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14.451 Lecture Notes Economic Growth

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Equivalently,<br />

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

<strong>14.451</strong> <strong>Lecture</strong> <strong>Notes</strong><br />

(ct,lt,kt+1,ct+1,lt+1, ) = arg max<br />

c,l,k0 ,c0 ,l0[U(c, l)+βU(c0 ,l 0 )]<br />

s.t. c + k 0 ≤ (1 − δ)kt + F (kt,l)<br />

c 0 + kt+2 ≤ (1 − δ)k 0 + F (k 0 ,l 0 )<br />

• We henceforth assume an interior solution. As long as kt > 0, interior solution is indeed<br />

ensured by the Inada conditions on F and U.<br />

• The FOC with respect to ct gives<br />

The FOC with respect to lt gives<br />

∂L0<br />

∂ct<br />

= β t ∂Ht<br />

∂ct<br />

=0⇔<br />

∂Ht<br />

∂ct<br />

=0⇔<br />

Uc(ct,zt) =λt<br />

∂L0<br />

∂lt<br />

t ∂Ht<br />

= β<br />

∂lt<br />

=0⇔<br />

∂Ht<br />

∂lt<br />

=0⇔<br />

Uz(ct,zt) =λtFL(kt,lt)<br />

Finally, the FOC with respect to kt+1 gives<br />

∂L0<br />

= β<br />

∂kt+1<br />

t<br />

·<br />

∂Ht<br />

+ β<br />

∂kt+1<br />

∂Ht+1<br />

¸<br />

=0⇔<br />

∂kt+1<br />

−λt + β ∂Ht+1<br />

=0⇔<br />

∂kt+1<br />

λt = β [1 − δ + FK(kt+1,lt+1)] λt+1<br />

47

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