diff --git a/lectures/olg.md b/lectures/olg.md index 23b97701..3fa56e38 100644 --- a/lectures/olg.md +++ b/lectures/olg.md @@ -18,7 +18,7 @@ is used by policy makers and researchers to examine * fiscal policy * monetary policy -* long run growth +* long-run growth and many other topics. @@ -57,7 +57,7 @@ prices, etc.) The OLG model takes up this challenge. We will present a simple version of the OLG model that clarifies the decision -problem of households and studies the implications for long run growth. +problem of households and studies the implications for long-run growth. Let's start with some imports. @@ -70,7 +70,7 @@ import matplotlib.pyplot as plt ## Environment -We assume that time is discrete, so that $t=0, 1, \ldots$, +We assume that time is discrete, so that $t=0, 1, \ldots$. An individual born at time $t$ lives for two periods, $t$ and $t + 1$. @@ -144,7 +144,7 @@ Here - $s_t$ is savings by an individual born at time $t$ - $w_t$ is the wage rate at time $t$ -- $R_{t+1}$ is the interest rate on savings invested at time $t$, paid at time $t+1$ +- $R_{t+1}$ is the gross interest rate on savings invested at time $t$, paid at time $t+1$ Since $u$ is strictly increasing, both of these constraints will hold as equalities at the maximum. @@ -225,7 +225,7 @@ economy. ## Demand for capital -First we describe the firm problem and then we write down an equation +First we describe the firm's problem and then we write down an equation describing demand for capital given prices. @@ -246,7 +246,7 @@ The profit maximization problem of the firm is ```{math} :label: opt_profit_olg - \max_{k_t, \ell_t} \{ k^{\alpha}_t \ell_t^{1-\alpha} - R_t k_t - \ell_t w_t \} + \max_{k_t, \ell_t} \{ k^{\alpha}_t \ell_t^{1-\alpha} - R_t k_t -w_t \ell_t \} ``` The first-order conditions are obtained by taking the derivative of the @@ -447,7 +447,7 @@ In particular, since $w_t = (1-\alpha)k_t^\alpha$, we have If we iterate on this equation, we get a sequence for capital stock. -Let's plot the 45 degree diagram of these dynamics, which we write as +Let's plot the 45-degree diagram of these dynamics, which we write as $$ k_{t+1} = g(k_t) @@ -463,12 +463,9 @@ def k_update(k, α, β): ```{code-cell} ipython3 α, β = 0.5, 0.9 kmin, kmax = 0, 0.1 -x = 1000 -k_grid = np.linspace(kmin, kmax, x) -k_grid_next = np.empty_like(k_grid) - -for i in range(x): - k_grid_next[i] = k_update(k_grid[i], α, β) +n = 1000 +k_grid = np.linspace(kmin, kmax, n) +k_grid_next = k_update(k_grid,α,β) fig, ax = plt.subplots(figsize=(6, 6)) @@ -520,7 +517,7 @@ R_star = (α/(1 - α)) * ((1 + β) / β) ### Time series -The 45 degree diagram above shows that time series of capital with positive initial conditions converge to this steady state. +The 45-degree diagram above shows that time series of capital with positive initial conditions converge to this steady state. Let's plot some time series that visualize this. @@ -554,6 +551,7 @@ fig, ax = plt.subplots() ax.plot(R_series, label="gross interest rate") ax.plot(range(ts_length), np.full(ts_length, R_star), 'k--', label="$R^*$") ax.set_ylim(0, 4) +ax.set_ylabel("gross interest rate") ax.set_xlabel("$t$") ax.legend() plt.show() @@ -629,7 +627,6 @@ def savings_crra(w, R, model): ``` ```{code-cell} ipython3 -R_vals = np.linspace(0.3, 1) model = create_olg_model() w = 2.0 @@ -685,7 +682,7 @@ In the exercise below, you will be asked to solve these equations numerically. Solve for the dynamics of equilibrium capital stock in the CRRA case numerically using [](law_of_motion_capital_crra). -Visualize the dynamics using a 45 degree diagram. +Visualize the dynamics using a 45-degree diagram. ``` @@ -735,15 +732,15 @@ def k_update(k, model): return optimize.newton(lambda k_prime: f(k_prime, k, model), 0.1) ``` -Finally, here is the 45 degree diagram. +Finally, here is the 45-degree diagram. ```{code-cell} ipython3 kmin, kmax = 0, 0.5 -x = 1000 -k_grid = np.linspace(kmin, kmax, x) +n = 1000 +k_grid = np.linspace(kmin, kmax, n) k_grid_next = np.empty_like(k_grid) -for i in range(x): +for i in range(n): k_grid_next[i] = k_update(k_grid[i], model) fig, ax = plt.subplots(figsize=(6, 6)) @@ -768,7 +765,7 @@ plt.show() ```{exercise} :label: olg_ex2 -The 45 degree diagram from the last exercise shows that there is a unique +The 45-degree diagram from the last exercise shows that there is a unique positive steady state. The positive steady state can be obtained by setting $k_{t+1} = k_t = k^*$ in [](law_of_motion_capital_crra), which yields