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Global Dynamics and Optimal Policy in the Ak Model with Anticipated Future Consumption

  • Manuel A. Gomez EMAIL logo
Veröffentlicht/Copyright: 13. März 2024

Abstract

This paper analyzes the Ak model with anticipated future consumption. In the model with internal anticipation individual’s utility depends on current consumption and a forward-looking reference level which is formed from individual’s own future consumption. The global dynamics of the economy is characterized by means of a qualitative phase diagram analysis. In the model with external anticipation the consumption reference level is formed from economy-wide average future consumption, which is taken as given by individuals and causes the competitive equilibrium to be inefficient. Characterizing the global dynamics of the economy is required to characterize an optimal fiscal policy that corrects the inefficiency brought about by this external effect.

JEL Classification: O41; H21

Corresponding author: Manuel A. Gomez, Department of Economics, Facultad de Economía y Empresa, Universidade da Coruña, Campus de Elviña, A Coruna, 15071, Spain; and ECOBAS, Universidade da Coruña, C+D Group, Departamento de Economía, 15071 A Coruña, España, E-mail:

Funding source: MCIN/AEI/10.13039/501100011033

Award Identifier / Grant number: PID2021-127599NB-I00

Funding source: European Regional Development Fund A way of making Europe

Award Identifier / Grant number: PID2021-127599NB-I00

Acknowledgments

I gratefully acknowledge the helpful comments of an anonymous referee. Grant PID2021-127599NB-I00 funded by MCIN/AEI/10.13039/501100011033 and by “ERDF A way of making Europe”.

Appendix A: The Condition sign det H u ( c , 1 ) = constant

To give an intuition on the meaning of condition P#6, let us note definition (14),

π ( c ) u A ( c , 1 ) u C ( c , 1 ) = u A ( C , A ) u C ( C , A ) d C d A u = c o n s t a n t ,

so −π(c) is the slope of the indifference curves of u depicted in the (A, C)–plane, and its absolute value is the marginal rate of substitution (MRS) of A for C. Differentiating π(c) we obtain

π ( c ) = u C A ( c , 1 ) u C ( c , 1 ) u C C ( c , 1 ) u A ( c , 1 ) u C ( c , 1 ) 2 = u C C ( c , 1 ) u A A ( c , 1 ) u C A ( c , 1 ) 2 v u C ( c , 1 ) 2 ,

so that sign π ( c ) = sign det H u ( c , 1 ) , where we have used that

u A ( c , 1 ) = c u C A ( c , 1 ) + u A A ( c , 1 ) v , u C ( c , 1 ) = c u C C ( c , 1 ) + u C A ( c , 1 ) v .

We have that

C u A ( c , 1 ) u C ( c , 1 ) = c C d d c u A ( c , 1 ) u C ( c , 1 ) = 1 A π ( c ) , A u A ( c , 1 ) u C ( c , 1 ) = c A d d c u A ( c , 1 ) u C ( c , 1 ) = 1 A c π ( c ) .

Hence, the convexity of the indifference curves can be studied from

d 2 C d A 2 = C u A ( c , 1 ) u C ( c , 1 ) d C d A A u A ( c , 1 ) u C ( c , 1 ) = 1 A π ( c ) c + π ( c ) ,

where P#3 entails that c + π(c) > 0, so sign d 2 C / d A 2 = sign π ( c ) .

If u A > 0, so both C and A are ‘goods’, the indifference curves are downward sloping and strictly convex (resp., concave) if π′(c) > 0 (resp., π′(c) < 0), and so, an increase in C, keeping A constant, causes the indifference curves to become steeper (flatter), and an increase in A, keeping C constant, causes the indifference curves to become flatter (steeper). If u A < 0, so A is a bad, the indifference curves are upward sloping and strictly concave (resp., convex) if π′(c) < 0 (resp., π′(c) > 0), and so, an increase in C, keeping A constant, causes the indifference curves to become flatter (steeper), and an increase in A, keeping C constant, causes the indifference curves to become steeper (flatter). Thus, the higher is the value of C, the higher is the increase in C needed to compensate a given increase in A to keep utility constant. Hence, the more meaningful situation seems to be that π′(c) > 0 if u A > 0 and π′(c) < 0 if u A < 0. The assumption P#6 that sign det H u ( c , 1 ) = sign π ( c ) is constant for all c simply states that the effect of an increase of C or A on the MRS goes always in the same direction so that the indifference curves are either strictly concave or strictly convex.

To illustrate the former discussion, let us consider the CES utility function

u ( C , A ) = 1 1 ϵ C ϕ + γ A ϕ 1 + γ ( 1 ϵ ) / ϕ , 0 < ϕ < 1 , ϵ > 1 ϕ , ϵ 1 , γ > 1 ,

which tends to the additive specification

u ( C , A ) = 1 1 ϵ C + γ A 1 + γ 1 ϵ

as ϕ → 1, and to the multiplicative specification

u ( C , A ) = ( C A γ ) ( 1 ϵ ) / ( 1 + γ ) 1 ϵ

as ϕ → 0. This specification implies that

u C ( C , A ) = C ϕ 1 1 + γ C ϕ + γ A ϕ 1 + γ ( 1 ϵ ϕ ) / ϕ , u A ( C , A ) = γ A ϕ 1 1 + γ C ϕ + γ A ϕ 1 + γ ( 1 ϵ ϕ ) / ϕ , π ( c ) = u A ( c , 1 ) u C ( c , 1 ) = γ c 1 ϕ ,

and so, sign π ( c ) = sign ( γ ) = sign ( u A ) . Hence, in the altruistic specification γ > 0 we have that π′(c) > 0, the utility function is strictly concave and the indifference curves are strictly decreasing and strictly convex. In the jealousy specification γ < 0 we have that π′(c) < 0 and the indifference curves are strictly increasing and strictly concave.

Appendix B: Dynamics of the Centrally Planned Economy

From (6) we have that

(B.1) λ = u C ( C , A ) / ( 1 + ρ q ) ,

(B.2) μ = q u C ( C , A ) / ( 1 + ρ q ) .

Using that q ̇ / q = μ ̇ / μ λ ̇ / λ , taking into account (7) and (8), together with (B.2) and the homogeneity of degree −v of u C and u A , we get Eq. (11).

Log-differentiating (B.1), after simplification we get

C ̇ C = u C ( C , A ) C u C C ( C , A ) λ ̇ λ A u C A ( C , A ) u C ( C , A ) A ̇ A + ρ q ̇ 1 + ρ q .

Now, using (3) and (7) and the homogeneity of degree −v of u C , we can obtain that

(B.3) C ̇ C = σ ( c ) B v A ̇ A ρ q ̇ 1 + ρ q β + A ̇ A ,

where σ(c) is defined in (13). From c ̇ / c = C ̇ / C A ̇ / A , using (B.3) and (3), we immediately get Eq. (10). Finally, Eq. (12) is obtained using that a ̇ / a = A ̇ / A K ̇ / K , taking into account (3) and (5).

Appendix C: Dynamics of the Market Economy with Government

The agent maximizes her utility (1) subject to the budget constraint (27), taking as given C ̃ and the initial condition on capital. The Hamiltonian of this problem is

H = u ( C , A ) + λ D ( 1 τ Y ) B K ( 1 + τ C ) C + S ,

where λ D is the shadow value of capital. The first-order conditions for an interior optimum, which are also sufficient, are

(C.1) H C = 0 u C ( C , A ) = ( 1 + τ C ) λ D ,

(C.2) H K = β λ D λ ̇ D ( 1 τ Y ) B = β λ ̇ D / λ D ,

together with the initial condition, K(0) = K 0, and the usual transversality condition lim t→∞eβt λ D K = 0. From (27) and (28), we get the resources’ constraint

(C.3) K ̇ = B K C .

In what follows we will use that C ̃ = C in a symmetric equilibrium. Log-differentiating (C.1), using (C.2), we get the Euler equation

C ̇ C = u C ( C , A ) C u C C ( C , A ) ( 1 τ Y ) B τ ̇ C 1 + τ C + u C A ( C , A ) u C ( C , A ) A ̇ β .

Denoting cC/A and aA/K, and using the homogeneity of degree −v of u C , this equation can be rewritten as

(C.4) C ̇ C = σ ( c ) ( 1 τ Y ) B τ ̇ C 1 + τ C v u C ( c , 1 ) + c u C C ( c , 1 ) u C ( c , 1 ) A ̇ A β = σ ( c ) ( 1 τ Y ) B τ ̇ C 1 + τ C v A ̇ A β + A ̇ A ,

where σ(c) is defined in (13).

From c ̇ / c = C ̇ / C A ̇ / A , using (C.4) and (3), we get (29), and from a ̇ / a = A ̇ / A K ̇ / K , using (C.3) and (3), we get (30).

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Received: 2023-07-28
Accepted: 2024-02-16
Published Online: 2024-03-13

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