[stationary_densities] Fix typos and align with style manual (#331)

* [stationary_densities] Fix typos and minor code issues

- Fix "for all s" -> "for all x" in state space remark
- Correct off-by-one comment on density dates (1,...,T)
- Remove stray trailing comma in boxplot data display
- "discrete-time" -> "discrete time" (noun usage)
- Add missing period after Markov operator definition
- Initialize X[0] in exercise 1 solution (was uninitialized np.empty)
- Boxplot titles: label initial condition as X_0, not t
- Comments: B ~ N(0, a_σ**2) since a_σ is the std dev

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* [stationary_densities] Align lecture with QuantEcon style manual

- Links: use intersphinx {doc} links for finite_markov references,
  normalize outdated python-intro.quantecon.org domain, auto-title
  link for arma
- Citation: use {cite:t} for in-sentence Hopenhayn-Rogerson reference
- Definitions: bold (not italic) for stochastic kernel, Markov
  operator, look-ahead, stationary, global stability, ergodicity
- Math: square brackets for expectation, \mathrm{LN} for lognormal,
  "CDFs" capitalization
- Figures: replace ax.set_title with mystnb caption on the main
  example, caption the static stability figure, drop unneeded
  figsize settings, format ex-3 panel titles to one decimal
- RNG: migrate to np.random.default_rng Generator API; seed scipy
  .rvs calls via random_state for reproducibility

Verified by executing the full lecture via jupytext --to py and
inspecting all five generated figures.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* FIX: disambiguate arma cross-reference to fix build

The auto-title link `[](arma)` was ambiguous: MyST found both a doc
target (Covariance Stationary Processes) and an equation labelled
`arma`, raising myst.xref_ambiguous. With warnings treated as errors
the book build failed. Use the explicit `{doc}` role to resolve to the
document while keeping the auto-generated title.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Co-authored-by: Matt McKay <mmcky@users.noreply.github.com>

Author: 3 people

lectures/stationary_densities.md

@@ -34,20 +34,20 @@ tags: [hide-output]
 
 ## Overview
 
-In a [previous lecture](https://python-intro.quantecon.org/finite_markov.html), we learned about finite Markov chains, a relatively elementary class of stochastic dynamic models.
+In {doc}`intermediate:finite_markov`, we learned about finite Markov chains, a relatively elementary class of stochastic dynamic models.
 
 The present lecture extends this analysis to continuous (i.e., uncountable) state Markov chains.
 
 Most stochastic dynamic models studied by economists either fit directly into this class or can be represented as continuous state Markov chains after minor modifications.
 
 In this lecture, our focus will be on continuous Markov models that
 
-* evolve in discrete-time
+* evolve in discrete time
 * are often nonlinear
 
 The fact that we accommodate nonlinear models here is significant, because
 linear stochastic models have their own highly developed toolset, as we'll
-see {doc}`later on <arma>`.
+see later in {doc}`arma`.
 
 The question that interests us most is: Given a particular stochastic dynamic
 model, how will the state of the system evolve over time?
@@ -96,7 +96,7 @@ Once we've built some intuition we'll cover the general case.
 
 ### Definitions and basic properties
 
-In our [lecture on finite Markov chains](https://python-intro.quantecon.org/finite_markov.html), we studied discrete-time Markov chains that evolve on a finite state space $S$.
+In our lecture {doc}`intermediate:finite_markov`, we studied discrete-time Markov chains that evolve on a finite state space $S$.
 
 In this setting, the dynamics of the model are described by a stochastic matrix --- a nonnegative square matrix $P = P[i, j]$ such that each row $P[i, \cdot]$ sums to one.
 
@@ -127,7 +127,7 @@ The family of discrete distributions $P[i, \cdot]$ will be replaced by a family
 
 Analogous to the finite state case, $p(x, \cdot)$ is to be understood as the distribution (density) of $X_{t+1}$ given $X_t = x$.
 
-More formally, a *stochastic kernel on* $S$ is a function $p \colon S \times S \to \mathbb R$ with the property that
+More formally, a **stochastic kernel on** $S$ is a function $p \colon S \times S \to \mathbb R$ with the property that
 
 1. $p(x, y) \geq 0$ for all $x, y \in S$
 1. $\int p(x, y) dy = 1$ for all $x \in S$
@@ -274,7 +274,7 @@ p(x, y)
 where $\phi$ is the density of $A_{t+1}$.
 
 (Regarding the state space $S$ for this model, a natural choice is $(0, \infty)$ --- in which case
-$\sigma(x) = s f(x)$ is strictly positive for all $s$ as required)
+$\sigma(x) = s f(x)$ is strictly positive for all $x$ as required)
 
 ### Distribution dynamics
 
@@ -322,7 +322,7 @@ defined by
 (\psi P)(y) = \int p(x,y) \psi(x) dx
 ```
 
-This operator is usually called the *Markov operator* corresponding to $p$
+This operator is usually called the **Markov operator** corresponding to $p$.
 
 ```{note}
 Unlike most operators, we write $P$ to the right of its argument,
@@ -368,7 +368,7 @@ $y$.
 
 Another possibility is to discretize the model, but this introduces errors of unknown size.
 
-A nicer alternative in the present setting is to combine simulation with an elegant estimator called the *look-ahead* estimator.
+A nicer alternative in the present setting is to combine simulation with an elegant estimator called the **look-ahead** estimator.
 
 Let's go over the ideas with reference to the growth model {ref}`discussed above <solow_swan>`, the dynamics of which we repeat here for convenience:
 
@@ -412,12 +412,12 @@ where $p$ is the growth model stochastic kernel in {eq}`statd_sssk`.
 
 What is the justification for this slightly surprising estimator?
 
-The idea is that, by the strong [law of large numbers](https://python-intro.quantecon.org/lln_clt.html#lln-ksl),
+The idea is that, by the strong [law of large numbers](https://python.quantecon.org/lln_clt.html#lln-ksl),
 
 $$
 \frac{1}{n} \sum_{i=1}^n p(k_{t-1}^i, y)
 \to
-\mathbb E p(k_{t-1}^i, y)
+\mathbb E \left[ p(k_{t-1}^i, y) \right]
 = \int p(x, y) \psi_{t-1}(x) \, dx
 = \psi_t(y)
 $$
@@ -462,13 +462,20 @@ The following code is an example of usage for the stochastic growth model {ref}`
 
 (stoch_growth)=
 ```{code-cell} python3
+---
+mystnb:
+  figure:
+    caption: Density of $k_1$ (lighter) to $k_T$ (darker)
+    name: fig-statd-density-sequence
+---
 # == Define parameters == #
 s = 0.2
 δ = 0.1
-a_σ = 0.4                    # A = exp(B) where B ~ N(0, a_σ)
+a_σ = 0.4                    # A = exp(B) where B ~ N(0, a_σ**2)
 α = 0.4                      # We set f(k) = k**α
 ψ_0 = beta(5, 5, scale=0.5)  # Initial distribution
 ϕ = lognorm(a_σ)
+rng = np.random.default_rng(1234)
 
 
 def p(x, y):
@@ -480,12 +487,12 @@ def p(x, y):
     return ϕ.pdf((y - (1 - δ) * x) / d) / d
 
 n = 10000    # Number of observations at each date t
-T = 30       # Compute density of k_t at 1,...,T+1
+T = 30       # Compute density of k_t at 1,...,T
 
 # == Generate matrix s.t. t-th column is n observations of k_t == #
 k = np.empty((n, T))
-A = ϕ.rvs((n, T))
-k[:, 0] = ψ_0.rvs(n)  # Draw first column from initial distribution
+A = ϕ.rvs((n, T), random_state=rng)
+k[:, 0] = ψ_0.rvs(n, random_state=rng)  # Draw first column from initial distribution
 for t in range(T-1):
     k[:, t+1] = s * A[:, t] * k[:, t]**α + (1 - δ) * k[:, t]
 
@@ -500,7 +507,6 @@ greys.reverse()
 for ψ, g in zip(laes, greys):
     ax.plot(ygrid, ψ(ygrid), color=g, lw=2, alpha=0.6)
 ax.set_xlabel('capital')
-ax.set_title(f'Density of $k_1$ (lighter) to $k_T$ (darker) for $T={T}$')
 plt.show()
 ```
 
@@ -531,7 +537,7 @@ We can, however, construct a fairly general theory using distribution functions.
 
 ### Example and definitions
 
-To illustrate the issues, recall that Hopenhayn and Rogerson {cite}`HopenhaynRogerson1993` study a model of firm dynamics where individual firm productivity follows the exogenous process
+To illustrate the issues, recall that {cite:t}`HopenhaynRogerson1993` study a model of firm dynamics where individual firm productivity follows the exogenous process
 
 $$
 X_{t+1} = a + \rho X_t + \xi_{t+1},
@@ -557,7 +563,7 @@ puts positive probability mass on 0 and 1.
 
 Hence it cannot be represented as a density.
 
-What we can do instead is use cumulative distribution functions (cdfs).
+What we can do instead is use cumulative distribution functions (CDFs).
 
 To this end, set
 
@@ -566,7 +572,7 @@ G(x, y) := \mathbb P \{ h(a + \rho x + \xi_{t+1}) \leq y \}
 \qquad (0 \leq x, y \leq 1)
 $$
 
-This family of cdfs $G(x, \cdot)$ plays a role analogous to the stochastic kernel in the density case.
+This family of CDFs $G(x, \cdot)$ plays a role analogous to the stochastic kernel in the density case.
 
 The distribution dynamics in {eq}`statd_fdd` are then replaced by
 
@@ -576,13 +582,13 @@ The distribution dynamics in {eq}`statd_fdd` are then replaced by
 F_{t+1}(y) = \int G(x,y) F_t(dx)
 ```
 
-Here $F_t$ and $F_{t+1}$ are cdfs representing the distribution of the current state and next period state.
+Here $F_t$ and $F_{t+1}$ are CDFs representing the distribution of the current state and next period state.
 
 The intuition behind {eq}`statd_fddc` is essentially the same as for {eq}`statd_fdd`.
 
 ### Computation
 
-If you wish to compute these cdfs, you cannot use the look-ahead estimator as before.
+If you wish to compute these CDFs, you cannot use the look-ahead estimator as before.
 
 Indeed, you should not use any density estimator, since the objects you are
 estimating/computing are not densities.
@@ -591,7 +597,7 @@ One good option is simulation as before, combined with the [empirical distributi
 
 ## Stability
 
-In our [lecture](https://python-intro.quantecon.org/finite_markov.html) on finite Markov chains, we also studied stationarity, stability and ergodicity.
+In our lecture {doc}`intermediate:finite_markov`, we also studied stationarity, stability and ergodicity.
 
 Here we will cover the same topics for the continuous case.
 
@@ -603,7 +609,7 @@ The general case is relatively similar --- references are given below.
 
 Analogous to [the finite case](https://python.quantecon.org/finite_markov.html#stationary-distributions), given a stochastic kernel $p$ and corresponding Markov operator as
 defined in {eq}`def_dmo`, a density $\psi^*$ on $S$ is called
-*stationary* for $P$ if it is a fixed point of the operator $P$.
+**stationary** for $P$ if it is a fixed point of the operator $P$.
 
 In other words,
 
@@ -640,9 +646,9 @@ With additional conditions, we can also get a unique stationary density ($\psi \
 ```
 
 This combination of existence, uniqueness and global convergence in the sense
-of {eq}`statd_dca` is often referred to as *global stability*.
+of {eq}`statd_dca` is often referred to as **global stability**.
 
-Under very similar conditions, we get *ergodicity*, which means that
+Under very similar conditions, we get **ergodicity**, which means that
 
 ```{math}
 :label: statd_lln
@@ -689,6 +695,7 @@ Here is such a figure.
 (statd_egs)=
 ```{figure} /_static/lecture_specific/stationary_densities/solution_statd_ex2.png
 
+Density sequences from four different initial conditions
 ```
 
 All sequences are converging towards the same limit, regardless of their initial condition.
@@ -818,6 +825,7 @@ Try running at `n = 10, 100, 1000, 10000` to get an idea of the speed of converg
 ϕ = norm()
 n = 500
 θ = 0.8
+rng = np.random.default_rng(1234)
 # == Frequently used constants == #
 d = np.sqrt(1 - θ**2)
 δ = θ / d
@@ -830,14 +838,15 @@ def p(x, y):
     "Stochastic kernel for the TAR model."
     return ϕ.pdf((y - θ * np.abs(x)) / d) / d
 
-Z = ϕ.rvs(n)
+Z = ϕ.rvs(n, random_state=rng)
 X = np.empty(n)
+X[0] = 0
 for t in range(n-1):
     X[t+1] = θ * np.abs(X[t]) + d * Z[t]
 ψ_est = LAE(p, X)
 k_est = gaussian_kde(X)
 
-fig, ax = plt.subplots(figsize=(10, 7))
+fig, ax = plt.subplots()
 ys = np.linspace(-3, 3, 200)
 ax.plot(ys, ψ_star(ys), 'b-', lw=2, alpha=0.6, label='true')
 ax.plot(ys, ψ_est(ys), 'g-', lw=2, alpha=0.6, label='look-ahead estimate')
@@ -882,10 +891,11 @@ Here's one program that does the job
 # == Define parameters == #
 s = 0.2
 δ = 0.1
-a_σ = 0.4                                # A = exp(B) where B ~ N(0, a_σ)
+a_σ = 0.4                                # A = exp(B) where B ~ N(0, a_σ**2)
 α = 0.4                                  # f(k) = k**α
 
 ϕ = lognorm(a_σ)
+rng = np.random.default_rng(1234)
 
 def p(x, y):
     "Stochastic kernel, vectorized in x.  Both x and y must be positive."
@@ -906,8 +916,8 @@ for i in range(4):
 
     # == Generate matrix s.t. t-th column is n observations of k_t == #
     k = np.empty((n, T))
-    A = ϕ.rvs((n, T))
-    k[:, 0] = ψ_0.rvs(n)
+    A = ϕ.rvs((n, T), random_state=rng)
+    k[:, 0] = ψ_0.rvs(n, random_state=rng)
     for t in range(T-1):
         k[:, t+1] = s * A[:,t] * k[:, t]**α + (1 - δ) * k[:, t]
 
@@ -938,22 +948,23 @@ To illustrate, let's generate three artificial data sets and compare them with a
 The three data sets we will use are:
 
 $$
-\{ X_1, \ldots, X_n \} \sim LN(0, 1), \;\;
+\{ X_1, \ldots, X_n \} \sim \mathrm{LN}(0, 1), \;\;
 \{ Y_1, \ldots, Y_n \} \sim N(2, 1), \;\;
 \text{ and } \;
-\{ Z_1, \ldots, Z_n \} \sim N(4, 1), \;
+\{ Z_1, \ldots, Z_n \} \sim N(4, 1)
 $$
 
 Here is the code and figure:
 
 ```{code-cell} python3
 n = 500
-x = np.random.randn(n)        # N(0, 1)
-x = np.exp(x)                 # Map x to lognormal
-y = np.random.randn(n) + 2.0  # N(2, 1)
-z = np.random.randn(n) + 4.0  # N(4, 1)
+rng = np.random.default_rng(1234)
+x = rng.standard_normal(n)        # N(0, 1)
+x = np.exp(x)                     # Map x to lognormal
+y = rng.standard_normal(n) + 2.0  # N(2, 1)
+z = rng.standard_normal(n) + 4.0  # N(4, 1)
 
-fig, ax = plt.subplots(figsize=(10, 6.6))
+fig, ax = plt.subplots()
 ax.boxplot([x, y, z])
 ax.set_xticks((1, 2, 3))
 ax.set_ylim(-2, 14)
@@ -1010,6 +1021,7 @@ J = 8
 θ = 0.9
 d = np.sqrt(1 - θ**2)
 δ = θ / d
+rng = np.random.default_rng(1234)
 
 fig, axes = plt.subplots(J, 1, figsize=(10, 4*J))
 initial_conditions = np.linspace(8, 0, J)
@@ -1018,9 +1030,9 @@ X = np.empty((k, n))
 for j in range(J):
 
     axes[j].set_ylim(-4, 8)
-    axes[j].set_title(f'time series from t = {initial_conditions[j]}')
+    axes[j].set_title(f'time series from $X_0 = {initial_conditions[j]:.1f}$')
 
-    Z = np.random.randn(k, n)
+    Z = rng.standard_normal((k, n))
     X[:, 0] = initial_conditions[j]
     for t in range(1, n):
         X[:, t] = θ * np.abs(X[:, t-1]) + d * Z[:, t]