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14.451 Lecture Notes Economic Growth

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• Define λt ≡ β t µ t and<br />

George-Marios Angeletos<br />

Ht ≡ H(kt,kt+1,ct,lt,λt) ≡<br />

≡ U(ct, 1 − lt)+λt [(1 − δ)kt + F (kt,lt) − kt+1 − ct]<br />

H is called the Hamiltonian of the problem.<br />

• We can rewrite the Lagrangian as<br />

∞X<br />

L0 =<br />

=<br />

β<br />

t=0<br />

t {U(ct, 1 − lt)+λt [(1 − δ)kt + F (kt,lt) − kt+1 − ct]} =<br />

∞X<br />

t=0<br />

or, in recursive form<br />

β t Ht<br />

Lt = Ht + βLt+1.<br />

• Given kt, ct and lt enter only the period t utility and resource constraint; (ct,lt) thus<br />

appears only in Ht. Similarly, kt,enter only the period t and t +1utility and resource<br />

constraints; they thus appear only in Ht and Ht+1. Therefore,<br />

Lemma 9 If {ct,lt,kt+1} ∞ t=0 is the optimum and {λt} ∞ t=0 the associated multipliers, then<br />

taking (kt,kt+1) as given, and<br />

kt+1 =argmax<br />

k 0<br />

taking (kt,kt+2) as given.<br />

(ct,lt) =argmax<br />

c,l<br />

Ht<br />

z }| {<br />

H(kt,kt+1,c,l,λt)<br />

Ht + βHt+1<br />

z }| {<br />

H(kt,k 0 ,ct,lt,λt)+βH(k 0 ,kt+2,ct+1,lt+1,λt+1)<br />

46

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