The centralized channel case. The whole channel supply chain profit function can be rewritten as
$$\begin{aligned} \max _{p,\zeta ,\theta } \pi _\mathrm{SC}=(p-c)(a-bp+\alpha \theta +\beta \zeta )-\frac{1}{2}L\zeta ^2-\frac{1}{2}K\theta ^2. \end{aligned}$$
(9)
The first-order condition
$$\begin{aligned} \frac{ \partial \pi _\mathrm{SC}}{ \partial p}=\beta \zeta +\alpha \theta -bp+a-(p-c)b, \end{aligned}$$
(10)
$$\begin{aligned} \frac{ \partial \pi _\mathrm{SC}}{ \partial \zeta }=p \beta -c \beta -L \zeta \end{aligned}$$
(11)
and
$$\begin{aligned} \frac{ \partial \pi _\mathrm{SC}}{ \partial \theta }=p \alpha -c \alpha -K \theta . \end{aligned}$$
(12)
The second-order condition
$$\begin{aligned}&\frac{ \partial ^2 \pi _\mathrm{SC}}{ \partial p^2}=-2b, \end{aligned}$$
(13)
$$\begin{aligned}&\frac{ \partial ^2 \pi _\mathrm{SC}}{ \partial \zeta ^2}=-L \end{aligned}$$
(14)
and
$$\begin{aligned} \frac{ \partial ^2 \pi _\mathrm{SC}}{ \partial \theta ^2}=-K. \end{aligned}$$
(15)
Moreover, we can also obtain
$$\begin{aligned} \frac{ \partial ^2 \pi _\mathrm{SC}}{ \partial p\partial \zeta }=\beta ,\quad \frac{ \partial ^2 \pi _\mathrm{SC}}{ \partial p\partial \theta }=\alpha ,\quad \frac{ \partial ^2 \pi _\mathrm{SC}}{ \partial \zeta \partial \theta }=0. \end{aligned}$$
Thus, the Hessian matrix is
$$\begin{aligned} \left[ \begin{array}{ccc} -2b &{}\beta &{}\alpha \\ \beta &{} -L&{}0 \\ \alpha &{}0&{}-K\\ \end{array} \right] . \end{aligned}$$
(16)
We substitute Eqs. (
21)–(
22) into the retailer’s profit function (
1) and derive
$$\begin{aligned} \max _{m,\zeta }\pi _\mathrm{R}=\frac{L\zeta ^2\alpha ^2-2bk[mb(c+m)-m(\beta \zeta +a)+L \zeta ^2]}{4Kb-2\alpha ^2}. \end{aligned}$$
By computing the first-order condition
$$\begin{aligned}&\frac{ \partial \pi _\mathrm{R}}{ \partial m}=\frac{bK[a+\beta \zeta -bc-2mb]}{2Kb-\alpha ^2}, \\&\frac{ \partial \pi _\mathrm{R}}{ \partial \zeta }=\frac{L\zeta \alpha ^2+bk(\beta m-2L\zeta )}{2Kb-\alpha ^2}, \end{aligned}$$
and the second-order condition
$$\begin{aligned}&\frac{ \partial ^2 \pi _\mathrm{R}}{ \partial m^2}=\frac{-2Kb^2}{2Kb-\alpha ^2}, \\&\frac{ \partial ^2 \pi _\mathrm{R}}{ \partial \zeta ^2}=\frac{L\alpha ^2-2LbK}{2Kb-\alpha ^2}, \\&\frac{ \partial ^2 \pi _\mathrm{R}}{ \partial m \partial \zeta }=\frac{\beta bK}{2Kb-\alpha ^2}, \end{aligned}$$
we can write the Hessian matrix of
\(\pi _\mathrm{R}\) as
$$\begin{aligned} \left[ \begin{array}{cc} \frac{-2Kb^2}{2Kb-\alpha ^2} &{}\frac{\beta bK}{2Kb-\alpha ^2} \\ \frac{\beta bK}{2Kb-\alpha ^2} &{} \frac{L\alpha ^2-2LbK}{2Kb-\alpha ^2} \\ \end{array} \right] . \end{aligned}$$
(23)
Through computing the Hessian matrix, the retailer’s profit function is joint concave with respect to the decision variables when
\(\frac{b^2K(4LbK-\beta ^2K-2L\alpha ^2)}{(2Kb-\alpha ^2)^2}>0\), i.e.,
\(4LbK-\beta ^2K-2L\alpha ^2>0\). Let the first-order condition be 0. We get
$$\begin{aligned} \zeta ^{\mathrm{D}^{*}}=\frac{\beta K(a-bc)}{4LbK-\beta ^2K-2L\alpha ^2} \end{aligned}$$
and
$$\begin{aligned} m^{\mathrm{D}^{*}}=\frac{L(\alpha ^2 bc+2Kab-a\alpha ^2-2Kb^2c)}{b(4LbK-\beta ^2K-2L\alpha ^2)}. \end{aligned}$$
Then, we have
$$\begin{aligned}&\theta ^{\mathrm{D}^{*}}=\frac{L\alpha (a-bc)}{4LbK-\beta ^2K-2L\alpha ^2}, \\&w^{\mathrm{D}^{*}}=\frac{[L(3bc+a)-c\beta ^2]K-2Lc\alpha ^2}{4LbK-\beta ^2K-2L\alpha ^2}, \\&p^{\mathrm{D}^{*}}=w^{\mathrm{D}^{*}}+m^{\mathrm{D}^{*}}=\frac{KLb^2c-Kb\beta ^2c-L\alpha ^2bc+3KLab-La\alpha ^2}{b(4LbK-\beta ^2K-2L\alpha ^2)}, \\&\pi _\mathrm{R}^{\mathrm{D}^{*}}=\frac{KL(a-bc)^2}{2(4LbK-\beta ^2K-2L\alpha ^2)}, \\&\pi _\mathrm{F}^{\mathrm{D}^{*}}=\frac{KL^2(a-bc)^2(2Kb-\alpha ^2)}{2(4LbK-\beta ^2K-2L\alpha ^2)^2}, \\&\pi _\mathrm{SC}^{\mathrm{D}^{*}}=\frac{KL(-bc+a)^2(6KLb-K\beta ^2-3L\alpha ^2)}{2(4KLb-K\beta ^2-2L\alpha ^2)^2}. \end{aligned}$$
Proof of Proposition2The model with cost sharing. Due to the game structure, we solve for the farmer’s profit function first:
$$\begin{aligned} \max _{w,\theta }\pi _\mathrm{F}=(w-c)q-\frac{1}{2}K(1-\psi )\theta ^2. \end{aligned}$$
The first-order condition
$$\begin{aligned}&\frac{ \partial \pi _\mathrm{F}}{ \partial w}=a+\alpha \theta +\beta \zeta +cb-2bw-bm,\\&\frac{\partial \pi _\mathrm{F}}{ \partial \theta }=(w-c)\alpha -K(1-\psi ) \theta , \end{aligned}$$
and the second-order condition
$$\begin{aligned}&\frac{ \partial ^2 \pi _\mathrm{F}}{ \partial w^2}=-2b,\\&\frac{\partial ^2 \pi _\mathrm{F}}{ \partial \theta ^2}=-K(1-\psi ) ,\\&\frac{ \partial ^2 \pi _\mathrm{F}}{ \partial w\partial \theta }=\alpha . \end{aligned}$$
We can write the Hessian matrix as
$$\begin{aligned} \left[ \begin{array}{cc} -2b &{}\alpha \\ \alpha &{} -K(1-\psi ) \\ \end{array} \right] . \end{aligned}$$
Since
$$\begin{aligned} \frac{\partial ^2 \pi _\mathrm{F}}{ \partial w^2}=-2b<0, \end{aligned}$$
then the Hessian matrix is negative definite if
$$\begin{aligned} K(1-\psi )\cdot 2b -\alpha \cdot \alpha >0. \end{aligned}$$
Thus, the farmer’s profit function is jointly concave in
w and
\(\theta \). Let the first condition be zero, we obtain
$$\begin{aligned}&\theta (\zeta ,m)=-{\frac{\alpha \, \left( \beta \zeta -bc-bm+a \right) }{2\,Kb\psi -2\,Kb+{\alpha }^{2}}}, \end{aligned}$$
(24)
$$\begin{aligned}&w(\zeta ,m)=\frac{K(1-\psi )(bm-bc-\beta \zeta -a)+\alpha ^2c}{2Kb\psi -2Kb+\alpha ^2}. \end{aligned}$$
(25)
Substituting Eqs. (
24–
25) into (
5), we can derive
$$\begin{aligned} \begin{aligned}&\max _{m,\zeta }\pi _\mathrm{R}\\&\quad = m\left( a-b \left( {\frac{ ( ( c-m ) b+\beta \zeta +a)( \psi -1) K+{\alpha }^{2} c}{2\,Kb\psi -2Kb+{\alpha }^{2}}}+m \right) \right. \\&\qquad \left. -\,{\frac{{\alpha }^{2} ( \beta \zeta -bc-bm+a) }{2\,Kb\psi -2\,K b+{\alpha }^{2}}}+\beta \zeta \right) \\&\qquad -\,\frac{1}{2}L{\zeta }^ {2}-\frac{1}{2}{\frac{K{\alpha }^{2} ( \beta \zeta -bc -bm+a ) ^{2}\psi }{ ( 2\,Kb\psi -2\,Kb+{\alpha }^{2}) ^ {2}}}. \end{aligned} \end{aligned}$$
(26)
According to the first-order condition
$$\begin{aligned} \begin{aligned} \frac{\partial \pi _\mathrm{R}}{ \partial m}&=a-b \left( {\frac{ \left( \left( c-m \right) b+\beta \, \left( \zeta \right) +a \right) \left( \psi -1 \right) K+{\alpha }^{2}c}{2\,Kb\psi - 2\,Kb+{\alpha }^{2}}}+m \right) \\&\quad -{\frac{{\alpha }^{2} \left( \beta \, \left( \zeta \right) -bc-bm+a \right) }{2\,Kb\psi -2\,Kb+{\alpha }^{2}} }+\beta \zeta \\&\quad +m \left( -b \left( -{\frac{b \left( \psi -1 \right) K}{2\,Kb\psi -2\,Kb+{\alpha }^{2}}}+1 \right) \right. \\&\quad +\left. {\frac{{ \alpha }^{2}b}{2\,Kb\psi -2\,Kb+{\alpha }^{2}}} \right) \\&\quad +{\frac{K{\alpha }^{2} \left( \beta \, \left( \zeta \right) -bc-bm+a \right) \psi \,b}{ \left( 2\,Kb\psi -2\,Kb+{\alpha }^{2} \right) ^{2}}},\\ \frac{\partial \pi _\mathrm{R}}{ \partial \zeta }&= m\left( -\frac{b\beta (\psi -1)K}{2Kb\psi -2Kb+\alpha ^2}\right. \\&\left. \quad -\frac{\alpha ^2\beta }{2Kb\psi -2Kb+\alpha ^2}+\beta \right) \\&\quad -L\zeta -\frac{K\alpha ^2(\beta \zeta -bc-bm+a)\psi \beta }{(2Kb\psi -2Kb+\alpha ^2)^2}, \end{aligned} \end{aligned}$$
and the second-order condition
$$\begin{aligned} \frac{\partial ^2 \pi _\mathrm{R}}{ \partial m^2}= & {} -2b\left( -\frac{b(\psi -1)K}{2Kb\psi -2Kb+\alpha ^2}+1\right) \\&+\frac{2 \alpha ^2b}{2Kb\psi -2Kb+\alpha ^2}\\&-\frac{K\alpha ^2b^2\psi }{(2Kb\psi -2Kb+\alpha ^2)^2},\\ \frac{\partial ^2 \pi _\mathrm{R}}{ \partial \zeta ^2}= & {} -L-{\frac{K{\alpha }^{2}\beta ^2\psi }{ \left( 2\,Kb\psi +{\alpha }^{2}-2\,Kb \right) ^{2}}},\\ \frac{\partial ^2 \pi _\mathrm{R}}{\partial m\partial \zeta }= & {} -\frac{b\beta (\psi -1)K}{2Kb\psi -2Kb+\alpha ^2}\!-\! \frac{\alpha ^2\beta }{2Kb\psi -2Kb+\alpha ^2}+\beta \\&+\frac{K\alpha ^2\beta \psi b}{(2Kb\psi -2Kb+\alpha ^2)^2}. \end{aligned}$$
Then, the Hessian matrix is negative definite if
$$\begin{aligned}&-2b\left( -\frac{b(\psi -1)K}{2Kb\psi -2Kb+\alpha ^2}+1\right) +\frac{2\alpha ^2b}{2Kb \psi -2Kb+\alpha ^2}\\&\quad -\frac{K\alpha ^2b^2\psi }{(2Kb\psi -2Kb+\alpha ^2)^2}<0 \end{aligned}$$
and
$$\begin{aligned} \frac{b^2K(4KLb\psi ^2-K\beta ^2\psi ^2-8KLb\psi +2K\beta ^2\psi +3L\alpha ^2 \psi +4KLb-\beta ^2K-2L\alpha ^2)}{(2Kb\psi -2Kb+\alpha ^2)^2}>0. \end{aligned}$$
Equivalent to
$$\begin{aligned} -\frac{b^2K(4Kb\psi ^2-8Kb\psi +3\alpha ^2\psi +4Kb-2\alpha ^2)}{(2Kb \psi -2Kb+\alpha ^2)^2}<0 \end{aligned}$$
and
$$\begin{aligned}&4KLb\psi ^2-K\beta ^2\psi ^2-8KLb\psi \\&\quad +2K\beta ^2\psi +3L\alpha ^2 \psi +4KLb-\beta ^2K-2L\alpha ^2>0, \end{aligned}$$
which implies
$$\begin{aligned} 4Kb\psi ^2-8Kb\psi +3\alpha ^2\psi +4Kb-2\alpha ^2>0 \end{aligned}$$
and
$$\begin{aligned} L(4Kb\psi ^2-8Kb\psi +3\alpha ^2\psi +4Kb-2\alpha ^2)-K\beta ^2(\psi -1)^2>0. \end{aligned}$$
By the first-order condition, we have
$$\begin{aligned}&m=\frac{A_4}{b(4KLb\psi ^2-K\beta ^2\psi ^2-8KLb\psi +2K\beta ^2\psi +3L\alpha ^2 \psi +4KLb-K\beta ^2-2L\alpha ^2)}, \\&\zeta =\frac{K\beta (-bc\psi ^2+a\psi ^2+2bc\psi -2a\psi -bc+a)}{4KLb\psi ^2-K\beta ^2\psi ^2-8KLb\psi +2K\beta ^2\psi +3L\alpha ^2\psi +4KLb-K \beta ^2-2L\alpha ^2}. \end{aligned}$$
Substituting
m and
\(\zeta \) into Eq. (
26), we can obtain the retailer’s profit function with respect to
\(\psi \)$$\begin{aligned} \pi _\mathrm{R}(\psi )=\frac{KL(\psi -1)^2(-bc+a)^2}{2(4KLb\psi ^2-K\beta ^2 \psi ^2-8KLb\psi +2K\beta ^2\psi +3L\alpha ^2\psi +4KLb-\beta ^2K-2L\alpha ^2)}. \end{aligned}$$
Then, the first-order condition
$$\begin{aligned} \frac{\mathrm{d}\pi _\mathrm{R}}{\mathrm{d}\psi }=\frac{KL^2(\psi -1)(-bc+a)^2\alpha ^2(3\psi -1)}{2(4KLb \psi ^2-K\beta ^2\psi ^2-8KLb\psi +2K\beta ^2\psi +3L\alpha ^2 \psi +4KLb-\beta ^2K-2L\alpha ^2)^2} \end{aligned}$$
and the second-order condition
$$\begin{aligned} \frac{\mathrm{d}^2\pi _\mathrm{R}}{\mathrm{d}\psi ^2}=\frac{-KL^2(-bc+a)^2\alpha ^2A_9}{(4KLb\psi ^2-K\beta ^2\psi ^2-8KLb\psi +2K\beta ^2\psi +3L\alpha ^2 \psi +4KLb-\beta ^2K-2L\alpha ^2)^3}. \end{aligned}$$
The second-order condition is negative if
\(A_9>0.\) Thus,
\(\pi _\mathrm{R}(\psi )\) is concave in
\(\psi \) for the above condition. Solving the first-order condition for
\(\psi \) gives
\(\psi =\frac{1}{3}\) or
\(\psi =1\).
Bargaining cost sharing contract model. Since the farmer and the retailer decide the
\(\psi \) via bargaining, we formulate the objective function as
$$\begin{aligned} \max _{\psi }\pi _\mathrm{F}\pi _\mathrm{R} \end{aligned}$$
(27)
Due to the game structure, we solve the farmer’s optimization problem first:
$$\begin{aligned} \max _{w,\theta }\pi _\mathrm{F}=(w-c)q-\frac{1}{2}K(1-\psi )\theta ^2 \end{aligned}$$
The first-order condition
$$\begin{aligned}&\frac{ \partial \pi _\mathrm{F}}{ \partial w}=a-b(w+m)+\alpha \theta +\beta \zeta -(w-c)b,\\&\frac{\partial \pi _\mathrm{F}}{ \partial \theta }=(w-c)\alpha -K\theta (1-\psi ), \end{aligned}$$
and the second-order condition
$$\begin{aligned}&\frac{ \partial ^2 \pi _\mathrm{F}}{ \partial w^2}=-2b, \\&\frac{\partial ^2 \pi _\mathrm{F}}{ \partial \theta ^2}=-K(1-\psi ) , \\&\frac{ \partial ^2 \pi _\mathrm{R}}{ \partial p\partial \zeta }=\alpha . \end{aligned}$$
We can write the Hessian matrix as
$$\begin{aligned} \left[ \begin{array}{cc} -2b &{}\alpha \\ \alpha &{} -K(1-\psi ) \\ \end{array} \right] . \end{aligned}$$
(28)
The Hessian matrix is negative definite if
$$\begin{aligned} 2bK(1-\psi )-\alpha ^2 > 0. \end{aligned}$$
Under this assumption, the farmer’s profit function is joint concave in
p and
\(\zeta \). Let the first-order condition be zero and solve equations for
p and
\(\zeta \). We can obtain
$$\begin{aligned} w(m,\zeta )=\frac{K(1-\psi )(bm-bc-\beta \zeta -a)+\alpha ^2c}{2Kb\psi -2Kb+\alpha ^2} \end{aligned}$$
(29)
and
$$\begin{aligned} \theta (\zeta ,m)=-{\frac{\alpha \, \left( \beta \zeta -bc-bm+a \right) }{2\,Kb\psi -2\,Kb+{\alpha }^{2}}}. \end{aligned}$$
(30)
Substituting Eqs. (
29–
30) into the retailer’s profit function, we get
$$\begin{aligned} \max _{m,\zeta }\pi _\mathrm{R}\,=\,& {} m\left( a-b \left( {\frac{ ( ( c-m ) b+\beta \zeta +a)( \psi -1) K+{\alpha }^{2} c}{2\,Kb\psi -2Kb+{\alpha }^{2}}}+m \right) \right. \\&\left. -{\frac{{\alpha }^{2} ( \beta \zeta -bc-bm+a) }{2\,Kb\psi -2\,K b+{\alpha }^{2}}}+\beta \zeta \right) \\&-\frac{1}{2}L{\zeta }^ {2}-\frac{1}{2}{\frac{K{\alpha }^{2} ( \beta \zeta -bc -bm+a ) ^{2}\psi }{ ( 2\,Kb\psi -2\,Kb+{\alpha }^{2}) ^ {2}}}. \end{aligned}$$
The first-order condition
$$\begin{aligned} \frac{\partial \pi _\mathrm{R}}{ \partial m}\,=\, & {} a-b \left( {\frac{ \left( \left( c-m \right) b+\beta \, \left( \zeta \right) +a \right) \left( \psi -1 \right) K+{\alpha }^{2}c}{2\,Kb\psi - 2\,Kb+{\alpha }^{2}}}+m \right) \\&- {\frac{{\alpha }^{2} \left( \beta \, \left( \zeta \right) -bc-bm+a \right) }{2\,Kb\psi -2\,Kb+{\alpha }^{2}} }+\beta \zeta \\&+m \left( -b \left( -{\frac{b \left( \psi -1 \right) K}{2\,Kb\psi -2\,Kb+{\alpha }^{2}}}+1 \right) \right. \\&+\left. {\frac{{ \alpha }^{2}b}{2\,Kb\psi -2\,Kb+{\alpha }^{2}}} \right) \\&+{\frac{K{\alpha }^{2} \left( \beta \, \left( \zeta \right) -bc-bm+a \right) \psi \,b}{ \left( 2\,Kb\psi -2\,Kb+{\alpha }^{2} \right) ^{2}}},\\ \frac{\partial \pi _\mathrm{R}}{ \partial \zeta }= & {} m\left( -\frac{b\beta (\psi -1)K}{2Kb\psi -2Kb+\alpha ^2}\right. \\&\left. - \frac{\alpha ^2\beta }{2Kb\psi -2Kb+\alpha ^2}+\beta \right) -L\zeta \\&-\frac{K\alpha ^2(\beta \zeta -bc-bm+a)\psi \beta }{(2Kb\psi -2Kb+\alpha ^2)^2}, \end{aligned}$$
and the second-order condition
$$\begin{aligned} \frac{\partial ^2 \pi _\mathrm{R}}{ \partial m^2}\,=\, & {} -2b\left( -\frac{b(\psi -1)K}{2Kb\psi -2Kb+\alpha ^2}+1\right) \\&+\frac{2\alpha ^2b}{2Kb\psi -2Kb+\alpha ^2}\\&-\frac{K\alpha ^2b^2\psi }{(2Kb\psi -2Kb+\alpha ^2)^2},\\ \frac{\partial ^2 \pi _\mathrm{R}}{ \partial \zeta ^2}= & {} -L-{\frac{K{\alpha }^{2}\beta ^2\psi }{ \left( 2\,Kb\psi +{\alpha }^{2}-2\,Kb \right) ^{2}}},\\ \frac{\partial ^2 \pi _\mathrm{R}}{ \partial m\partial \zeta }= & {} -\frac{b\beta (\psi -1)K}{2Kb\psi -2Kb+\alpha ^2}\\&- \frac{\alpha ^2\beta }{2Kb\psi -2Kb+\alpha ^2}+\beta +\frac{K\alpha ^2\beta \psi b}{(2Kb\psi -2Kb+\alpha ^2)^2}. \end{aligned}$$
Then, the Hessian matrix is negative definite if
$$\begin{aligned}&-2b\left( -\frac{b(\psi -1)K}{2Kb\psi -2Kb+\alpha ^2}+1\right) +\frac{2\alpha ^2b}{2Kb \psi -2Kb+\alpha ^2}\\&\quad -\frac{K\alpha ^2b^2\psi }{(2Kb\psi -2Kb+\alpha ^2)^2}<0 \end{aligned}$$
and
$$\begin{aligned} \frac{b^2K(4KLb\psi ^2-K\beta ^2\psi ^2-8KLb\psi +2K\beta ^2\psi +3L\alpha ^2 \psi +4KLb-\beta ^2K-2L\alpha ^2)}{(2Kb\psi -2Kb+\alpha ^2)^2}>0. \end{aligned}$$
Equivalent to
$$\begin{aligned} -\frac{b^2K(4Kb\psi ^2-8Kb\psi +3\alpha ^2\psi +4Kb-2\alpha ^2)}{(2Kb \psi -2Kb+\alpha ^2)^2}<0 \end{aligned}$$
and
$$\begin{aligned}&4KLb\psi ^2-K\beta ^2\psi ^2-8KLb\psi \\&\quad +2K\beta ^2\psi +3L\alpha ^2 \psi +4KLb-\beta ^2K-2L\alpha ^2>0 \end{aligned}$$
which implies
$$\begin{aligned} 4Kb\psi ^2-8Kb\psi +3\alpha ^2\psi +4Kb-2\alpha ^2>0 \end{aligned}$$
and
$$\begin{aligned} L(4Kb\psi ^2-8Kb\psi +3\alpha ^2\psi +4Kb-2\alpha ^2)-K\beta ^2(\psi -1)^2>0. \end{aligned}$$
Let the first-order condition be zero and solve equations for
m and
\(\zeta \). We can get
$$\begin{aligned}&m=\frac{A_4}{b(4KLb\psi ^2-K\beta ^2\psi ^2-8KLb\psi +2K\beta ^2\psi +3L\alpha ^2 \psi +4KLb-K\beta ^2-2L\alpha ^2)}, \\&\zeta =\frac{K\beta (-bc\psi ^2+a\psi ^2+2bc\psi -2a\psi -bc+a)}{4KLb\psi ^2-K\beta ^2\psi ^2-8KLb\psi +2K\beta ^2\psi +3L\alpha ^2 \psi +4KLb-K\beta ^2-2L\alpha ^2}. \end{aligned}$$
Substituting
m and
\(\zeta \) into Eqs. (
29–
30) and the Nash bargaining function (
27). By computing the first-order condition and the second-order condition of the bargaining model
$$\begin{aligned} \max _{\psi }\pi _\mathrm{F}\pi _\mathrm{R}. \end{aligned}$$
The first-order condition
$$\begin{aligned} \frac{\mathrm{d}\pi _\mathrm{B}}{\mathrm{d}\psi }=\frac{K^2L^2(\psi -1)^4(-cb+a)^4\alpha ^2A_1}{4(4KLb\psi ^2-K\beta ^2\psi ^2-8KLb\psi +2K\beta ^2\psi +3L\alpha ^2 \psi +4KLb-K\beta ^2-2L\alpha ^2)^4}. \end{aligned}$$
The second-order condition
$$\begin{aligned} \frac{\mathrm{d}^{2} \pi _\mathrm{B}}{\mathrm d\psi ^2}=-A_2\cdot \frac{K^2\alpha ^2L^3(-bc+a)^4}{2(4KLb\psi -2K\beta ^2\psi -4KLb+2K\beta ^2+L\alpha ^2)^5}. \end{aligned}$$
The second-order condition is negative if
\(A_2<0.\) Thus, the objective function is concave in
\(\psi \) for the above condition. Let the first-order condition be 0. We obtain
\(\psi =1\),
$$\begin{aligned} \psi =\frac{8KLb+K\beta ^2-3L\alpha ^2+A_3}{K(14Lb+\beta ^2)}, \end{aligned}$$
or
$$\begin{aligned} \psi =\frac{8KLb+K\beta ^2-3L\alpha ^2-A_3}{K(14Lb+\beta ^2)}. \end{aligned}$$
When
\(\psi =1\),
\(2Kb(1-\psi )-\alpha ^2>0\) can be deduced to
\(-\alpha ^2>0\), which is a contradiction. Meanwhile, if
$$\begin{aligned} \psi =\frac{8KLb+K\beta ^2-3L\alpha ^2+A_3}{K(14Lb+\beta ^2)}, \end{aligned}$$
then the
\(2Kb(1-\psi )-\alpha ^2>0\) is not satisfied. When
$$\begin{aligned} \psi\,=\,& {} \frac{8KLb+K\beta ^2-3L\alpha ^2+A_3}{K(14Lb+\beta ^2)},\\ 2Kb(1-\psi )-\alpha ^2\,=\, & {} \frac{12KLb^2-8L\alpha ^2b-\alpha ^2\beta ^2-2bA_3}{14Lb+\beta ^2}. \end{aligned}$$
If
\(12KLb^2-8L\alpha ^2b-\alpha ^2\beta ^2<0\), then we can get
\(2Kb(1-\psi )-\alpha ^2<0\); if
\(12KLb^2-8L\alpha ^2b-\alpha ^2\beta ^2>0\), we only need to check
\((12KLb^2-8L\alpha ^2b-\alpha ^2\beta ^2)^2-(2bA_3)^2\) is greater than or less than 0. By algebra,
$$\begin{aligned}&(12KLb^2-8L\alpha ^2b-\alpha ^2\beta ^2)^2-(2bA_3)^2\\&\quad =-\alpha ^2(14Lb+ \beta ^2)(4KLb^2-2L\alpha ^2b-\alpha ^2\beta ^2). \end{aligned}$$
The problem transfers to verify whether
\(4KLb^2-2L\alpha ^2b-\alpha ^2\beta ^2\) is greater or less than 0. Since we have
\(2KLb-\beta ^2K-L\alpha ^2>0\) and
\(2Kb-\alpha ^2>0\),
\(2b(2KLb-\beta ^2K-L\alpha ^2)>0\) and
\(4KLb^2-2L\alpha ^2b-2\beta ^2Kb>0\). Owing to
\(2Kb-\alpha ^2>0\), we can derive
\(4KLb^2-2L\alpha ^2b-\alpha ^2\beta ^2>0\). Thus,
\(2Kb(1-\psi )-\alpha ^2<0\) when
$$\begin{aligned} \psi =\frac{8KLb+K\beta ^2-3L\alpha ^2+A_3}{K(14Lb+\beta ^2)}. \end{aligned}$$
Obviously, the
$$\begin{aligned} \psi =\frac{8KLb+K\beta ^2-3L\alpha ^2+A_3}{K(14Lb+\beta ^2)} \end{aligned}$$
should be omitted. Analogously, we can also prove that
\(2Kb(1-\psi )-\alpha ^2>0\) can be satisfied when
$$\begin{aligned} \psi =\frac{8KLb+K\beta ^2-3L\alpha ^2-A_3}{K(14Lb+\beta ^2)}. \end{aligned}$$
Thus, the equilibrium
$$\begin{aligned} \psi ^{\mathrm{B}^{*}}=\frac{8KLb+K\beta ^2-3L\alpha ^2-A_3}{K(14Lb+\beta ^2)}. \end{aligned}$$
The equilibrium decisions and profit functions are
$$\begin{aligned}&\zeta ^{\mathrm{B}^{*}}\,=\,\frac{K\beta (-bc+a)(\psi ^{\mathrm{B}^{*}}-1)^2}{4KLb{\psi ^{\mathrm{B}^{*}}}^2-K\beta ^2{\psi ^{\mathrm{B}^{*}}}^2-8KLb{\psi ^{\mathrm{B}^{*}}}+2K\beta ^2{\psi ^{\mathrm{B}^{*}}}+ 3L\alpha ^2{\psi ^{\mathrm{B}^{*}}}+4KLb-K\beta ^2-2L\alpha ^2}, \\&m^{\mathrm{B}^{*}}\,=\,\frac{A_4}{4KLb{\psi ^{\mathrm{B}^{*}}}^2-K\beta ^2{\psi ^{\mathrm{B}^{*}}}^2-8KLb{\psi ^{\mathrm{B}^{*}}}+2K\beta ^2{\psi ^{\mathrm{B}^{*}}}+ 3L\alpha ^2{\psi ^{\mathrm{B}^{*}}}+4KLb-K\beta ^2-2L\alpha ^2}, \\&\theta ^{\mathrm{B}^{*}}\,=\,\frac{(-bc+a)(1-\psi ^{\mathrm{B}^{*}})}{4KLb{\psi ^{\mathrm{B}^{*}}}^2-K\beta ^2{\psi ^{\mathrm{B}^{*}}}^2-8KLb{\psi ^{\mathrm{B}^{*}}}+2K\beta ^2{\psi ^{\mathrm{B}^{*}}}+ 3L\alpha ^2{\psi ^{\mathrm{B}^{*}}}+4KLb-K\beta ^2-2L\alpha ^2}, \\&w^{\mathrm{B}^{*}}\,=\,\frac{(3KLbc-K\beta ^2c+KLa)(\psi ^{\mathrm{B}^{*}}-1)^2+3L\alpha ^2c(\psi _B^{*}-1)+L\alpha ^2c}{4KLb{\psi ^{\mathrm{B}^{*}}}^2-K\beta ^2{\psi ^{\mathrm{B}^{*}}}^2-8KLb{\psi ^{\mathrm{B}^{*}}}+2K\beta ^2{\psi ^{\mathrm{B}^{*}}}+ 3L\alpha ^2{\psi ^{\mathrm{B}^{*}}}+4KLb-K\beta ^2-2L\alpha ^2}, \\&\pi _\mathrm{R}^{\mathrm{B}^{*}}\,=\,\frac{KL(-bc+a)^2(\psi ^{\mathrm{B}^{*}}-1)^2}{2(4KLb{\psi ^{\mathrm{B}^{*}}}^2-K\beta ^2{\psi ^{\mathrm{B}^{*}}}^2-8KLb{\psi ^{\mathrm{B}^{*}}}+2K\beta ^2{\psi ^{\mathrm{B}^{*}}}+ 3L\alpha ^2{\psi ^{\mathrm{B}^{*}}}+4KLb-K\beta ^2-2L\alpha ^2)}, \\&\pi _\mathrm{F}^{\mathrm{B}^{*}}\,=\,\frac{(\psi ^{\mathrm{B}^{*}}-1)^3(-bc+a)^2(2Kb\psi ^{\mathrm{B}^{*}}-2Kb+\alpha ^2)KL^2}{2(4KLb{\psi ^{\mathrm{B}^{*}}}^2-K\beta ^2{\psi ^{\mathrm{B}^{*}}}^2-8KLb{\psi ^{\mathrm{B}^{*}}}+2K\beta ^2{\psi ^{\mathrm{B}^{*}}}+ 3L\alpha ^2{\psi ^{\mathrm{B}^{*}}}+4KLb-K\beta ^2-2L\alpha ^2)^2}, \\&\pi _\mathrm{SC}^{\mathrm{B}^{*}}\,=\,\frac{(\psi ^{\mathrm{B}^{*}}-1)^2(-bc+a)^2 A_5 KL}{2(4KLb{\psi ^{\mathrm{B}^{*}}}^2-K\beta ^2{\psi ^{\mathrm{B}^{*}}}^2-8KLb{\psi ^{\mathrm{B}^{*}}}+2K\beta ^2{\psi ^{\mathrm{B}^{*}}}+ 3L\alpha ^2{\psi ^{\mathrm{B}^{*}}}+4KLb-K\beta ^2-2L\alpha ^2)^2}. \end{aligned}$$