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The role of FDI along transitional dynamics of the host country in an endogenous growth model

Thanh Tam NGUYEN-HUU EM Normandie Business School, Métis Lab (France). Email: tnguyenhuu@em-normandie.fr. Phone: +33 (0)2 78 34 04 61. Address: EM Normandie (campus Havre), 20 Quai Frissard, 76600 Le Havre, France.    Ngoc-Sang PHAM EM Normandie Business School, Métis Lab (France). Email: npham@em-normandie.fr. Phone: +33 (0)2 50 32 04 08. Address: EM Normandie (campus Caen), 9 Rue Claude Bloch, 14000 Caen, France.
(August 6, 2025)
Abstract

We investigate the role of foreign direct investment (FDI) in the transitional dynamics of host countries by using an optimal growth model. FDI may be beneficial for the host country because local people can work for multinational firms to get a favorable salary. However, if the host country only focuses on FDI, it may face a middle-income trap. We show that if the host country invests in research and development, its economy may have sustained growth. Moreover, in this case, FDI helps the host country only at the first stages of its development process.

Keywords: Optimal growth, FDI, R&D, fixed cost.
JEL Classifications: D15, F23, F4, O3, O4.

1 Introduction

Over the past few decades, opening up to the global economy and attracting foreign direct investment (FDI) have been significant policy priorities in developing countries to promote their economic development. The main argument is that multinational enterprises (MNEs) would boost investment, bring new technologies and (management) skills, and generate FDI spillovers on domestic firms. However, the effects of FDI on the host country’s development are far from clear.

Overall, empirical literature finds that the effect of FDI on the host country’s economic growth is relatively weak (Carkovic and Levine, 2005; Gunby et al., 2017) and the link between FDI and growth varies over time (Bénétrix et al., 2023). More precisely, whether this effect is significant or not depends on local conditions such as the host country’s income levels, institutional strength (Baiashvili and Gattini, 2020), the level of human capital (Li and Liu, 2005), or the development of local financial markets (Alfaro et al., 2004, 2010). For instance, the impact is positive in countries with high development level of human capital or financial markets (Borensztein et al., 1998; Alfaro et al., 2004). Besides, there is an inverted-U sharped connection between the extent of FDI impact and the host country’s income-levels. The effect is larger for low- to middle-income countries, before declining from transition to high-income ones (Baiashvili and Gattini, 2020).111See, for example, Almfraji and Almsafir (2014) for a literature review on the FDI-growth’s relationship.

Despite substantial empirical research on the nexus FDI-growth,222Over the five decades of research on FDI, the link FDI-GDP (economic growth) is the most often investigated. Indeed, 107 of 500 published articles considered in Paul and Feliciano-Cestero (2021) study the impacts of FDI on the host country’s economic growth. there is still a lack of theoretical analysis. Hence, our paper aims to understand the role of FDI along the transitional dynamics of the host economy by investigating fundamental questions:

  • (i)

    What is the optimal strategy of a country receiving FDI?

  • (ii)

    How can FDI help the host country escape the middle-income trap and potentially achieve economic growth in the long run?

We address these questions by using an optimal growth model with FDI and endogenous growth. Let us briefly describe the ingredients of our model. The host country is assumed to be a small open economy with three goods: consumption, physical capital, and new goods. These commodities are freely tradable with the rest of the world. There are two agents: a representative agent of the host country and an MNE. The representative agent has three choices for investment: (1) buy physical capital to produce consumption good, (2) invest in training to improve her/his skills and then work for MNE in order to get a salary, and (3) invest in research and development (R&D) to get innovations. If innovations are good enough, they will improve the productivity of domestic firms.

First, we show that if the host country has a low initial resource and a weak research process efficiency, it should never invest in R&D but focus on FDI. In this case, its economy converges to a steady state, which is higher than that of the economy without FDI.

Second, consider a low-income country so that the country cannot immediately invest in R&D and new technology (because the fixed cost in R&D is so high). In this situation, we prove that if the leverage of new technology is high enough or the country has good potential in R&D, the optimal strategy of the country should be as follows:

  • -

    Stage 1: the country should train specific workers.

  • -

    Stage 2: specific workers will work for the MNE to get a favorable salary and improve the country’s income and capital accumulation.

  • -

    Stage 3: once the country’s income is high enough, it should focus on R&D to create new technology that increases the domestic firms’ total factor productivity (TFP). Thanks to this, its economy may have economic growth in the long run.

Our model also shows that a country may get economic growth in the long run without FDI. Our analyses suggest that FDI only plays a role as a catalyst for the host country’s economic growth, especially in the first stages of the host country’s development process.

This research has two significant contributions to the literature. First, we theoretically advance the understanding of how FDI affects economic growth. The existing literature provides some theoretical models to study the effect of FDI on growth. Looking back to history, Findlay (1978) examines the role of FDI in a dynamic framework by assuming that the sequences of domestic and foreign firms’ capital stocks are determined by a continuous time dynamical system.333This system’s parameters include domestic and foreign firms’ technological efficiency that are exogenous. A key insight in Findlay (1978) is his assumption of the ‘contagion’ effect: the level of efficiency of domestic firms depends on (but is lower than) that of the advanced part of the world. Wang (1990) develops this idea by assuming that there is technology diffusion: the host country’s human capital stock is an increasing function of the ratio of foreign investment to domestically owned capital. By using this modeling of FDI and a two-country model with free capital mobility and exogenous propensities to save, Wang (1990) shows that opening to FDI has beneficial implications for the host country.

Notice, nevertheless, that in Wang (1990), the propensity to save is exogenous. Some other research considers models with endogenous saving rates. In a continuous time model with a continuum of varieties of capital goods,444For this kind of growth models, see Romer (1990), Grossman and Helpman (1991). Borensztein et al. (1998) model FDI as the fraction of varieties produced by foreign firms in the total varieties of products. Under specific setups (Cobb-Douglas production and CRRA utility functions), they compute the rate of growth in the steady state equilibrium, which is an increasing function of the fraction of varieties produced by foreign firms in the total varieties of products. Berthélemy and Démurger (2000) extend Borensztein et al. (1998)’s model by endogenizing the numbers of varieties produced by domestic and foreign firms. As in Borensztein et al. (1998), Berthélemy and Démurger (2000) focus on the steady state equilibrium and compute the growth rate of the host country in the case of Cobb-Douglas production and CRRA utility functions. Using a continuous time product variety-based endogenous growth, Alfaro et al. (2010) study the role of local financial markets in enabling FDI to promote growth through backward linkages.555In Alfaro et al. (2010), the development level of the local financial market is modeled by the difference between the instantaneous borrowing rate and the lending rate. They focus on the balanced growth path and their calibration shows that an increase in FDI leads to higher growth rates in financially developed countries compared to those observed in financially poorly developed ones.

Unlike the above studies, we focus neither on the steady state nor on the balanced growth path with specific functions. Instead, we explore the global and transitional dynamics of the optimal paths in endogenous growth models with FDI and without restrictions on the utility function.666Nguyen-Huu and Pham (2018, 2024) study the nexus between FDI spillovers and industrial policy of a host country in a two-period and an exogenous growth models respectively. However, they do not consider endogenous growth. To be best of our knowledge, we are the first to do so. Consequently, our results seem to be more robust. Moreover, our analyses of transitional dynamics allow us to better understand the optimal strategy for the host countries (as we have explained), while the existing literature does not.

Second, our paper also contributes to the literature on optimal growth with thresholds (see Azariadis and Drazen, 1990; Bruno et al., 2009; Le Van et al., 2010, 2016 among others) and increasing returns (see Romer, 1986; Jones and Manuelli, 1990; Kamihigashi and Roy, 2007 among others). Our added value is to show the role of FDI. We point out that FDI may partially contribute to the capital accumulation of the host country and hence enable the country to overcome the threshold at the first stage of its development process. However, whether a host country can obtain growth in the long run does not depend on FDI but on the local conditions (mainly its innovation capacity and the efficiency of its investment in R&D). From a technical point of view, our analysis is far from trivial because of the presence of both domestic and foreign firms. For instance, the method used in Bruno et al. (2009), Le Van et al. (2010) cannot be directly applied in our model.

The paper is structured as follows. Section 2 introduces an endogenous growth model with FDI. Section 3 investigates the interplay between FDI, R&D, and economic growth of the host country. Section 4 concludes. Formal proofs are presented in the Appendix.

2 An endogenous growth model with FDI

Let us start with a benchmark model in which there is a small open economy with three kinds of goods: consumption, physical capital, and so-called new goods. The consumption good is taken as numéraire. The price (in terms of consumption good) of physical capital is exogenous and denoted by pp.

In each period, there is an MNE in the host country. It produces the new good by using two inputs: physical capital and specific labor. We assume that there is no domestic firm in this sector.

At each date tt, the foreign firm (without market power) chooses Ke,tK_{e,t} units of physical capital and Le,tDL^{D}_{e,t} units of specific labor in order to maximize its profit:

(Ft):πe,t=\displaystyle(F_{t}):\quad\pi_{e,t}= maxKe,t,Le,tD0[pnFte(Ke,t,Le,tD)pKe,twtLe,tD]\displaystyle\max\limits_{K_{e,t},L^{D}_{e,t}\geq 0}\Big{[}p_{n}F^{e}_{t}(K_{e,t},L^{D}_{e,t})-pK_{e,t}-w_{t}L^{D}_{e,t}\Big{]} (1)

where pnp_{n} is the exogenous price (in terms of consumption good) of the new good.

Assumption 1.

We assume that Fte(Ke,t,Le,tD)=AeKe,tαe(Le,tD)1αeF^{e}_{t}(K_{e,t},L^{D}_{e,t})=A_{e}K_{e,t}^{\alpha_{e}}(L^{D}_{e,t})^{1-\alpha_{e}}, where αe(0,1)\alpha_{e}\in(0,1).

There is a representative agent in the host country. Taking prices and wages as given, the agent chooses the allocation of resources to maximize the intertemporal welfare of the whole population.

The host country has three choices for investment at each date tt. First, it can buy Kc,t+1K_{c,t+1} units of physical capital to produce AcKc,t+1αA_{c}K_{c,t+1}^{\alpha} units of the consumption good at period t+1t+1, where α(0,1)\alpha\in(0,1).

Second, it can use Ht+1H_{t+1} units of the consumption good for training to generate AhHt+1αhA_{h}H_{t+1}^{\alpha_{h}} units of specific labor, where αh(0,1)\alpha_{h}\in(0,1). The latter works for the MNE to get a total wage of wt+1AhHt+1αhw_{t+1}A_{h}H_{t+1}^{\alpha_{h}} (units of the consumption good).

The last choice is to invest in R&D to create new technology: If the host country invests Nt+1N_{t+1} units of the consumption good in R&D at period tt, it will obtain bNt+1σbN_{t+1}^{\sigma} units of new technology in period t+1t+1, where bb represents the efficiency of the research process. We assume that σ(0,1)\sigma\in(0,1). The new technologies can improve the old sector’s productivity, but only if the amount of investment in R&D exceeds a critical threshold such that bNt+1σ>x¯bN_{t+1}^{\sigma}>\bar{x}, where x¯>0\bar{x}>0 represents a fixed cost. In this case, the productivity goes up to Ac+a(bNt+1σx¯)A_{c}+a(bN_{t+1}^{\sigma}-\bar{x}) where the parameter aa indicates the efficiency or the leverage of the new technology.777To introduce R&D, we can also write, for example, Ac+γ((Nt+1N)+)σA_{c}+\gamma((N_{t+1}-N^{*})^{+})^{\sigma} instead of Ac+a(bNt+1σx¯)+A_{c}+a(bN_{t+1}^{\sigma}-\bar{x})^{+}. However, the main results have similar insights.

To sum up, the representative agent solves the dynamic growth problem below:

(P):\displaystyle(P):\quad max(ct,Kc,t,Ht,Nt,Le,t)t=0+[t=0+βtu(ct)]\displaystyle\max\limits_{\big{(}c_{t},K_{c,t},H_{t},N_{t},L_{e,t}\big{)}_{t=0}^{+\infty}}\Big{[}\sum\limits_{t=0}^{+\infty}\beta^{t}u(c_{t})\Big{]} (2)

subject to

0\displaystyle 0 ct,Kc,t,Ht,Le,t,Nt\displaystyle\leq c_{t},K_{c,t},H_{t},L_{e,t},N_{t} (3a)
ct+pKc,t+1+Nt+1+Ht+1\displaystyle c_{t}+pK_{c,t+1}+N_{t+1}+H_{t+1} (Ac+a(bNtσx¯)+)Kc,tα+wtLe,t\displaystyle\leq\Big{(}A_{c}+a(bN_{t}^{\sigma}-\bar{x})^{+}\Big{)}K_{c,t}^{\alpha}+w_{t}L_{e,t} (3b)
Le,t\displaystyle L_{e,t} AhHtαh.\displaystyle\leq A_{h}H_{t}^{\alpha_{h}}. (3c)

for every t1t\geq 1. Here, β(0,1)\beta\in(0,1) is a rate of time preference while uu is the instantaneous utility function.

We require the following assumption.

Assumption 2.

The utility function uu is in C1C^{1}, strictly increasing, concave, and u(0)=u^{\prime}(0)=\infty. Assume that Ac>0,Ah>0,α(0,1),αh(0,1)A_{c}>0,A_{h}>0,\alpha\in(0,1),\alpha_{h}\in(0,1).

We assume that ax¯>Aca\bar{x}>A_{c}, implying the fixed cost x¯\bar{x} is not too low.

At initial date, assume that N0=0N_{0}=0 while Kc,0,Le,0>0K_{c,0},L_{e,0}>0 are given.

We provide a formal definition of equilibrium.

Definition 1.

An intertemporal equilibrium is a list (ct,Kt,Ht,Nt,Le,t,Le,tD,Ke,tD,wt)t=0(c_{t},K_{t},H_{t},N_{t},L_{e,t},L^{D}_{e,t},K_{e,t}^{D},w_{t})_{t=0}^{\infty} satisfying 3 conditions:

  • (i)

    Given (wt)t=0(w_{t})_{t=0}^{\infty}, (ct,Kt,Ht,Nt,Le,t)t=0(c_{t},K_{t},H_{t},N_{t},L_{e,t})_{t=0}^{\infty} is a solution of the problem (P)(P),

  • (ii)

    Given wtw_{t}, (Le,tD,Ke,tD)(L^{D}_{e,t},K_{e,t}^{D}) is a solution of the problem (Ft)(F_{t})

  • (iii)

    Labor market clears: Le,tD=Le,tL^{D}_{e,t}=L_{e,t}.

At equilibrium, we have Le,tD=Le,t>0L^{D}_{e,t}=L_{e,t}>0. Hence, the first order conditions of the problem (Ft)(F_{t}) imply that, for every tt:

wt=w:=(αeαe(1αe)1αepnAepαe)11αe.\displaystyle w_{t}=w:=\Big{(}\alpha_{e}^{\alpha_{e}}(1-\alpha_{e})^{1-\alpha_{e}}\frac{p_{n}A_{e}}{p^{\alpha_{e}}}\Big{)}^{\frac{1}{1-\alpha_{e}}}. (4)

Wage ww depends not only on the foreign firm TFP but also on the prices of physical capital and new goods.

Denote St+1=pKc,t+1+Nt+1+Ht+1S_{t+1}=pK_{c,t+1}+N_{t+1}+H_{t+1} the total savings of the host country. In our framework, it is equal to the aggregate investment. By using Equation (4), the problem (P)(P) can be rewritten as follows:

(P):\displaystyle(P^{\prime}): max(ct,St+1)t=0+[t=0+βtu(ct)] subject to: ct,St0,ct+St+1G(St)\displaystyle\quad\max\limits_{(c_{t},S_{t+1})_{t=0}^{+\infty}}\Big{[}\sum\limits_{t=0}^{+\infty}\beta^{t}u(c_{t})\Big{]}\text{ subject to: }c_{t},S_{t}\geq 0,\quad c_{t}+S_{t+1}\leq G(S_{t}) (5)

for any t1t\geq 1, and c0+S1X0,c_{0}+S_{1}\leq X_{0}, where X0AcKc,0α+w0Le,0X_{0}\equiv A_{c}K_{c,0}^{\alpha}+w_{0}L_{e,0} and G(S)G(S) is defined by

(GS):G(S)\displaystyle(G_{S}):G(S) maxKc,N,H{g(Kc,N,H):pKc+N+HS;Kc,N,H0}\displaystyle\equiv\max\limits_{K_{c},N,H}\Big{\{}g(K_{c},N,H):pK_{c}+N+H\leq S;K_{c},N,H\geq 0\Big{\}} (6a)
where g(Kc,N,H)(Ac+a(bNσx¯)+)Kcα+wAhHαh.\displaystyle g(K_{c},N,H)\equiv\Big{(}A_{c}+a(bN^{\sigma}-\bar{x})^{+}\Big{)}K_{c}^{\alpha}+wA_{h}H^{\alpha_{h}}. (6b)

Notice that the function G()G(\cdot) is continuous, strictly increasing and G(0)=0G(0)=0. However, it may be non-concave and non-smooth.

Remark 1.

In the absence of the MNE and R&D, we recover an economy without FDI. In this case, the problem (P)(P) becomes the standard Ramsey optimal growth model with the budget constraint: ct+pKc,t+1AcKc,tαc_{t}+pK_{c,t+1}\leq A_{c}K_{c,t}^{\alpha} t\forall t. We can prove that limtSt=Sa\lim\limits_{t\rightarrow\infty}S_{t}={S}_{a}, where Sa{S}_{a} is defined by Sa1α=αβAc/pα{S}_{a}^{1-\alpha}=\alpha\beta A_{c}/p^{\alpha}.

Let us now consider a case where there is the MNE but no R&D. In this case, the problem (P)(P) becomes

(P1):\displaystyle(P_{1}^{\prime}):\quad max(ct,St+1)t=0+[t=0+βtu(ct)] subject to ct,St0,ct+St+1F(St)\displaystyle\max\limits_{(c_{t},S_{t+1})_{t=0}^{+\infty}}\Big{[}\sum\limits_{t=0}^{+\infty}\beta^{t}u(c_{t})\Big{]}\text{ subject to }c_{t},S_{t}\geq 0,\quad c_{t}+S_{t+1}\leq F(S_{t}) (7)

where the function F(S)F(S) is defined by

F(S)maxpKc+HS,Kc0,H0{AcKcα+wAhHαh}.\displaystyle F(S)\equiv\max\limits_{pK_{c}+H\leq S,K_{c}\geq 0,H\geq 0}\{A_{c}K_{c}^{\alpha}+wA_{h}H^{\alpha_{h}}\}. (8)

We can check that F(S)F(S) is strictly increasing, strictly concave, smooth and satisfies Inada condition F(0)=F^{\prime}(0)=\infty. By using the standard argument in the dynamic programming, we obtain the following result.

Proposition 1.

Assume that there is the MNE, but the country does not invest in R&D. Then, StS_{t} converges to Sb{S}_{b} defined by

βF(Sb)=1.\displaystyle\beta F^{\prime}(S_{b})=1. (9)

Moreover, SbS_{b} increases in Ac,w,AhA_{c},w,A_{h}, and Sb>SaS_{b}>S_{a}.

In a particular case where α=αh\alpha=\alpha_{h}, the value SbS_{b} can be explicitly computed by:

Sb1α=αβA where A((Acpα)11α+(wAh)11α)1α.\displaystyle{S}_{b}^{1-\alpha}=\alpha\beta A\text{ where }A\equiv\big{(}(\dfrac{A_{c}}{p^{\alpha}})^{\frac{1}{1-\alpha}}+(wA_{h})^{\frac{1}{1-\alpha}}\big{)}^{1-\alpha}. (10)

The property Sb>Sa{S}_{b}>{S}_{a} means that with the presence of the MNE, the economy’s investment stock converges to a steady state which is higher than that of the economy without FDI. Moreover, the steady state level SbS_{b} is increasing in the TFP of the old sector, wage, as well as the TFP of the MNE. It implies that the effect of FDI on the steady state output depends on both FDI and the host country’s circumstances. This is consistent with several studies mentioned in the Introduction section.

3 Roles of FDI on the economy’s dynamics

We now investigate the global dynamics of the allocation to explore the role of FDI. We first provide some static analysis (Subsection 3.1) and then global dynamic analysis (Subsection 3.2).

3.1 Static analysis

In this subsection, given the savings SS, we study the optimal allocation of the host country. Formally, we look at the optimization problem (GS)(G_{S}). First, it is easy to see that this problem has a solution. However, since the objective function is not concave, the uniqueness of solutions may not be ensured.

We start our exposition by the following result.

Proposition 2.
  • (i)

    If bSσx¯bS^{\sigma}\leq\bar{x} then at optimum, we have N=0N=0 for any aa.

  • (ii)

    If bSσ>x¯bS^{\sigma}>\bar{x} and [Ac+a((b1σS2+x¯1σ2)σx¯)]1pα(S2x¯1σ2b1σ)α>F(S)\Big{[}A_{c}+{a}\Big{(}\big{(}b^{\frac{1}{\sigma}}\frac{S}{2}+\frac{\bar{x}^{\frac{1}{\sigma}}}{2}\big{)}^{\sigma}-\bar{x}\Big{)}\Big{]}\frac{1}{p^{\alpha}}\Big{(}\frac{S}{2}-\frac{\bar{x}^{\frac{1}{\sigma}}}{2b^{\frac{1}{\sigma}}}\Big{)}^{\alpha}>F(S), then N>0N>0 at optimum.

Proof.

See Appendix A.1. ∎

Point (i) of Proposition 2 indicates that if either the efficiency of the research process or the initial resource is low or the fixed cost is high, the host country may not invest in R&D. Besides, point (ii) implies that the country invests in R&D when aa and bb are high enough (because F(S)F(S) depends neither on aa nor bb).

If we have increasing return to scale (i.e., σ+α1\sigma+\alpha\geq 1), then condition in point (ii) in Proposition 2 is satisfied for any SS high enough. It suggests that the host country should invest in R&D once it is rich enough. We complement Proposition 2 by the following result.

Proposition 3.

Assume that α+σ1\alpha+\sigma\geq 1.

(1) There exists a unique SS^{*} such that: (i) G(S)F(S)=0G(S)-F(S)=0 for any SSS\leq S^{*}, and (ii) G(S)>F(S)G(S)>F(S) and N>0N>0 at optimum for any S>SS>S^{*}.

(2) We also have b(S)σx¯>0b(S^{*})^{\sigma}-\bar{x}>0. Moreover, we have G(S)=F(S)G^{\prime}(S)=F^{\prime}(S) if S<SS<S^{*}, and G(S)=G0(S)>F(S)G^{\prime}(S)=G_{0}^{\prime}(S)>F^{\prime}(S) if S>SS>S^{*}. At S=SS=S^{*}, the left derivative is F(S)F^{\prime}(S^{*}) and the right derivative is G0(S)G_{0}^{\prime}(S^{*}).

Proof.

See Appendix A.1. ∎

Proposition 3 plays a crucial role in our analysis. It is in line with Lemma 3 in Bruno et al. (2009). However, notice that the method used in Bruno et al. (2009), Le Van et al. (2010) cannot be directly applied in our model.888Indeed, their method relies on the set BB defined on page 291 of Bruno et al. (2009). In our model with FDI and ααh\alpha\not=\alpha_{h}, this trick no longer works.

Let us provide a sketch of our proof. First, we introduce functions g0g_{0} and G0G_{0}

g0(Kc,N,H)\displaystyle g_{0}(K_{c},N,H) (Ac+a(bNσx¯))Kcα+wAhHαh\displaystyle\equiv\big{(}A_{c}+a(bN^{\sigma}-\bar{x})\big{)}K_{c}^{\alpha}+wA_{h}H^{\alpha_{h}}
G0(S)\displaystyle G_{0}(S) =max{g0(Kc,N,H):pKc+N+HS;Kc,N,H0;bNσx¯}\displaystyle=\max\limits\{g_{0}(K_{c},N,H):pK_{c}+N+H\leq S;K_{c},N,H\geq 0;bN^{\sigma}\geq\bar{x}\}

Observe that G0(S)G(S)G_{0}(S)\leq G(S). More importantly, we have that

G(S)F(S)\displaystyle G(S)-F(S) =max{F(S),G0(S)}F(S)=max{0,G0(S)F(S)}\displaystyle=\max\limits\{F(S),G_{0}(S)\}-F(S)=\max\limits\{0,G_{0}(S)-F(S)\} (11)

Second, we prove that G0(S)F(S)G_{0}(S)-F(S) is strictly increasing in SS. The value SS^{*} is in fact the unique solution of the equation G0(S)=F(S)G_{0}(S^{*})=F(S^{*}).

3.2 Global dynamic analysis

In this subsection, we explore the global dynamics of equilibrium. First, we have the monotonicity of the savings path (St)(S_{t}).

Proposition 4.

The optimal path (St)t(S_{t})_{t} is monotonic. Moreover, StS_{t} does not converge to zero.

Proof.

See Appendix A.2. ∎

Second, we study the boundedness of the allocation. Let us define the sequence (xt)(x_{t}) as x0=X0,xt+1=F(xt)x_{0}=X_{0},x_{t+1}=F(x_{t}), where the function FF is given in (8). Denote xx^{*} and S¯\bar{S} be uniquely defined by:

F(x)=x and S¯:=max{X0,x}.\displaystyle F(x^{*})=x^{*}\text{ and }\bar{S}:=\max\limits\{X_{0},x^{*}\}. (12)

Notice that xx^{*} and S¯\bar{S} depend on (i) the productivity AcA_{c} and capital elasticity α\alpha of the consumption good sector, (ii) the efficiency of specific labor training Ah,αhA_{h},\alpha_{h}, and (iii) wage ww,999If αh=α\alpha_{h}=\alpha, we can explicitly compute that x=(Acpα)11α+(wAh)11α.x^{*}=(\frac{A_{c}}{p^{\alpha}})^{\frac{1}{1-\alpha}}+(wA_{h})^{\frac{1}{1-\alpha}}. but not on the TFP AdA_{d} of the potential domestic firm in the new sector.

It is important to mention some properties of the function FF and the threshold S¯\bar{S}.

Lemma 1.

(1) F(x)F(x)=xF(x)\leq F(x^{*})=x^{*} for every xxx\leq x^{*} and F(x)xF(x)\leq x for every xxx\geq x^{*}.

(2) In equilibrium, we have StxtS¯S_{t}\leq x_{t}\leq\bar{S} t\forall t

Proof.

See Appendix A.2. ∎

By consequence, we obtain the following result.

Proposition 5 (middle income trap).

If X0AcKc,0α+w0Le,0xX_{0}\equiv A_{c}K_{c,0}^{\alpha}+w_{0}L_{e,0}\leq x^{*} and b(x)σx¯b(x^{*})^{\sigma}\leq\bar{x}, where xx^{*} is defined by (12), then Nt=0N_{t}=0 for any tt. In this case, we have limtSt=Sb\lim\limits_{t\rightarrow\infty}S_{t}=S_{b} (SbS_{b} is defined in Proposition 1).

Proof.

See Appendix A.2. ∎

Proposition 5 indicates that when the host country has both a low initial resource (in the sense that X0AcKc,0α+w0Le,0xX_{0}\equiv A_{c}K_{c,0}^{\alpha}+w_{0}L_{e,0}\leq x^{*}) and a weak research process efficiency (in the sense that b(x)σx¯b(x^{*})^{\sigma}\leq\bar{x}), it never invests in R&D (Nt=0N_{t}=0 for t\forall t). In this case, both savings StS_{t} and the output are bounded from above (this can be viewed as a middle income trap). More precisely, StS_{t} converges to the same value SbS_{b}, defined by (9), as in the economy with FDI but without investment in R&D.

We now study the case under which the economy may grow without bound.

Proposition 6 (convergence and growth with increasing return to scale).

Assume that α+σ1\alpha+\sigma\geq 1, αh+1α2\alpha_{h}+\frac{1}{\alpha}\geq 2, and

βmin(F(S),Γ(a,b,x¯))>1\displaystyle\beta\min\Big{(}F^{\prime}(S^{*}),\Gamma(a,b,\bar{x})\Big{)}>1 (13)
where Γ(a,b,x¯)(αAcpσ)αx¯(1α)(1σ)σa1αb1ασ(1+ασ+(αhwAh(pσ)ασ(αAc)α)11αh1a1α1αhbαhασ(1αh))α\displaystyle\text{where }\Gamma(a,b,\bar{x})\equiv\frac{\big{(}\frac{\alpha A_{c}}{p\sigma}\big{)}^{\alpha}\bar{x}^{\frac{-(1-\alpha)(1-\sigma)}{\sigma}}a^{1-\alpha}b^{\frac{1-\alpha}{\sigma}}}{\Big{(}1+\frac{\alpha}{\sigma}+\Big{(}\frac{\alpha_{h}wA_{h}(p\sigma)^{\alpha}}{\sigma(\alpha A_{c})^{\alpha}}\Big{)}^{\frac{1}{1-\alpha_{h}}}\frac{1}{a^{\frac{1-\alpha}{1-\alpha_{h}}}b^{\frac{\alpha_{h}-\alpha}{\sigma(1-\alpha_{h})}}}\Big{)}^{\alpha}} (14)

Then, for any level of initial resource, we have limtSt=\lim\limits_{t\rightarrow\infty}S_{t}=\infty. Moreover,

limtNtSt=σα+σ,limtpKc,tSt=αα+σ,limtHtSt=0.\displaystyle\lim\limits_{t\rightarrow\infty}\frac{N_{t}}{S_{t}}=\frac{\sigma}{\alpha+\sigma},\quad\lim\limits_{t\rightarrow\infty}\frac{pK_{c,t}}{S_{t}}=\frac{\alpha}{\alpha+\sigma},\quad\lim\limits_{t\rightarrow\infty}\frac{H_{t}}{S_{t}}=0. (15)
Proof.

See Appendix A.2. ∎

Condition (13) ensures that the marginal productivity of function GG is high enough in the sense that βD+G(S)>1\beta D^{+}G(S)>1 S>0\forall S>0, where D+G(S)D^{+}G(S) is the Dini derivative of function GG.101010The Dini derivatives of a function ff are defined by D+f(x)=lim supϵ0f(x+ϵ)f(x)ϵD^{+}f(x)=\limsup\limits_{\epsilon\downarrow 0}\frac{f(x+\epsilon)-f(x)}{\epsilon} and Df(x)=lim infϵ0f(x)f(xϵ)ϵD^{-}f(x)=\liminf\limits_{\epsilon\downarrow 0}\frac{f(x)-f(x-\epsilon)}{\epsilon}. This happens if aa and bb are high enough because the function Γ(a,b,x¯)\Gamma(a,b,\bar{x}) is strictly increasing in aa and bb.

Notice that the conditions given in Proposition 6 do not depend on the initial resource X0AcKc,0α+w0Le,0X_{0}\equiv A_{c}K_{c,0}^{\alpha}+w_{0}L_{e,0} which is less than xx^{*}. So, our theoretical results lead to an interesting implication: Consider a low-income country characterized by condition bX0α<x¯bX_{0}^{\alpha}<\bar{x}. According to Proposition 2, we have N1=0N_{1}=0, i.e., the country cannot immediately improve the local firm TFP. Now, suppose that the leverage of new technology aa is high enough and conditions in Proposition 6 hold. In this case, the country obtains a sustained growth (in the sense that limtSt=\lim_{t\rightarrow\infty}S_{t}=\infty). Moreover, the sequence StS_{t} is increasing in time. According to point (ii) of Proposition 2, there is a date t0t_{0} along the optimal path such that the country should focus on R&D from date t0t_{0} on (i.e., Nt=0N_{t}=0 tt0\forall t\leq t_{0} and Nt>0N_{t}>0 for any t>t0t>t_{0}). Therefore, the optimal strategy of the country should be as follows.

  • -

    First, the country should train specific workers.

  • -

    Second, specific workers will work for the MNE to improve the country’s income and capital accumulation.

  • -

    Third, once the country’s resource is high enough, it should focus on R&D to create new technology that increases the country’s TFP. Hence, its economy may grow faster and converge to a high-income country.

Proposition 6 is related to the economic growth literature with increasing return to scale (Romer, 1986; Jones and Manuelli, 1990; Bruno et al., 2009; Le Van et al., 2010). Our main contribution is to introduce and study the role of FDI in an optimal growth model. In our model, FDI is beneficial to the host country but only at the first stages of its development process. Moreover, the property limtSt=\lim_{t\rightarrow\infty}S_{t}=\infty and condition (15) indicate that in the long run, when the host country’s resource is high enough, it should focus on domestic investment in physical capital and R&D but not on FDI.

Remark 2 (growth without FDI).

It is interesting to note that conditions in Proposition 6 can be satisfied even if Ae=w=0A_{e}=w=0. In other words, a host country may get economic growth in the long run even in the absence of FDI. The key factors for such growth are the efficiency of investment in R&D (parameter bb), the new technology’s leverage on the firm TFP (parameter aa), and increasing return to scale.

In the case of decreasing return to scale, the capital stock may converge to a finite steady state, which is higher than that of an economy described in Proposition 1. Formally, we have the following result.

Proposition 7 (decreasing return to scale).

Let X0X_{0} be such that X0<SbX_{0}<S_{b}. Assume that α+σ<1\alpha+\sigma<1. The optimal path (St)(S_{t}) increasingly converges to a finite value StS_{t} and SdSbS_{d}\geq S_{b}. Moreover, Sd>SbS_{d}>S_{b} if aa and bb are high enough.

Proof.

See Appendix A.2. ∎

So far, we have provided several theoretical results to show the role of FDI on the host country. In general, the host country benefits from FDI. However, the effect of FDI on economic growth depends not only on the nature of FDI but, more importantly, on the circumstances of the host country (initial resources, domestic firms’ TFP, education system, efficiency of R&D process, …). Indeed, look back at Proposition 1, if the host country only focuses on FDI, the steady state SbS_{b}, that is higher than the steady state of the economy without FDI, is increasing in the local conditions (the domestic TFP AcA_{c}, the efficiency of the training process AhA_{h}) and the TFP of the MNE. Moreover, according to Propositions 6 and 7, if the country invests in R&D and the local conditions are good enough, the host country may get a sustained growth in the long run. This may happen even the country does not receive FDI.

Our point concerning the conditional impact of FDI on economic growth is supported by several empirical studies (Borensztein et al., 1998; Berthélemy and Démurger, 2000; Li and Liu, 2005; Alfaro et al., 2004, 2010).

4 Conclusion

We have investigated the nexus between FDI, R&D and growth in a host country by using infinite-horizon optimal growth models. According to our results, the very question does not rely on whether or not developing countries should attract inward FDI, but instead on how they implement policies to benefit from FDI spillovers. We have proved that FDI can act as a catalyst, helping a host developing country to avoid a middle income trap and potentially attain a higher income. However, to reach sustained economic growth in the long run, the host country should focus on domestic investment and R&D.

Appendix A Formal proofs

A.1 Static analysis

Proof of Proposition 2.

Let x:=bSσx¯x:=bS^{\sigma}-\bar{x}. Since x>0x>0, there exists αn(0,1)\alpha_{n}\in(0,1) such that bSσαnσ=x¯bS^{\sigma}\alpha_{n}^{\sigma}=\bar{x}. Define Kc,N,HK_{c},N,H by

N\displaystyle N =(αn+ϵ)S,pKc=ϵS,H=0\displaystyle=(\alpha_{n}+\epsilon)S,\quad pK_{c}=\epsilon S,\quad H=0 (A.1)

where ϵ>0\epsilon>0 such that αn+2ϵ=1\alpha_{n}+2\epsilon=1 (so that N+pKc=SN+pK_{c}=S). Precisely, ϵ=12(1(x¯bSσ)1σ)\epsilon=\frac{1}{2}\Big{(}1-\big{(}\frac{\bar{x}}{bS^{\sigma}}\big{)}^{\frac{1}{\sigma}}\Big{)}. With such N,KcN,K_{c}, we have bNσ>x¯bN^{\sigma}>\bar{x}, and hence, we can verify that

g(Kc,N,H)\displaystyle g(K_{c},N,H) =[Ac+a((b1σS2+x¯1σ2)σx¯)]1pα(S2x¯1σ2b1σ)α\displaystyle=\Big{[}A_{c}+{a}\Big{(}\big{(}b^{\frac{1}{\sigma}}\frac{S}{2}+\frac{\bar{x}^{\frac{1}{\sigma}}}{2}\big{)}^{\sigma}-\bar{x}\Big{)}\Big{]}\frac{1}{p^{\alpha}}\Big{(}\frac{S}{2}-\frac{\bar{x}^{\frac{1}{\sigma}}}{2b^{\frac{1}{\sigma}}}\Big{)}^{\alpha} (A.2)

g(Kc,N,H)g(K_{c},N,H) is increasing in aa and bb. It will be higher than F(S)F(S) when aa and bb are high enough because F(S)F(S) does not depend on (a,b)(a,b).

Proof of Proposition 3.

We need an intermediate step.

Claim 1.

Assume that ax¯>Aca\bar{x}>A_{c}. Denote N1(x¯/b)1/σN_{1}\equiv(\bar{x}/b)^{1/\sigma}, x1(α+σσασax¯Acax¯)N1x_{1}\equiv\big{(}\frac{\alpha+\sigma}{\sigma}-\frac{\alpha}{\sigma}\frac{a\bar{x}-A_{c}}{a\bar{x}}\big{)}N_{1} and

G1(x)=max{(Ac+a(bNσx¯))Kcα:Kc,N0,pKc+Nx,bNσx¯}.G_{1}(x)=\max\{\big{(}A_{c}+a(bN^{\sigma}-\bar{x})\big{)}K_{c}^{\alpha}:K_{c},N\geq 0,pK_{c}+N\leq x,bN^{\sigma}\geq\bar{x}\}.

We have that x1N1x_{1}\geq N_{1}. The function G1G_{1} is well-defined on the interval [N1,)[N_{1},\infty), and G1(N1)=0G_{1}(N_{1})=0. On the interval (N1,)(N_{1},\infty), the function G1G_{1} is strictly increasing, continuously differentiable and G1(x)>αAxα1pα.G_{1}^{\prime}(x)>\frac{\alpha Ax^{\alpha-1}}{p^{\alpha}}. G1(x)Acxα/pαG_{1}(x)-A_{c}x^{\alpha}/p^{\alpha} is strictly increasing in xx and there exists a unique x2x_{2} such that G1(x2)=Acx2α/pαG_{1}(x_{2})=A_{c}x_{2}^{\alpha}/p^{\alpha}. Moreover, bx2σx¯>0bx_{2}^{\sigma}-\bar{x}>0.

Proof of Claim 1.

If xN1x\leq N_{1}, then condition bNσx¯bN^{\sigma}\geq\bar{x} implies that NN1xNN\geq N_{1}\geq x\geq N. Then x=Nx=N, pKc=xN=0pK_{c}=x-N=0 and hence G1(x)=0G_{1}(x)=0.

Consider the case x>N1x>N_{1}. Let (Kc,N)(K_{c},N) be an optimal point. It is clear that Kc>0K_{c}>0 and N<xN<x.

If NN1N\leq N_{1}, then N=N1N=N_{1}, G1(x)=AcKcα=Ac(xN)α/pα=Ac(xN1)α/pαG_{1}(x)=A_{c}K_{c}^{\alpha}=A_{c}(x-N)^{\alpha}/p^{\alpha}=A_{c}(x-N_{1})^{\alpha}/p^{\alpha} and G1(x)=αAc(xN1)α1/pα>αAcxα1/pαG_{1}^{\prime}(x)=\alpha A_{c}(x-N_{1})^{\alpha-1}/p^{\alpha}>\alpha A_{c}x^{\alpha-1}/p^{\alpha}.

If N(N1,x)N\in(N_{1},x) at optimal, we write FOCs

σabNσ1Kcα\displaystyle\sigma abN^{\sigma-1}K_{c}^{\alpha} =λ\displaystyle=\lambda (A.3)
(Ac+a(bNσx¯))αKcα1\displaystyle\big{(}A_{c}+a(bN^{\sigma}-\bar{x})\big{)}\alpha K_{c}^{\alpha-1} =pλ=pσabNσ1Kcα.\displaystyle=p\lambda=p\sigma abN^{\sigma-1}K_{c}^{\alpha}. (A.4)

It follows that (Ac+a(bNσx¯))α=σabNσ1pKc\big{(}A_{c}+a(bN^{\sigma}-\bar{x})\big{)}\alpha=\sigma abN^{\sigma-1}pK_{c} or equivalently

σabxNN+α(ax¯Ac)Nσαab=0.\displaystyle\sigma ab\frac{x-N}{N}+\frac{\alpha(a\bar{x}-A_{c})}{N^{\sigma}}-\alpha ab=0. (A.5)

The left hand side (LHS) is strictly decreasing in NN because ax¯Ac>0a\bar{x}-A_{c}>0. When N=xN=x, the LHS equals α(ax¯Ac)xσαab<0\frac{\alpha(a\bar{x}-A_{c})}{x^{\sigma}}-\alpha ab<0 because x>N1x>N_{1}. When N=N1N=N_{1}, the LHS equals

LHS(N1)σabxN1N1+α(ax¯Ac)(N1)σαab.\displaystyle LHS(N_{1})\equiv\sigma ab\frac{x-N_{1}}{N_{1}}+\frac{\alpha(a\bar{x}-A_{c})}{(N_{1})^{\sigma}}-\alpha ab. (A.6)

Observe that LHS(N1)0LHS(N_{1})\geq 0 if and only if

xx1α+σσN1ασN1ax¯Acax¯=N1+α(N1)1σσab(αab(N1)σax¯+Ac).x\geq x_{1}\equiv\frac{\alpha+\sigma}{\sigma}N_{1}-\frac{\alpha}{\sigma}N_{1}\frac{a\bar{x}-A_{c}}{a\bar{x}}=N_{1}+\frac{\alpha(N_{1})^{1-\sigma}}{\sigma ab}\Big{(}\alpha ab(N_{1})^{\sigma}-a\bar{x}+A_{c}\Big{)}.

Therefore, we get that:

  1. 1.

    If xx1x\leq x_{1}, then N=N1N=N_{1} and G1(x)=Ac(xN1)α/pαG_{1}(x)=A_{c}(x-N_{1})^{\alpha}/p^{\alpha}. In this case, we have

    G1(x)=αAc(xN1)α1pα>αAxα1pαG_{1}^{\prime}(x)=\frac{\alpha A_{c}(x-N_{1})^{\alpha-1}}{p^{\alpha}}>\frac{\alpha Ax^{\alpha-1}}{p^{\alpha}}

    because N1>0N_{1}>0 and α1<0\alpha-1<0.

  2. 2.

    If x>x1x>x_{1}, then the equation (A.5) has a unique NxN_{x} in the interval (N1,x)(N_{1},x). The optimal point (Kc,N)(K_{c},N) is given by N=NxN=N_{x} and pKc+N=SpK_{c}+N=S. Moreover, when xx increases, we have NxN_{x}, xNxx-N_{x} and xNxNx\frac{x-N_{x}}{N_{x}} increase.

    We have G1(x)=(Ac+a(bNσx¯))KcαG_{1}(x)=\big{(}A_{c}+a(bN^{\sigma}-\bar{x})\big{)}K_{c}^{\alpha} where NN is uniquely given by (A.5) and pKc=SNpK_{c}=S-N. By computing directly or using the envelop theorem, we have

    G1(x)=(Ac+a(bNσx¯))αKcα1>αAxα1pα\displaystyle G_{1}^{\prime}(x)=\big{(}A_{c}+a(bN^{\sigma}-\bar{x})\big{)}\alpha K_{c}^{\alpha-1}>\frac{\alpha Ax^{\alpha-1}}{p^{\alpha}} (A.7)

    because bNσx¯>0bN^{\sigma}-\bar{x}>0 and pKc<SpK_{c}<S.

  3. 3.

    When xx tends to x1x_{1}, we have NxN_{x} tends to N1N_{1} and therefore G1(x)=(Ac+a(bNσx¯))αKcα1G_{1}^{\prime}(x)=\big{(}A_{c}+a(bN^{\sigma}-\bar{x})\big{)}\alpha K_{c}^{\alpha-1} tends to αAc(xN1)α1pα>αAcxα1pα\frac{\alpha A_{c}(x-N_{1})^{\alpha-1}}{p^{\alpha}}>\frac{\alpha A_{c}x^{\alpha-1}}{p^{\alpha}} because α<1\alpha<1.

To sum up, the function G1G_{1} is continuously differentiable and G1(x)>αAxα1pα.G_{1}^{\prime}(x)>\frac{\alpha Ax^{\alpha-1}}{p^{\alpha}}.

We now prove Proposition 3. Observe that

G0(S)\displaystyle G_{0}(S) maxKc,N,H{G1(x)+wAhHαh:x+HS;x,H0}.\displaystyle\equiv\max\limits_{K_{c},N,H}\Big{\{}G_{1}(x)+wA_{h}H^{\alpha_{h}}:x+H\leq S;x,H\geq 0\Big{\}}. (A.8)

Let (xg,Hg)(x_{g},H_{g}) be an optimal point. Since G1G_{1} is differentiable, we have the FOC

G1(xg)αhwAh(Sxg)αh1=0.G_{1}^{\prime}(x_{g})-\alpha_{h}wA_{h}(S-x_{g})^{\alpha_{h}-1}=0.

Let (x,Sx)(x,S-x) be the unique pair such that Ac(xp)α+wAhHαh=F(S)A_{c}(\frac{x}{p})^{\alpha}+wA_{h}H^{\alpha_{h}}=F(S). Then, we have αAxα1pααhwAh(Sx)αh1=0\frac{\alpha Ax^{\alpha-1}}{p^{\alpha}}-\alpha_{h}wA_{h}(S-x)^{\alpha_{h}-1}=0.

Since G1(xg)>αA(xg)α1pαG_{1}^{\prime}(x_{g})>\frac{\alpha A(x_{g})^{\alpha-1}}{p^{\alpha}}, we have 0>αA(xg)α1pααhwAh(Sxg)αh10>\frac{\alpha A(x_{g})^{\alpha-1}}{p^{\alpha}}-\alpha_{h}wA_{h}(S-x_{g})^{\alpha_{h}-1}. Therefore, we have xg>xx_{g}>x and hence Hg<HH_{g}<H. It follows that

G0(S)\displaystyle G_{0}^{\prime}(S) =αhwAhHgαh1>αhwAhHgαh1=F(S).\displaystyle=\alpha_{h}wA_{h}H_{g}^{\alpha_{h}-1}>\alpha_{h}wA_{h}H_{g}^{\alpha_{h}-1}=F^{\prime}(S). (A.9)

So, G0(S)F(S)G_{0}(S)-F(S) is strictly increasing.

When SS is small enough, G0(S)F(S)G_{0}(S)-F(S) is negative. When SS is high enough, G0(S)F(S)G_{0}(S)-F(S) is positive (see, for example, point (ii) of Proposition 2). So, there exists a unique SS^{*} such that G0(S)F(S)=0G_{0}(S^{*})-F(S^{*})=0.

According to (11) and G(S)G0(S)G(S)\geq G_{0}(S) S\forall S, we have G(S)F(S)=0G(S)-F(S)=0 SS\forall S\leq S^{*}, and G(S)F(S)>0G(S)-F(S)>0 S>S\forall S>S^{*}.

A.2 Dynamic analysis

Proof of Proposition 4.

Since the function G()G(\cdot) is continuous, strictly increasing, we can use the standard argument in dynamic programming (Amir, 1996) to prove that the optimal path (St)t(S_{t})_{t} is monotonic.

We now prove that StS_{t} does not converge to zero. First, following Kamihigashi and Roy (2007), we have Euler condition in the form of inequality

βu(ct+1)Df(St+1)u(ct)βu(ct+1)D+f(St+1).\displaystyle\beta u^{\prime}(c_{t+1})D^{-}f(S_{t+1})\geq u^{\prime}(c_{t})\geq\beta u^{\prime}(c_{t+1})D^{+}f(S_{t+1}). (A.10)

where the Dini derivatives of function ff are defined by D+f(x)=lim supϵ0f(x+ϵ)f(x)ϵD^{+}f(x)=\limsup\limits_{\epsilon\downarrow 0}\frac{f(x+\epsilon)-f(x)}{\epsilon} and Df(x)=lim infϵ0f(x)f(xϵ)ϵD^{-}f(x)=\liminf\limits_{\epsilon\downarrow 0}\frac{f(x)-f(x-\epsilon)}{\epsilon}.

Suppose that limtSt=0\lim_{t\to\infty}S_{t}=0. According to budget constraints and the fact that G(0)=0G(0)=0, we have limtct=0\lim_{t\to\infty}c_{t}=0. Since limt+St=0\lim\limits_{t\rightarrow+\infty}S_{t}=0, there exists t0t_{0} such that βD+G(St+1)>1\beta D^{+}G(S_{t+1})>1 for every tt0t\geq t_{0}. Consequently, u(ct)u(ct+1)u^{\prime}(c_{t})\geq u^{\prime}(c_{t+1}) and hence ctct+1c_{t}\leq c_{t+1} for every tt0t\geq t_{0}. This leads to a contradiction to the fact that limt+ct=0\lim\limits_{t\rightarrow+\infty}c_{t}=0. ∎

Proof of Lemma 1.

(1) If x<xx<x^{*}, then F(x)<F(x)=xF(x)<F(x^{*})=x^{*}. If x>xx>x^{*}, then F(x)xF(x)x=1\frac{F(x)}{x}\leq\frac{F(x^{*})}{x^{*}}=1 since FF is concave.

(2) It is obvious that StxttS_{t}\leq x_{t}\forall t. We prove xtS¯x_{t}\leq\bar{S} t\forall t by induction argument. First, we see that x0S¯.x_{0}\leq\bar{S}. Second, assume that xsS¯x_{s}\leq\bar{S} st\forall s\leq t. If X0xX_{0}\leq x^{*}, then xtS¯=xx_{t}\leq\bar{S}=x^{*}, then xt+1=F(xt)F(x)=x=S¯x_{t+1}=F(x_{t})\leq F(x^{*})=x^{*}=\bar{S}. If X0>xX_{0}>x^{*}, then xtS¯=X0x_{t}\leq\bar{S}=X_{0} and hence xt+1=F(xt)=F(x0)x1S¯x_{t+1}=F(x_{t})=F(x_{0})\leq x_{1}\leq\bar{S}.

Proof of Proposition 5.

We will prove, by induction argument, that bx¯tσx¯b\bar{x}_{t}^{\sigma}\leq\bar{x} and Stx1S_{t}\leq x_{1} t1\forall t\geq 1.

When t=1t=1. We have N1S1X0x1N_{1}\leq S_{1}\leq X_{0}\leq x_{1}, So, bN1σbS¯1σx¯bN_{1}^{\sigma}\leq b\bar{S}_{1}^{\sigma}\leq\bar{x}.

Assume that bx¯tσx¯b\bar{x}_{t}^{\sigma}\leq\bar{x} and Stx1S_{t}\leq x_{1} tT\forall t\leq T. This implies that NT=0N_{T}=0, because otherwise we can reduce NTN_{T} and augment Kc,TK_{c,T} in order to get a higher utility, which is a contradiction.

Since NT=0N_{T}=0, we have that G(ST)=F(ST)G(S_{T})=F(S_{T}). Since STx1S_{T}\leq x_{1}, we have F(ST)F(x)=xF(S_{T})\leq F(x^{*})=x^{*}.

Hence, ST+1G(ST)xS_{T+1}\leq G(S_{T})\leq x^{*} and therefore bx¯T+1σbST+1σb(x)σx¯b\bar{x}_{T+1}^{\sigma}\leq bS_{T+1}^{\sigma}\leq b(x^{*})^{\sigma}\leq\bar{x}. We have finished our proof.

Proof of Proposition 6.

We need intermediate steps.

Lemma 2.

Assume that α+σ1\alpha+\sigma\geq 1. For any solution Kc,N,HK_{c},N,H of the problem (GS)(G_{S}), there exists the following limits

limSθc=αα+σ,limSθn=σα+σ,limSθh=0.\displaystyle\lim\limits_{S\rightarrow\infty}\theta_{c}=\frac{\alpha}{\alpha+\sigma},\lim\limits_{S\rightarrow\infty}\theta_{n}=\frac{\sigma}{\alpha+\sigma},\lim\limits_{S\rightarrow\infty}\theta_{h}=0. (A.11)
Proof of Lemma 2.

Observe that, when SS is high enough, we have bNσx¯>0bN^{\sigma}-\bar{x}>0 at optimal. It is also to see that θc,θh>0\theta_{c},\theta_{h}>0. By consequence, we can write FOCs for the problem (G)(G^{\prime}) as follows (we have FOCs even the objective function is not concave):

αhwAhSαhθhαh1\displaystyle\alpha_{h}wA_{h}S^{\alpha_{h}}\theta_{h}^{\alpha_{h}-1} =λ\displaystyle=\lambda (A.12)
(Ac+a(bSσθnσx¯)+)αpαθcα1Sα\displaystyle\Big{(}A_{c}+a(bS^{\sigma}\theta_{n}^{\sigma}-\bar{x})^{+}\Big{)}\frac{\alpha}{p^{\alpha}}\theta_{c}^{\alpha-1}S^{\alpha} =λ\displaystyle=\lambda (A.13)
abσSσ+αθnσ1(θcp)α\displaystyle ab\sigma S^{\sigma+\alpha}\theta_{n}^{\sigma-1}\big{(}\frac{\theta_{c}}{p}\big{)}^{\alpha} =λ\displaystyle=\lambda (A.14)

where λ\lambda is the multiplier associated to the constraint θc+θn+θh1.\theta_{c}+\theta_{n}+\theta_{h}\leq 1. The first and the third equations imply that

αhwAhpαabσ=Sσ+ααhθnσ1θcαθh1αh=(Sθn)σ1(Sθc)α(Sθh)1αh\displaystyle\frac{\alpha_{h}wA_{h}p^{\alpha}}{ab\sigma}=S^{\sigma+\alpha-\alpha_{h}}\theta_{n}^{\sigma-1}\theta_{c}^{\alpha}\theta_{h}^{1-\alpha_{h}}=(S\theta_{n})^{\sigma-1}(S\theta_{c})^{\alpha}(S\theta_{h})^{1-\alpha_{h}} (A.15)

while the second and third conditions imply that

(Ac+a(bSσθnσx¯)+)α=abσSσθnσ1θc.\displaystyle\Big{(}A_{c}+a(bS^{\sigma}\theta_{n}^{\sigma}-\bar{x})^{+}\Big{)}\alpha=ab\sigma S^{\sigma}\theta_{n}^{\sigma-1}\theta_{c}. (A.16)

By consequence, we obtain

θc=ασθn+αθn1σ(Acax¯)abσSσ\displaystyle\theta_{c}=\frac{\alpha}{\sigma}\theta_{n}+\frac{\alpha\theta_{n}^{1-\sigma}(A_{c}-a\bar{x})}{ab\sigma S^{\sigma}} (A.17)
or equivalently SθcSθn=ασ+α(Acax¯)σab(Sθn)σ.\displaystyle\text{or equivalently }\frac{S\theta_{c}}{S\theta_{n}}=\frac{\alpha}{\sigma}+\frac{\alpha(A_{c}-a\bar{x})}{\sigma ab(S\theta_{n})^{\sigma}}. (A.18)

From this, we get limS(σθcαθn1)θnσ=0\lim_{S\rightarrow\infty}(\frac{\sigma\theta_{c}}{\alpha\theta_{n}}-1)\theta_{n}^{\sigma}=0. By combining this with the fact that σ1\sigma\leq 1, we obtain limS(θcασθn)=0\lim_{S\rightarrow\infty}(\theta_{c}-\frac{\alpha}{\sigma}\theta_{n})=0.

Notice that b(Sθn)σ>Nb(S\theta_{n})^{\sigma}>N for SS high enough.

We will prove that when SS tends to infinity, SθhS\theta_{h} is bounded from above, and hence limSθh=0\lim_{S\rightarrow\infty}\theta_{h}=0. To do so, we firstly prove that lim infS(Sθc)α(Sθn)1σ>0\liminf_{S\rightarrow\infty}\frac{(S\theta_{c})^{\alpha}}{(S\theta_{n})^{1-\sigma}}>0. Indeed, according to (A.18), we have

(Sθc)α(Sθn)1σ=(Sθn)α+σ1(ασ+α(Acax¯)σab(Sθn)σ)α\displaystyle\frac{(S\theta_{c})^{\alpha}}{(S\theta_{n})^{1-\sigma}}=(S\theta_{n})^{\alpha+\sigma-1}\Big{(}\frac{\alpha}{\sigma}+\frac{\alpha(A_{c}-a\bar{x})}{\sigma ab(S\theta_{n})^{\sigma}}\Big{)}^{\alpha} (A.19)

Suppose that there is a sequence (Sk)(S_{k}) tends to infinity such that limk(Skθc)α(Skθn)1σ=0\lim_{k\rightarrow\infty}\frac{(S_{k}\theta_{c})^{\alpha}}{(S_{k}\theta_{n})^{1-\sigma}}=0. Notice that

(Sθc)α(Sθn)1σ=1(Sθn)(1σ)(1α)(ασab[Ac+a(b(Sθn)σx¯)])α1(Sθn)(1σ)(1α)(ασabAc)α\displaystyle\frac{(S\theta_{c})^{\alpha}}{(S\theta_{n})^{1-\sigma}}=\frac{1}{(S\theta_{n})^{(1-\sigma)(1-\alpha)}}\Big{(}\frac{\alpha}{\sigma ab}\big{[}A_{c}+a(b(S\theta_{n})^{\sigma}-\bar{x})\big{]}\Big{)}^{\alpha}\geq\frac{1}{(S\theta_{n})^{(1-\sigma)(1-\alpha)}}\Big{(}\frac{\alpha}{\sigma ab}A_{c}\Big{)}^{\alpha}

for any SS high enough, which implies that limkSkθn=\lim_{k\rightarrow\infty}S_{k}\theta_{n}=\infty. However, this will follow that

(Skθc)α(Skθn)1σ=(Skθn)α+σ1(ασ+α(Acax¯)σab(Skθn)σ)α\displaystyle\frac{(S_{k}\theta_{c})^{\alpha}}{(S_{k}\theta_{n})^{1-\sigma}}=(S_{k}\theta_{n})^{\alpha+\sigma-1}\Big{(}\frac{\alpha}{\sigma}+\frac{\alpha(A_{c}-a\bar{x})}{\sigma ab(S_{k}\theta_{n})^{\sigma}}\Big{)}^{\alpha} (A.20)

is bounded away from zero (because α+σ1\alpha+\sigma\geq 1), a contradiction.

So, we get that lim infS(Sθc)α(Sθn)1σ>0\liminf_{S\rightarrow\infty}\frac{(S\theta_{c})^{\alpha}}{(S\theta_{n})^{1-\sigma}}>0. By combining this with (A.15), we have that SθhS\theta_{h} is bounded from above and hence limSθh=0\lim_{S\rightarrow\infty}\theta_{h}=0. Combining with (A.17), we obtain (A.11).

Lemma 3.

Assume that ax¯Ac0a\bar{x}-A_{c}\geq 0. We have

D+G(S)=lim supϵ0G(S+ϵ)G(S)ϵmin(F(S),Γ(a,b,x¯))\displaystyle D^{+}G(S)=\limsup\limits_{\epsilon\downarrow 0}\dfrac{G(S+\epsilon)-G(S)}{\epsilon}\geq\min\Big{(}F^{\prime}(S^{*}),\Gamma(a,b,\bar{x})\Big{)} (A.21)
where Γ(a,b,x¯)(αAcpσ)αx¯(1α)(1σ)σa1αb1ασ(1+ασ+(αhwAh(pσ)ασ(αAc)α)11αh1a1α1αhbαhασ(1αh))α\displaystyle\text{where }\Gamma(a,b,\bar{x})\equiv\frac{\big{(}\frac{\alpha A_{c}}{p\sigma}\big{)}^{\alpha}\bar{x}^{\frac{-(1-\alpha)(1-\sigma)}{\sigma}}a^{1-\alpha}b^{\frac{1-\alpha}{\sigma}}}{\Big{(}1+\frac{\alpha}{\sigma}+\Big{(}\frac{\alpha_{h}wA_{h}(p\sigma)^{\alpha}}{\sigma(\alpha A_{c})^{\alpha}}\Big{)}^{\frac{1}{1-\alpha_{h}}}\frac{1}{a^{\frac{1-\alpha}{1-\alpha_{h}}}b^{\frac{\alpha_{h}-\alpha}{\sigma(1-\alpha_{h})}}}\Big{)}^{\alpha}} (A.22)

By consequence, when aa and bb are high enough and αh+1α2\alpha_{h}+\frac{1}{\alpha}\geq 2, we have βD+G(S)>1\beta D^{+}G(S)>1 S>0\forall S>0.

Proof of Lemma 3.

Part 1. We prove (A.21). Let S>SS>S^{*}. Consider the function

(G0):G0(S)\displaystyle(G_{0}):\quad G_{0}(S) maxKc,N,H{(Ac+a(bNσx¯))Kcα+wAhHαh}\displaystyle\equiv\max\limits_{K_{c},N,H}\Big{\{}\big{(}A_{c}+a(bN^{\sigma}-\bar{x})\big{)}K_{c}^{\alpha}+wA_{h}H^{\alpha_{h}}\Big{\}} (A.23a)
subject to: pKc+N+HS,bNσx¯ and Kc,N,H0.\displaystyle pK_{c}+N+H\leq S,\quad bN^{\sigma}\geq\bar{x}\text{ and }K_{c},N,H\geq 0. (A.23b)

When S>SS>S^{*}, we have G(S)=G0(S)G(S)=G_{0}(S) and bNσ>x¯bN^{\sigma}>\bar{x} at optimal. We will quantify G0(S)G_{0}^{\prime}(S).

Let λ\lambda be the multiplier associated to the constraint pKc+N+HSpK_{c}+N+H\leq S, we have FOCs

(abNσ(ax¯Ac))αKcα1\displaystyle(abN^{\sigma}-(a\bar{x}-A_{c}))\alpha K_{c}^{\alpha-1} =pλ\displaystyle=p\lambda (A.24)
abσNσ1Kcα\displaystyle ab\sigma N^{\sigma-1}K_{c}^{\alpha} =λ\displaystyle=\lambda (A.25)
αhwAhHαh1\displaystyle\alpha_{h}wA_{h}H^{\alpha_{h}-1} =λ.\displaystyle=\lambda. (A.26)

FOCs imply that α(abNσ(ax¯Ac))=pabσNσ1Kc\alpha(abN^{\sigma}-(a\bar{x}-A_{c}))=pab\sigma N^{\sigma-1}K_{c} and hence

ασNpKc\displaystyle\frac{\alpha}{\sigma}N\geq pK_{c} =ασN(1ax¯AcabNσ)>NαAcσax¯\displaystyle=\frac{\alpha}{\sigma}N\Big{(}1-\frac{a\bar{x}-A_{c}}{abN^{\sigma}}\Big{)}>N\frac{\alpha A_{c}}{\sigma a\bar{x}} (A.27)

because ax¯Ac0a\bar{x}-A_{c}\geq 0 and bNσx¯bN^{\sigma}\geq\bar{x}.

Since abσNσ1Kcα=αhwAhHαh1ab\sigma N^{\sigma-1}K_{c}^{\alpha}=\alpha_{h}wA_{h}H^{\alpha_{h}-1}, we have

H1αhN1αh\displaystyle\frac{H^{1-\alpha_{h}}}{N^{1-\alpha_{h}}} =αhwAhabσN1σ(1αh)Kcα\displaystyle=\frac{\alpha_{h}wA_{h}}{ab\sigma}N^{1-\sigma-(1-\alpha_{h})}K_{c}^{-\alpha} (A.28)
αhwAhabσN1σ(1αh)(NαAcpσax¯)α=αhwAhabσ(pσax¯αAc)αN(α+σαh)\displaystyle\leq\frac{\alpha_{h}wA_{h}}{ab\sigma}N^{1-\sigma-(1-\alpha_{h})}\Big{(}N\frac{\alpha A_{c}}{p\sigma a\bar{x}}\Big{)}^{-\alpha}=\frac{\alpha_{h}wA_{h}}{ab\sigma}\Big{(}\frac{p\sigma a\bar{x}}{\alpha A_{c}}\Big{)}^{\alpha}N^{-(\alpha+\sigma-\alpha_{h})} (A.29)
αhwAhabσ(pσax¯αAc)α(x¯b)α+σαhσ=αhwAh(pσ)ασ(αAc)α1a1αbαhασ\displaystyle\leq\frac{\alpha_{h}wA_{h}}{ab\sigma}\Big{(}\frac{p\sigma a\bar{x}}{\alpha A_{c}}\Big{)}^{\alpha}(\frac{\bar{x}}{b})^{-\frac{\alpha+\sigma-\alpha_{h}}{\sigma}}=\frac{\alpha_{h}wA_{h}(p\sigma)^{\alpha}}{\sigma(\alpha A_{c})^{\alpha}}\frac{1}{a^{1-\alpha}b^{\frac{\alpha_{h}-\alpha}{\sigma}}} (A.30)

Thus, we get that

HN(αhwAh(pσ)ασ(αAc)α)11αh1a1α1αhbαhασ(1αh)\displaystyle H\leq N\Big{(}\frac{\alpha_{h}wA_{h}(p\sigma)^{\alpha}}{\sigma(\alpha A_{c})^{\alpha}}\Big{)}^{\frac{1}{1-\alpha_{h}}}\frac{1}{a^{\frac{1-\alpha}{1-\alpha_{h}}}b^{\frac{\alpha_{h}-\alpha}{\sigma(1-\alpha_{h})}}} (A.31)

Since S=N+pKc+HS=N+pK_{c}+H, we have

SN+Nασ+N(αhwAh(pσ)ασ(αAc)α)11αh1a1α1αhbαhασ(1αh)\displaystyle S\leq N+N\frac{\alpha}{\sigma}+N\Big{(}\frac{\alpha_{h}wA_{h}(p\sigma)^{\alpha}}{\sigma(\alpha A_{c})^{\alpha}}\Big{)}^{\frac{1}{1-\alpha_{h}}}\frac{1}{a^{\frac{1-\alpha}{1-\alpha_{h}}}b^{\frac{\alpha_{h}-\alpha}{\sigma(1-\alpha_{h})}}} (A.32)

which implies that

N(1+ασ+(αhwAh(pσ)ασ(αAc)α)11αh1a1α1αhbαhασ(1αh))S(x¯b)1σ\displaystyle N\Big{(}1+\frac{\alpha}{\sigma}+\Big{(}\frac{\alpha_{h}wA_{h}(p\sigma)^{\alpha}}{\sigma(\alpha A_{c})^{\alpha}}\Big{)}^{\frac{1}{1-\alpha_{h}}}\frac{1}{a^{\frac{1-\alpha}{1-\alpha_{h}}}b^{\frac{\alpha_{h}-\alpha}{\sigma(1-\alpha_{h})}}}\Big{)}\geq S\geq\Big{(}\frac{\bar{x}}{b}\Big{)}^{\frac{1}{\sigma}} (A.33)

Denote dax¯Ac0d\equiv a\bar{x}-A_{c}\geq 0. We have

G0(S)\displaystyle G_{0}(S) =(abNσd)Kcα+wAhHαh\displaystyle=(abN^{\sigma}-d)K_{c}^{\alpha}+wA_{h}H^{\alpha_{h}}
G0(S)\displaystyle G_{0}^{\prime}(S) =(abNσd)αKcα1Kc(S)+σabNσ1KcαN(S)+αhwAhHαh1H(S)\displaystyle=(abN^{\sigma}-d)\alpha K_{c}^{\alpha-1}K_{c}^{\prime}(S)+\sigma abN^{\sigma-1}K_{c}^{\alpha}N^{\prime}(S)+\alpha_{h}wA_{h}H^{\alpha_{h}-1}H^{\prime}(S)
=σabNσ1Kcα\displaystyle=\sigma abN^{\sigma-1}K_{c}^{\alpha}

because pKc(S)+N(S)+H(S)=1pK_{c}^{\prime}(S)+N^{\prime}(S)+H^{\prime}(S)=1.

By combining this with KcαAcpσax¯NK_{c}\geq\frac{\alpha A_{c}}{p\sigma a\bar{x}}N and σ<1\sigma<1, we have

G0(S)\displaystyle G_{0}^{\prime}(S) =abNσ1KcαabNσ+α1(αAcpσax¯)α\displaystyle=abN^{\sigma-1}K_{c}^{\alpha}\geq abN^{\sigma+\alpha-1}\Big{(}\frac{\alpha A_{c}}{p\sigma a\bar{x}}\Big{)}^{\alpha} (A.35)
(αAcpσx¯)αa1αb(x¯b)σ+α1σ1(1+ασ+(αhwAh(pσ)ασ(αAc)α)11αh1a1α1αhbαhασ(1αh))α\displaystyle\geq\Big{(}\frac{\alpha A_{c}}{p\sigma\bar{x}}\Big{)}^{\alpha}a^{1-\alpha}b\Big{(}\frac{\bar{x}}{b}\Big{)}^{\frac{\sigma+\alpha-1}{\sigma}}\frac{1}{\Big{(}1+\frac{\alpha}{\sigma}+\Big{(}\frac{\alpha_{h}wA_{h}(p\sigma)^{\alpha}}{\sigma(\alpha A_{c})^{\alpha}}\Big{)}^{\frac{1}{1-\alpha_{h}}}\frac{1}{a^{\frac{1-\alpha}{1-\alpha_{h}}}b^{\frac{\alpha_{h}-\alpha}{\sigma(1-\alpha_{h})}}}\Big{)}^{\alpha}} (A.36)
=(αAcpσ)αx¯(1α)(1σ)σa1αb1ασ(1+ασ+(αhwAh(pσ)ασ(αAc)α)11αh1a1α1αhbαhασ(1αh))αΓ(a,b,x¯)\displaystyle=\frac{\big{(}\frac{\alpha A_{c}}{p\sigma}\big{)}^{\alpha}\bar{x}^{\frac{-(1-\alpha)(1-\sigma)}{\sigma}}a^{1-\alpha}b^{\frac{1-\alpha}{\sigma}}}{\Big{(}1+\frac{\alpha}{\sigma}+\Big{(}\frac{\alpha_{h}wA_{h}(p\sigma)^{\alpha}}{\sigma(\alpha A_{c})^{\alpha}}\Big{)}^{\frac{1}{1-\alpha_{h}}}\frac{1}{a^{\frac{1-\alpha}{1-\alpha_{h}}}b^{\frac{\alpha_{h}-\alpha}{\sigma(1-\alpha_{h})}}}\Big{)}^{\alpha}}\equiv\Gamma(a,b,\bar{x}) (A.37)

At point SS^{*}, the right Dini derivative D+G(S)D^{+}G(S^{*}) of GG is

D+G(S)=lim supϵ0G(S+ϵ)G(S)ϵlim supϵ0F(S+ϵ)F(S)ϵ=F(S).\displaystyle D^{+}G(S^{*})=\limsup\limits_{\epsilon\downarrow 0}\dfrac{G(S^{*}+\epsilon)-G(S^{*})}{\epsilon}\geq\limsup\limits_{\epsilon\downarrow 0}\dfrac{F(S^{*}+\epsilon)-F(S^{*})}{\epsilon}=F^{\prime}(S^{*}). (A.38)

When S<SS<S^{*}, we have G(S)=F(S)G(S)=F(S) and hence G(S)=F(S)F(S)G^{\prime}(S)=F^{\prime}(S)\geq F^{\prime}(S^{*}) because FF^{\prime} is decreasing.

Part 2. We prove that, when aa and bb are high enough and αh+1α2\alpha_{h}+\frac{1}{\alpha}\geq 2, we have βD+G(S)>1\beta D^{+}G(S)>1 S>0\forall S>0.

Observe that Γ(a,b,x¯)\Gamma(a,b,\bar{x}) is increasing in aa and βΓ(a,b,x¯)>1\beta\Gamma(a,b,\bar{x})>1 when aa is high enough.

When αh+1α2\alpha_{h}+\frac{1}{\alpha}\geq 2, we have 1ασ+αhασ(1αh)0\frac{1-\alpha}{\sigma}+\frac{\alpha_{h}-\alpha}{\sigma(1-\alpha_{h})}\geq 0 and therefore Γ(a,b,x¯)\Gamma(a,b,\bar{x}) is strictly increasing in bb. In this case, it is easy to see that βΓ(a,b,x¯)>1\beta\Gamma(a,b,\bar{x})>1 when bb is high enough.

We now prove that βF(S)>1\beta F^{\prime}(S^{*})>1 when aa or bb is high enough. As in proof of point (ii) of Proposition 2, we have that: If bSσ>x¯bS^{\sigma}>\bar{x}, then

G(S)[Ac+a((b1σS2+x¯1σ2)σx¯)]1pα(S2x¯1σ2b1σ)α>0\displaystyle G(S)\geq\Big{[}A_{c}+{a}\Big{(}\big{(}b^{\frac{1}{\sigma}}\frac{S}{2}+\frac{\bar{x}^{\frac{1}{\sigma}}}{2}\big{)}^{\sigma}-\bar{x}\Big{)}\Big{]}\frac{1}{p^{\alpha}}\Big{(}\frac{S}{2}-\frac{\bar{x}^{\frac{1}{\sigma}}}{2b^{\frac{1}{\sigma}}}\Big{)}^{\alpha}>0 (A.39)

At point SS^{*} which depends on aa and bb, we have F(S)=G(S)F(S^{*})=G(S^{*}) and hence

[Ac+a((b1σS2+x¯1σ2)σx¯)]1pα(S2x¯1σ2b1σ)αF(S)max(Ac(S)α,whAhSαh).\displaystyle\Big{[}A_{c}+{a}\Big{(}\big{(}b^{\frac{1}{\sigma}}\frac{S^{*}}{2}+\frac{\bar{x}^{\frac{1}{\sigma}}}{2}\big{)}^{\sigma}-\bar{x}\Big{)}\Big{]}\frac{1}{p^{\alpha}}\Big{(}\frac{S^{*}}{2}-\frac{\bar{x}^{\frac{1}{\sigma}}}{2b^{\frac{1}{\sigma}}}\Big{)}^{\alpha}\leq F(S^{*})\leq\max(A_{c}(S^{*})^{\alpha},w_{h}A_{h}S^{\alpha_{h}}).

We prove that SS^{*} tends to zero when aa or bb goes to infinity. Indeed, let, for example, bb tend to infinity. If lim infbS>0\liminf_{b\rightarrow\infty}S^{*}>0, by using the property α+σ1>max(α,αh)\alpha+\sigma\geq 1>\max(\alpha,\alpha_{h}), we get that

[Ac+a((b1σS2+x¯1σ2)σx¯)]1pα(S2x¯1σ2b1σ)α>max(Ac(S)α,whAhSαh).\displaystyle\Big{[}A_{c}+{a}\Big{(}\big{(}b^{\frac{1}{\sigma}}\frac{S^{*}}{2}+\frac{\bar{x}^{\frac{1}{\sigma}}}{2}\big{)}^{\sigma}-\bar{x}\Big{)}\Big{]}\frac{1}{p^{\alpha}}\Big{(}\frac{S^{*}}{2}-\frac{\bar{x}^{\frac{1}{\sigma}}}{2b^{\frac{1}{\sigma}}}\Big{)}^{\alpha}>\max(A_{c}(S^{*})^{\alpha},w_{h}A_{h}S^{\alpha_{h}}).

where bb is high enough, a contradiction.

So, when aa or bb is high enough, SS^{*} is low enough and hence βF(S)>1\beta F^{\prime}(S^{*})>1 since F(0)=F^{\prime}(0)=\infty.

We are now ready to prove Proposition 6. According to Lemma 3, we have βD+G(S)>1\beta D^{+}G(S)>1 S>0\forall S>0 when aa and bb are high enough. According to Proposition 4.6 in Kamihigashi and Roy (2007), we have that limtSt=\lim\limits_{t\rightarrow\infty}S_{t}=\infty. According to Lemma 2, we obtain point 2 of Proposition 6.

Proof of Proposition 7.

We observe that

G(S)\displaystyle G(S) (Ac+abSσ)1pαSα+wAhSαh{Ac+abpα+wAh if S1(Ac+abpα+wAh)Smax(α+σ,αh) if S1.\displaystyle\leq(A_{c}+abS^{\sigma})\frac{1}{p^{\alpha}}S^{\alpha}+wA_{h}S^{\alpha_{h}}\leq\begin{cases}\frac{A_{c}+ab}{p^{\alpha}}+wA_{h}\text{ if }S\leq 1\\ \Big{(}\frac{A_{c}+ab}{p^{\alpha}}+wA_{h}\Big{)}S^{\max(\alpha+\sigma,\alpha_{h})}\text{ if }S\geq 1.\end{cases}

By using max(α+σ,αh)<1\max(\alpha+\sigma,\alpha_{h})<1, it is easy to prove that StS_{t} is bounded from above. Since StS_{t} is monotonic, it must converge to a finite value, say SdS_{d}. So, we have that βDG(Sd)1βD+G(Sd)\beta D^{-}G(S_{d})\geq 1\geq\beta D^{+}G(S_{d}), where the Dini derivatives of a function ff are defined by D+f(x)=lim supϵ0f(x+ϵ)f(x)ϵD^{+}f(x)=\limsup\limits_{\epsilon\downarrow 0}\frac{f(x+\epsilon)-f(x)}{\epsilon} and Df(x)=lim infϵ0f(x)f(xϵ)ϵD^{-}f(x)=\liminf\limits_{\epsilon\downarrow 0}\frac{f(x)-f(x-\epsilon)}{\epsilon} (see, for instance, Kamihigashi and Roy (2007)).

We consider three cases:

  1. 1.

    If Sd<SS_{d}<S^{*}, then GG is differentiable at SdS_{d} and βG(Sd)=1=βF(Sb)\beta G^{\prime}(S_{d})=1=\beta F^{\prime}(S_{b}) which in turn implies that Sd=SbS_{d}=S_{b}.

  2. 2.

    If Sd>SS_{d}>S^{*}, then GG is differentiable at SdS_{d} and βF(Sb)=1=βG(Sd)>βF(Sd)\beta F^{\prime}(S_{b})=1=\beta G^{\prime}(S_{d})>\beta F^{\prime}(S_{d}) which in turn implies that Sd>SbS_{d}>S_{b} (because F(S)F^{\prime}(S) is decreasing).

  3. 3.

    If Sd=SS_{d}=S^{*}, then we have βF(Sd)=βDG(Sd)1βD+G(Sd)βF(Sd).\beta F^{\prime}(S_{d})=\beta D^{-}G(S_{d})\geq 1\geq\beta D^{+}G(S_{d})\geq\beta F^{\prime}(S_{d}). So, S=S=SbS=S^{*}=S_{b}.

Suming up three cases, we have that SdSbS_{d}\geq S_{b} in any case. Since X0<SbX_{0}<S_{b}, we have S1<SbSS_{1}<S_{b}\leq S. Hence StS_{t} is increasing in tt. Moreover, when aa and bb are high enough, we have Sb>SS_{b}>S^{*}. In this case, we have Sd>SS_{d}>S^{*}. According to the three cases mentioned above, we must have Sd>SbS_{d}>S_{b}.

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