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lectures/ak2.md

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@@ -218,10 +218,10 @@ $$
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To maximize profits a firm equates marginal products to rental rates:
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$$
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\begin{align}
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\begin{aligneded}
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W_t & = (1-\alpha) K_t^\alpha L_t^{-\alpha} \\
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r_t & = \alpha K_t^\alpha L_t^{1-\alpha}
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\end{align}
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\end{aligneded}
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$$ (eq:firmfonc)
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Output can be consumed either by old people or young people; or sold to young people who use it to augment the capital stock; or sold to the government for uses that do not generate utility for the people in the model (i.e., ``it is thrown into the ocean'').
@@ -274,10 +274,10 @@ $$ (eq:utilfn)
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subject to the following budget constraints at times $t$ and $t+1$:
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$$
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\begin{align}
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\begin{aligned}
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C_{yt} + A_{t+1} & = W_t (1 - \tau_t) - \delta_{yt} \\
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C_{ot+1} & = (1+ r_{t+1} (1 - \tau_{t+1}))A_{t+1} - \delta_{ot}
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\end{align}
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\end{aligned}
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$$ (eq:twobudgetc)
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@@ -290,9 +290,9 @@ $$ (eq:onebudgetc)
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To solve the young person's choice problem, form a Lagrangian
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$$
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\begin{align}
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\begin{aligned}
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{\mathcal L} & = C_{yt}^\beta C_{o,t+1}^{1-\beta} \\ & + \lambda \Bigl[ C_{yt} + \frac{C_{ot+1}}{1 + r_{t+1}(1 - \tau_{t+1})} - W_t (1 - \tau_t) + \delta_{yt} + \frac{\delta_{ot}}{1 + r_{t+1}(1 - \tau_{t+1})}\Bigr],
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\end{align}
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\end{aligned}
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$$ (eq:lagC)
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where $\lambda$ is a Lagrange multiplier on the intertemporal budget constraint {eq}`eq:onebudgetc`.
@@ -302,10 +302,10 @@ After several lines of algebra, the intertemporal budget constraint {eq}`eq:oneb
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imply that an optimal consumption plan satisfies
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$$
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\begin{align}
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\begin{aligned}
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C_{yt} & = \beta \Bigl[ W_t (1 - \tau_t) - \delta_{yt} - \frac{\delta_{ot}}{1 + r_{t+1}(1 - \tau_{t+1})}\Bigr] \\
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\frac{C_{0t+1}}{1 + r_{t+1}(1-\tau_{t+1}) } & = (1-\beta) \Bigl[ W_t (1 - \tau_t) - \delta_{yt} - \frac{\delta_{ot}}{1 + r_{t+1}(1 - \tau_{t+1})}\Bigr]
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\end{align}
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\end{aligned}
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$$ (eq:optconsplan)
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The first-order condition for minimizing Lagrangian {eq}`eq:lagC` with respect to the Lagrange multipler $\lambda$ recovers the budget constraint {eq}`eq:onebudgetc`,
@@ -351,10 +351,10 @@ As our special case of {eq}`eq:optconsplan`, we compute the following consumptio
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$$
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\begin{align}
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\begin{aligned}
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C_{yt} & = \beta (1 - \tau_t) W_t \\
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A_{t+1} &= (1-\beta) (1- \tau_t) W_t
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\end{align}
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\end{aligned}
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$$
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Using {eq}`eq:firmfonc` and $A_t = K_t + D_t$, we obtain the following closed form transition law for capital:
@@ -368,20 +368,20 @@ $$ (eq:Klawclosed)
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From {eq}`eq:Klawclosed` and the government budget constraint {eq}`eq:govbudgetsequence`, we compute **time-invariant** or **steady state values** $\hat K, \hat D, \hat T$:
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$$
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\begin{align}
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\begin{aligned}
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\hat{K} &=\hat{K}\left(1-\hat{\tau}\right)\left(1-\alpha\right)\left(1-\beta\right) - \hat{D} \\
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\hat{D} &= (1 + \hat{r}) \hat{D} + \hat{G} - \hat{T} \\
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\hat{T} &= \hat{\tau} \hat{Y} + \hat{\tau} \hat{r} \hat{D} .
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\end{align}
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\end{aligned}
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$$ (eq:steadystates)
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These imply
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$$
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\begin{align}
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\begin{aligned}
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\hat{K} &= \left[\left(1-\hat{\tau}\right)\left(1-\alpha\right)\left(1-\beta\right)\right]^{\frac{1}{1-\alpha}} \\
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\hat{\tau} &= \frac{\hat{G} + \hat{r} \hat{D}}{\hat{Y} + \hat{r} \hat{D}}
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\end{align}
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\end{aligned}
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$$
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Let's take an example in which
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Our formulas for steady-state values tell us that
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$$
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\begin{align}
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\begin{aligned}
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\hat{D} &= 0 \\
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\hat{G} &= 0.15 \hat{Y} \\
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\hat{\tau} &= 0.15 \\
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\end{align}
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\end{aligned}
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$$
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@@ -694,12 +694,12 @@ To illustrate the power of `ClosedFormTrans`, let's first experiment with the fo
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The following equations completely characterize the equilibrium transition path originating from the initial steady state
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$$
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\begin{align}
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\begin{aligned}
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K_{t+1} &= K_{t}^{\alpha}\left(1-\tau_{t}\right)\left(1-\alpha\right)\left(1-\beta\right) - \bar{D} \\
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\tau_{0} &= (1-\frac{1}{3}) \hat{\tau} \\
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\bar{D} &= \hat{G} - \tau_0\hat{Y} \\
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\quad\tau_{t} & =\frac{\hat{G}+r_{t} \bar{D}}{\hat{Y}+r_{t} \bar{D}}
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\end{align}
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\end{aligned}
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$$
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We can simulate the transition for $20$ periods, after which the economy will be close to a new steady state.

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