โ
< ๐(๐)
and therefore more firms enter in equilibrium than in the social optimum, i.e., ๐๐ > ๐๐.
Advanced Microeconomic Theory 106
Endogenous Entry
Advanced Microeconomic Theory 107
Endogenous Entry
โข Example:
โ Consider a linear inverse demand ๐ ๐ = 1 โ ๐and no marginal costs.
โ The equilibrium quantity in a market with ๐ firms that compete a la Cournot is
๐ ๐ =1
๐+1
โ Letโs check if the three assumptions from above hold.
Advanced Microeconomic Theory 108
Endogenous Entry
โข Example (continued):โ First, individual output decreases with entry
๐๐ ๐
๐๐= โ
1
๐+1 2 < 0
โ Second, aggregate output ๐๐(๐) increases with entry
๐ ๐๐ ๐
๐๐=
1
๐+1 2 > 0
โ Third, price lies above marginal cost for any number of firms
๐ ๐ โ ๐ = 1 โ ๐ โ1
๐+1=
1
๐+1> 0 for all ๐
Advanced Microeconomic Theory 109
Endogenous Entry
โข Example (continued):
โ Every firm earns equilibrium profits of
๐ ๐ =1
๐ + 1
๐(๐)
1
๐ + 1๐(๐)
โ ๐น =1
๐ + 1 2โ ๐น
โ Since equilibrium profits after entry, 1
๐+1 2, is
smaller than 1 even if only one firm enters the industry, ๐ = 1, we assume that entry costs are lower than 1, i.e., ๐น < 1.
Advanced Microeconomic Theory 110
Endogenous Entry
โข Example (continued):
โ Social welfare is
๐ ๐ = เถฑ0
๐๐+1
(1 โ ๐ )๐๐ โ ๐ โ ๐น
= เธฌ๐ โ๐
2 0
๐๐+1
โ ๐ โ ๐น
=๐ ๐ + 2
2
1
๐ + 1
2
โ ๐ โ ๐น
Advanced Microeconomic Theory 111
Endogenous Entry
โข Example (continued):โ The number of firms entering the market in
equilibrium, ๐๐, is that solving ๐ ๐๐ = 0,1
๐๐ + 1 2โ ๐น = 0 โบ ๐๐ =
1
๐นโ 1
whereas the number of firms maximizing social welfare, i.e., ๐๐ solving ๐โฒ ๐๐ = 0,
๐โฒ ๐๐ =1
๐๐ + 1 3= 0 โบ ๐๐ =
13
๐นโ 1
where ๐๐ < ๐๐ for all admissible values of ๐น, i.e., ๐น โ 0,1 .
Advanced Microeconomic Theory 112
Entry costs, F
ne = โ 1 (Equilibrium)1
F ยฝ
no = โ 1 (Soc. Optimal)1
F โ
Number of firms
0
Endogenous Entry
โข Example (continued):
Advanced Microeconomic Theory 113