[MIT中级宏观经济学课件]chapter6

14.06Lecture Notes Intermediate Macroeconomics

George-Marios Angeletos

MIT Department of Economics

Spring2004

Chapter6

Endogenous Growth I:AK,H,and G

6.1The Simple AK Model

6.1.1Pareto Allocations

•Total output in the economy is given by

Y t=F(K t,L t)=AK t,

where A>0is an exogenous parameter.In intensive form,

y t=f(k t)=Ak t.

•The social planner’s problem is the same as in the Ramsey model,except for the fact that output is linear in capital:

∞X t=0u(c t)

max

s.t.c t+k t+1≤f(k t)+(1−δ)k t

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George-Marios Angeletos

•The Euler condition gives

u0(c t)

u0(c t+1)

=β(1+A−δ)

Assuming CEIS,this reduces to

c t+1

c t

=[β(1+A−δ)]θ

or

ln c t+1−ln c t≈θ(R−ρ)

where R=A−δis the net social return on capital.That is,consumption growth is proportional to the difference between the real return on capital and the discount rate.Note that now the real return is a constant,rather than diminishing with capital accumulation.

•Note that the resource constraint can be rewritten as

c t+k t+1=(1+A−δ)k t.

Since total resources(the RHS)are linear in k,an educated guess is that optimal consumption and investment are also linear in k.We thus propose

c t=(1−s)(1+A−δ)k t

k t+1=s(1+A−δ)k t

where the coefficient s is to be determined and must satisfy s∈(0,1)for the solution to exist.

•It follows that

c t+1

c t =

k t+1

k t

=

y t+1

y t 114

Lecture Notes

so that consumption,capital and income all grow at the same rate.To ensure perpetual growth,we thus need to impose

β(1+A−δ)>1,

or equivalently A−δ>ρ.If that condition were not satisfied,and instead A−δ<ρ, then the economy would shrink at a constant rate towards zero.

•From the resource constraint we then have

c t t +

k t+1

t

=(1+A−δ),

implying that the consumption-capital ratio is given by

c t

k t

=(1+A−δ)−[β(1+A−δ)]θ

Using c t=(1−s)(1+A−δ)k t and solving for s we conclude that the optimal saving rate is

s=βθ(1+A−δ)θ−1.

Equivalently,s=βθ(1+R)θ−1,where R=A−δis the net social return on capital. Note that the saving rate is increasing(decreasing)in the real return if and only if the EIS is higher(lower)than unit,and s=βforθ=1.Finally,to ensure s∈(0,1),we impose

βθ(1+A−δ)θ−1<1.

This is automatically ensured whenθ≤1andβ(1+A−δ)>1,as then s=βθ(1+A−δ)θ−1≤β<1.But whenθ>1,this puts an upper bound on A.If A exceeded that upper bound,then the social planner could attain infinite utility,and the problem is not well-defined.

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