misc (#699)

Author: jstac

lectures/mccall_model_with_sep_markov.md

@@ -34,8 +34,8 @@ kernelspec:
 This lecture builds on the job search model with separation presented in the
 {doc}`previous lecture <mccall_model_with_separation>`.
 
-The key difference is that wage offers now follow a **Markov chain** rather than
-being independent and identically distributed (IID).
+The key difference is that wage offers now follow a {doc}`Markov chain <finite_markov>` rather than
+being IID.
 
 This modification adds persistence to the wage offer process, meaning that
 today's wage offer provides information about tomorrow's offer.
@@ -264,8 +264,7 @@ def T(v: jnp.ndarray, model: Model) -> jnp.ndarray:
     return jnp.maximum(accept, reject)
 ```
 
-Here's a routine for value function iteration, as well as a second routine that
-computes the reservation wage directly from the value function.
+Here's a routine for value function iteration.
 
 ```{code-cell} ipython3
 @jax.jit
@@ -293,67 +292,65 @@ def vfi(
     v_final, error, i = final_loop_state
 
     return v_final
+```
 
+Here is a routine that computes the reservation wage from the value function.
 
+```{code-cell} ipython3
 @jax.jit
 def get_reservation_wage(v: jnp.ndarray, model: Model) -> float:
     """
-    Calculate the reservation wage directly from the value function.
+    Calculate the reservation wage from the unemployed agents 
+    value function v := v_u.
 
     The reservation wage is the lowest wage w where accepting (v_e(w))
-    is at least as good as rejecting (u(c) + β(Pv)(w)).
-
-    Parameters:
-    - v: Value function v_u
-    - model: Model instance containing parameters
+    is at least as good as rejecting (u(c) + β(Pv_u)(w)).
 
-    Returns:
-    - Reservation wage (lowest wage for which acceptance is optimal)
     """
     n, w_vals, P, P_cumsum, β, c, α, γ = model
 
     # Compute accept and reject values
     d = 1 / (1 - β * (1 - α))
-    accept = d * (u(w_vals, γ) + α * β * P @ v)
-    reject = u(c, γ) + β * P @ v
+    v_e = d * (u(w_vals, γ) + α * β * P @ v)
+    continuation_value = u(c, γ) + β * P @ v
 
     # Find where acceptance becomes optimal
-    should_accept = accept >= reject
-    first_accept_idx = jnp.argmax(should_accept)
+    accept_indices = v_e >= continuation_value
+    first_accept_idx = jnp.argmax(accept_indices)  # index of first True
 
     # If no acceptance (all False), return infinity
     # Otherwise return the wage at the first acceptance index
-    return jnp.where(jnp.any(should_accept), w_vals[first_accept_idx], jnp.inf)
+    return jnp.where(jnp.any(accept_indices), w_vals[first_accept_idx], jnp.inf)
 ```
 
+
 ## Computing the Solution
 
 Let's solve the model:
 
 ```{code-cell} ipython3
 model = create_js_with_sep_model()
 n, w_vals, P, P_cumsum, β, c, α, γ = model
-v_star = vfi(model)
-w_star = get_reservation_wage(v_star, model)
+v_u = vfi(model)
+w_bar = get_reservation_wage(v_u, model)
 ```
 
 Next we compute some related quantities for plotting.
 
 ```{code-cell} ipython3
 d = 1 / (1 - β * (1 - α))
-accept = d * (u(w_vals, γ) + α * β * P @ v_star)
-h_star = u(c, γ) + β * P @ v_star
+v_e = d * (u(w_vals, γ) + α * β * P @ v_u)
+h = u(c, γ) + β * P @ v_u
 ```
 
 Let's plot our results.
 
 ```{code-cell} ipython3
 fig, ax = plt.subplots(figsize=(9, 5.2))
-ax.plot(w_vals, h_star, linewidth=4, ls="--", alpha=0.4,
-        label="continuation value")
-ax.plot(w_vals, accept, linewidth=4, ls="--", alpha=0.4,
-        label="stopping value")
-ax.plot(w_vals, v_star, "k-", alpha=0.7, label=r"$v_u^*(w)$")
+ax.plot(w_vals, h, 'g-', linewidth=2, 
+        label="continuation value function $h$")
+ax.plot(w_vals, v_e, 'b-', linewidth=2, 
+        label="employment value function $v_e$")
 ax.legend(frameon=False)
 ax.set_xlabel(r"$w$")
 plt.show()
@@ -370,16 +367,16 @@ Let's examine how reservation wages change with the separation rate.
 ```{code-cell} ipython3
 α_vals: jnp.ndarray = jnp.linspace(0.0, 1.0, 10)
 
-w_star_vec = []
+w_bar_vec = []
 for α in α_vals:
     model = create_js_with_sep_model(α=α)
-    v_star = vfi(model)
-    w_star = get_reservation_wage(v_star, model)
-    w_star_vec.append(w_star)
+    v_u = vfi(model)
+    w_bar = get_reservation_wage(v_u, model)
+    w_bar_vec.append(w_bar)
 
 fig, ax = plt.subplots(figsize=(9, 5.2))
 ax.plot(
-    α_vals, w_star_vec, linewidth=2, alpha=0.6, label="reservation wage"
+    α_vals, w_bar_vec, linewidth=2, alpha=0.6, label="reservation wage"
 )
 ax.legend(frameon=False)
 ax.set_xlabel(r"$\alpha$")
@@ -414,7 +411,7 @@ unemployed, 1 if employed) and $w_t$ is
 * their current wage, if employed. 
 
 ```{code-cell} ipython3
-def update_agent(key, status, wage_idx, model, w_star):
+def update_agent(key, status, wage_idx, model, w_bar):
     """
     Updates an agent's employment status and current wage.
 
@@ -423,7 +420,7 @@ def update_agent(key, status, wage_idx, model, w_star):
     - status: Current employment status (0 or 1)
     - wage_idx: Current wage, recorded as an array index
     - model: Model instance
-    - w_star: Reservation wage
+    - w_bar: Reservation wage
 
     """
     n, w_vals, P, P_cumsum, β, c, α, γ = model
@@ -436,7 +433,7 @@ def update_agent(key, status, wage_idx, model, w_star):
     )
     separation_occurs = jax.random.uniform(key2) < α
     # Accept if current wage meets or exceeds reservation wage
-    accepts = w_vals[wage_idx] >= w_star
+    accepts = w_vals[wage_idx] >= w_bar
 
     # If employed: status = 1 if no separation, 0 if separation
     # If unemployed: status = 1 if accepts, 0 if rejects
@@ -462,7 +459,7 @@ Here's a function to simulate the employment path of a single agent.
 ```{code-cell} ipython3
 def simulate_employment_path(
         model: Model,     # Model details
-        w_star: float,    # Reservation wage
+        w_bar: float,    # Reservation wage
         T: int = 2_000,   # Simulation length
         seed: int = 42    # Set seed for simulation
     ):
@@ -487,7 +484,7 @@ def simulate_employment_path(
 
         key, subkey = jax.random.split(key)
         status, wage_idx = update_agent(
-            subkey, status, wage_idx, model, w_star
+            subkey, status, wage_idx, model, w_bar
         )
 
     return jnp.array(wage_path), jnp.array(status_path)
@@ -499,10 +496,10 @@ Let's create a comprehensive plot of the employment simulation:
 model = create_js_with_sep_model()
 
 # Calculate reservation wage for plotting
-v_star = vfi(model)
-w_star = get_reservation_wage(v_star, model)
+v_u = vfi(model)
+w_bar = get_reservation_wage(v_u, model)
 
-wage_path, employment_status = simulate_employment_path(model, w_star)
+wage_path, employment_status = simulate_employment_path(model, w_bar)
 
 fig, (ax1, ax2, ax3) = plt.subplots(3, 1, figsize=(8, 6))
 
@@ -518,8 +515,8 @@ ax1.set_ylim(-0.1, 1.1)
 
 # Plot wage path with employment status coloring
 ax2.plot(wage_path, 'b-', alpha=0.7, linewidth=1)
-ax2.axhline(y=w_star, color='black', linestyle='--', alpha=0.8,
-           label=f'Reservation wage: {w_star:.2f}')
+ax2.axhline(y=w_bar, color='black', linestyle='--', alpha=0.8,
+           label=f'Reservation wage: {w_bar:.2f}')
 ax2.set_xlabel('time')
 ax2.set_ylabel('wage')
 ax2.set_title('Wage path (actual and offers)')
@@ -620,7 +617,7 @@ We first create a vectorized version of `update_agent` to efficiently update all
 
 ```{code-cell} ipython3
 # Create vectorized version of update_agent
-# The last parameter is now w_star (scalar) instead of σ (array)
+# The last parameter is now w_bar (scalar) instead of σ (array)
 update_agents_vmap = jax.vmap(
     update_agent, in_axes=(0, 0, 0, None, None)
 )
@@ -633,7 +630,7 @@ Next we define the core simulation function, which uses `lax.fori_loop` to effic
 def _simulate_cross_section_compiled(
         key: jnp.ndarray,
         model: Model,
-        w_star: float,
+        w_bar: float,
         n_agents: int,
         T: int
     ):
@@ -653,7 +650,7 @@ def _simulate_cross_section_compiled(
         agent_keys = jax.random.split(subkey, n_agents)
 
         status, wage_indices = update_agents_vmap(
-            agent_keys, status, wage_indices, model, w_star
+            agent_keys, status, wage_indices, model, w_bar
         )
 
         return key, status, wage_indices
@@ -688,12 +685,12 @@ def simulate_cross_section(
     key = jax.random.PRNGKey(seed)
 
     # Solve for optimal reservation wage
-    v_star = vfi(model)
-    w_star = get_reservation_wage(v_star, model)
+    v_u = vfi(model)
+    w_bar = get_reservation_wage(v_u, model)
 
     # Run JIT-compiled simulation
     final_status = _simulate_cross_section_compiled(
-        key, model, w_star, n_agents, T
+        key, model, w_bar, n_agents, T
     )
 
     # Calculate unemployment rate at final period
@@ -717,10 +714,10 @@ def plot_cross_sectional_unemployment(model: Model, t_snapshot: int = 200,
     """
     # Get final employment state directly
     key = jax.random.PRNGKey(42)
-    v_star = vfi(model)
-    w_star = get_reservation_wage(v_star, model)
+    v_u = vfi(model)
+    w_bar = get_reservation_wage(v_u, model)
     final_status = _simulate_cross_section_compiled(
-        key, model, w_star, n_agents, t_snapshot
+        key, model, w_bar, n_agents, t_snapshot
     )
 
     # Calculate unemployment rate