@@ -3,14 +3,13 @@ jupytext:
33 text_representation :
44 extension : .md
55 format_name : myst
6- format_version : 0.13
7- jupytext_version : 1.14.1
86kernelspec :
9- display_name : Python 3 (ipykernel)
7+ display_name : Python 3
108 language : python
119 name : python3
1210---
1311
12+ (solow)=
1413# The Solow-Swan Growth Model
1514
1615In this lecture we review a famous model due
5150
5251$$
5352 F(\lambda K, \lambda L) = \lambda F(K, L)
54- \quad \text{for all } \lambda \geq 0.
53+ \quad \text{for all } \lambda \geq 0
5554$$
5655
5756Production functions with this property include
@@ -73,7 +72,7 @@ becomes $1-\delta$ units tomorrow.
7372Thus,
7473
7574$$
76- K_{t+1} = s F(K_t, L_t) + (1 - \delta) K_t.
75+ K_{t+1} = s F(K_t, L_t) + (1 - \delta) K_t
7776$$
7877
7978
@@ -84,7 +83,7 @@ Setting $k_t := K_t / L$ and using homogeneity of degree one now yields
8483$$
8584 k_{t+1}
8685 = s \frac{F(K_t, L)}{L} + (1 - \delta) k_t
87- = s F(k_t, 1) + (1 - \delta) k_t.
86+ = s F(k_t, 1) + (1 - \delta) k_t
8887$$
8988
9089
@@ -93,7 +92,7 @@ With $f(k) := F(k, 1)$, the final expression for capital dynamics is
9392``` {math}
9493:label: solow
9594 k_{t+1} = g(k_t)
96- \text{ where } g(k) := s f(k) + (1 - \delta) k.
95+ \text{ where } g(k) := s f(k) + (1 - \delta) k
9796```
9897
9998Our aim is to learn about the evolution of $k_t$ over time,
@@ -114,7 +113,6 @@ $\alpha=0.3$, $s=0.3$ and $\delta=0.4$.
114113The function $g$ from {eq}` solow ` is then plotted, along with the 45
115114degree line.
116115
117- +++
118116
119117Let's define the constants.
120118
@@ -195,14 +193,12 @@ It solves $k = s Ak^{\alpha} + (1-\delta)k$ and hence is given by
195193
196194``` {math}
197195:label: kstarss
198- k^* := \left( \frac{s A}{\delta} \right)^{1/(1 - \alpha)}.
196+ k^* := \left( \frac{s A}{\delta} \right)^{1/(1 - \alpha)}
199197```
200198If initial capital is below $k^* $, then capital increases over time.
201199
202200If initial capital is above this level, then the reverse is true.
203201
204- +++
205-
206202Let's plot the 45 degree diagram to show the $k^* $ in the plot
207203
208204``` {code-cell} ipython3
@@ -222,8 +218,6 @@ three distinct initial conditions, under the parameterization listed above.
222218
223219At this parameterization, $k^* \approx 1.78$.
224220
225- +++
226-
227221Let's define the constants and three distinct intital conditions
228222
229223``` {code-cell} ipython3
@@ -268,8 +262,6 @@ simulate_ts(x0, ts_length)
268262
269263As expected, the time paths in the figure both converge to this value.
270264
271- +++
272-
273265## Growth in Continuous Time
274266
275267In this section we investigate a continuous time version of the Solow--Swan
@@ -287,7 +279,7 @@ A simple rearrangement gives the rate of change per unit of time:
287279$$
288280 \Delta k_t = s f(k_t) - \delta k_t
289281 \quad \text{where} \quad
290- \Delta k_t := k_{t+1} - k_t.
282+ \Delta k_t := k_{t+1} - k_t
291283$$
292284
293285Taking the time step to zero gives the continuous time limit
@@ -296,7 +288,7 @@ Taking the time step to zero gives the continuous time limit
296288:label: solowc
297289 k'_t = s f(k_t) - \delta k_t
298290 \qquad \text{with} \qquad
299- k'_t := \frac{d}{dt} k_t.
291+ k'_t := \frac{d}{dt} k_t
300292```
301293
302294Our aim is to learn about the evolution of $k_t$ over time,
@@ -324,8 +316,6 @@ When $g(k) < 0$, the opposite occurs. Once again, high marginal returns to
324316savings at low levels of capital combined with low rates of return at high
325317levels of capital combine to yield global stability.
326318
327- +++
328-
329319To see this in a figure, let's define the constants
330320
331321``` {code-cell} ipython3
@@ -395,7 +385,7 @@ linear differential equation
395385
396386``` {math}
397387:label: xsolow
398- x'_t = (1-\alpha) (sA - \delta x_t).
388+ x'_t = (1-\alpha) (sA - \delta x_t)
399389```
400390
401391This equation has the exact solution
406396 k_0^{1-\alpha} - \frac{sA}{\delta}
407397 \right)
408398 \mathrm{e}^{-\delta (1-\alpha) t} +
409- \frac{sA}{\delta}.
399+ \frac{sA}{\delta}
410400$$
411401
412402(You can confirm that this function $x_t$ satisfies {eq}` xsolow ` by
@@ -424,41 +414,34 @@ Converting back to $k_t$ yields
424414 \right)
425415 \mathrm{e}^{-\delta (1-\alpha) t} +
426416 \frac{sA}{\delta}
427- \right]^{1/(1-\alpha)}.
417+ \right]^{1/(1-\alpha)}
428418```
429419
430420Since $\delta > 0$ and $\alpha \in (0, 1)$, we see immediately that $k_t \to
431421k^* $ as $t \to \infty$ independent of $k_0$.
432422
433423Thus, global stability holds.
434424
435- +++
436-
437425## Exercises
438426
439- ``` {exercise-start}
440- :label: ex1
441- ```
442- ### Exercise 1
427+ ``` {exercise}
428+ :label: solow_ex1
443429
444- plot per capita consumption $c$ at the steady state, as a function of the savings rate $s$, where $0 \leq s \leq 1$.
430+ Plot per capita consumption $c$ at the steady state, as a function of the savings rate $s$, where $0 \leq s \leq 1$.
445431
446432Use the Cobb--Douglas specification $f(k) = A k^\alpha$.
447433
448434Set $A=2.0, \alpha=0.3,$ and $\delta=0.5$
449435
450436Also, find the approximate value of $s$ that maximizes the $C^*(s)$ and show it in the plot.
451437
452- ``` {exercise-end}
453438```
454439
455- ``` {solution-start} ex1
440+ ``` {solution-start} solow_ex1
456441:class: dropdown
457442```
458443
459- +++
460-
461- Steady state consumption at savings rate $s$ is given by
444+ Steady state consumption at savings rate $s$ is given by
462445
463446
464447$$
@@ -515,14 +498,10 @@ Incidentally, the rate of savings which maximizes steady state level of per capi
515498``` {solution-end}
516499```
517500
518- +++
519-
520501``` {exercise-start}
521- :label: ex2
502+ :label: solow_ex2
522503```
523- ### Exercise 2
524-
525- #### Stochastic Productivity
504+ ** Stochastic Productivity**
526505
527506To bring the Solow--Swan model closer to data, we need to think about handling
528507random fluctuations in aggregate quantities.
@@ -540,7 +519,7 @@ Dynamics are now
540519
541520``` {math}
542521:label: solowran
543- k_{t+1} = s A_{t+1} f(k_t) + (1 - \delta) k_t.
522+ k_{t+1} = s A_{t+1} f(k_t) + (1 - \delta) k_t
544523```
545524
546525We suppose $f$ is Cobb--Douglas and $(A_t)$ is IID and lognormal.
@@ -555,9 +534,7 @@ Generate and plot the time series $k_t$.
555534``` {exercise-end}
556535```
557536
558- +++
559-
560- ``` {solution-start} ex2
537+ ``` {solution-start} solow_ex2
561538:class: dropdown
562539```
563540
@@ -614,7 +591,3 @@ ts_plot(x0, 50)
614591
615592``` {solution-end}
616593```
617-
618- ``` {code-cell} ipython3
619-
620- ```
0 commit comments