|
170 | 170 | "\n", |
171 | 171 | "In period $ t $ and history $ s^t $, let\n", |
172 | 172 | "\n", |
173 | | - "- $ b_{t+1}(s^t) $ be the amount of the time $ t+1 $ consumption good that at time $ t $ the government promised to pay \n", |
| 173 | + "- $ b_{t+1}(s^t) $ be the amount of the time $ t+1 $ consumption good that at time $ t $, history $ s^t $ the government promised to pay \n", |
174 | 174 | "- $ R_t(s^t) $ be the gross interest rate on risk-free one-period debt between periods $ t $ and $ t+1 $ \n", |
175 | | - "- $ T_t(s^t) $ be a non-negative lump-sum transfer to the representative household <sup><a href=#fn-a id=fn-a-link>[1]</a></sup> \n", |
| 175 | + "- $ T_t(s^t) $ be a non-negative lump-sum *transfer* to the representative household <sup><a href=#fn-a id=fn-a-link>[1]</a></sup> \n", |
176 | 176 | "\n", |
177 | 177 | "\n", |
178 | 178 | "That $ b_{t+1}(s^t) $ is the same for all realizations of $ s_{t+1} $ captures its *risk-free* character.\n", |
|
216 | 216 | " { u_c(s^{t+1}) \\over u_c(s^{t}) } \\; b_{t+1}(s^t) \\tag{5}\n", |
217 | 217 | "$$\n", |
218 | 218 | "\n", |
219 | | - "Components of $ z(s^t) $ on the right side depend on $ s^t $, but the left side is required to depend on $ s^{t-1} $ only.\n", |
| 219 | + "Components of $ z(s^t) $ on the right side depend on $ s^t $, but the left side is required to depend only\n", |
| 220 | + "on $ s^{t-1} $ .\n", |
220 | 221 | "\n", |
221 | 222 | "**This is what it means for one-period government debt to be risk-free**.\n", |
222 | 223 | "\n", |
|
248 | 249 | " \\end{aligned} \\tag{6}\n", |
249 | 250 | "$$\n", |
250 | 251 | "\n", |
| 252 | + "Notice how the conditioning sets in equation [(6)](#equation-ts-gov-wo3) differ: they are $ s^{t-1} $ on the left side and\n", |
| 253 | + "$ s^t $ on the right side.\n", |
| 254 | + "\n", |
251 | 255 | "Now let’s\n", |
252 | 256 | "\n", |
253 | 257 | "- substitute the resource constraint into the net-of-interest government surplus, and \n", |
|
264 | 268 | " -g_t(s_t) - T_t(s^t)\\,. \\tag{7}\n", |
265 | 269 | "$$\n", |
266 | 270 | "\n", |
267 | | - "If we substitute the appropriate versions of the right side of [(7)](#equation-amss-44-2) for $ z(s^{t+j}) $ into equation [(6)](#equation-ts-gov-wo3),\n", |
| 271 | + "If we substitute appropriate versions of the right side of [(7)](#equation-amss-44-2) for $ z(s^{t+j}) $ into equation [(6)](#equation-ts-gov-wo3),\n", |
268 | 272 | "we obtain a sequence of *implementability constraints* on a Ramsey allocation in an AMSS economy.\n", |
269 | 273 | "\n", |
270 | 274 | "Expression [(6)](#equation-ts-gov-wo3) at time $ t=0 $ and initial state $ s^0 $\n", |
|
297 | 301 | "source": [ |
298 | 302 | "### Comparison with Lucas-Stokey Economy\n", |
299 | 303 | "\n", |
300 | | - "The expression on the right side of [(9)](#equation-ts-gov-wo4a) in the Lucas-Stokey (1983) economy would equal the present value of a continuation stream of government surpluses evaluated at what would be competitive equilibrium Arrow-Debreu prices at date $ t $.\n", |
| 304 | + "The expression on the right side of [(9)](#equation-ts-gov-wo4a) in the Lucas-Stokey (1983) economy would equal the present value of a continuation stream of government net-of-interest surpluses evaluated at what would be competitive equilibrium Arrow-Debreu prices at date $ t $.\n", |
301 | 305 | "\n", |
302 | 306 | "In the Lucas-Stokey economy, that present value is measurable with respect to $ s^t $.\n", |
303 | 307 | "\n", |
304 | 308 | "In the AMSS economy, the restriction that government debt be risk-free imposes that that same present value must be measurable with respect to $ s^{t-1} $.\n", |
305 | 309 | "\n", |
306 | | - "In a language used in the literature on incomplete markets models, it can be said that the AMSS model requires that at each $ (t, s^t) $ what would be the present value of continuation government surpluses in the Lucas-Stokey model must belong to the **marketable subspace** of the AMSS model." |
| 310 | + "In a language used in the literature on incomplete markets models, it can be said that the AMSS model requires that at each $ (t, s^t) $ what would be the present value of continuation government net-of-interest surpluses in the Lucas-Stokey model must belong to the **marketable subspace** of the AMSS model." |
307 | 311 | ] |
308 | 312 | }, |
309 | 313 | { |
|
337 | 341 | "\\mathbb E_{t} \\sum_{j=0}^\\infty \\beta^j\n", |
338 | 342 | " { u_c(s^{t+j}) \\over u_c(s^{t}) } \\;\n", |
339 | 343 | " z(s^{t+j}) = b_t(s^{t-1})\n", |
340 | | - " \\quad \\forall \\, s^t \\tag{11}\n", |
| 344 | + " \\quad \\forall \\, t, s^t \\tag{11}\n", |
341 | 345 | "$$\n", |
342 | 346 | "\n", |
343 | 347 | "given $ b_0(s^{-1}) $." |
|
354 | 358 | "As in the Lucas-Stokey economy, this multiplier is strictly positive when the government must resort to\n", |
355 | 359 | "distortionary taxation; otherwise it equals zero.\n", |
356 | 360 | "\n", |
357 | | - "A consequence of the assumption that there are no markets in state-contingent securities and that a market exists only in a risk-free security is that we have to attach stochastic processes $ \\{\\gamma_t(s^t)\\}_{t=1}^\\infty $ of\n", |
| 361 | + "A consequence of the assumption that there are no markets in state-contingent securities and that a market exists only in a risk-free security is that we have to attach a stochastic process $ \\{\\gamma_t(s^t)\\}_{t=1}^\\infty $ of\n", |
358 | 362 | "Lagrange multipliers to the implementability constraints [(11)](#equation-amss-46).\n", |
359 | 363 | "\n", |
360 | 364 | "Depending on how the constraints bind, these multipliers can be positive or negative:\n", |
361 | 365 | "\n", |
362 | 366 | "$$\n", |
363 | 367 | "\\begin{aligned}\n", |
364 | 368 | " \\gamma_t(s^t)\n", |
365 | | - " &\\;\\geq\\; (\\leq)\\;\\, 0 \\quad \\text{if the constraint binds in this direction }\n", |
| 369 | + " &\\;\\geq\\; (\\leq)\\;\\, 0 \\quad \\text{if the constraint binds in the following direction }\n", |
366 | 370 | " \\\\\n", |
367 | 371 | " & \\mathbb E_{t} \\sum_{j=0}^\\infty \\beta^j\n", |
368 | 372 | " { u_c(s^{t+j}) \\over u_c(s^{t}) } \\;z(s^{t+j}) \\;\\geq \\;(\\leq)\\;\\, b_t(s^{t-1})\n", |
|
464 | 468 | " while the multiplier $ \\Phi $ in the Lucas-Stokey economy is time-invariant. \n", |
465 | 469 | "\n", |
466 | 470 | "\n", |
467 | | - "We need some code from our [an earlier lecture](https://python-programming.quantecon.org/opt_tax_recur.html)\n", |
| 471 | + "We need some code from [an earlier lecture](https://python-programming.quantecon.org/opt_tax_recur.html)\n", |
468 | 472 | "on optimal taxation with state-contingent debt sequential allocation implementation:" |
469 | 473 | ] |
470 | 474 | }, |
|
895 | 899 | "- a counterpart to $ V_x(x,s) $ is time-invariant and equal to\n", |
896 | 900 | " the Lagrange multiplier on the Lucas-Stokey implementability constraint \n", |
897 | 901 | "- time invariance of $ V_x(x,s) $ is the source of a key\n", |
898 | | - " feature of the Lucas-Stokey model, namely, state variable degeneracy\n", |
899 | | - " (i.e., $ x_t $ is an exact function of $ s_t $) \n", |
| 902 | + " feature of the Lucas-Stokey model, namely, **state variable degeneracy** in which $ x_t $ is an exact time-invariant function of $ s_t $) \n", |
900 | 903 | "\n", |
901 | 904 | "\n", |
902 | 905 | "That $ V_x(x,s) $ varies over time according to a twisted martingale\n", |
|
930 | 933 | "expenditure function $ g(s) $ are such that $ g_t $ is perpetually\n", |
931 | 934 | "random, $ V_x(x, s) $ almost surely converges to zero.\n", |
932 | 935 | "\n", |
933 | | - "For quasi-linear preferences, the first-order condition with respect to $ n(s) $ becomes\n", |
| 936 | + "For quasi-linear preferences, the first-order condition for maximizing [(22)](#equation-eqn-amssapp5) subject to [(23)](#equation-eqn-amssapp6) with respect to $ n(s) $ becomes\n", |
934 | 937 | "\n", |
935 | 938 | "$$\n", |
936 | 939 | "(1-\\mu(s|s_-) ) (1 - u_l(s)) + \\mu(s|s_-) n(s) u_{ll}(s) =0\n", |
|
1552 | 1555 | "\n", |
1553 | 1556 | "- the government purchases an Arrow security that pays off when $ g_3 = g_h $ \n", |
1554 | 1557 | "- the government sells an Arrow security that pays off when $ g_3 = g_l $ \n", |
1555 | | - "- These purchases are designed in such a way that regardless of whether or not there is a war at $ t=3 $, the government will begin period $ t=4 $ with the *same* government debt \n", |
| 1558 | + "- these purchases are designed in such a way that regardless of whether or not there is a war at $ t=3 $, the government will begin period $ t=4 $ with the *same* government debt \n", |
1556 | 1559 | "\n", |
1557 | 1560 | "\n", |
1558 | 1561 | "This pattern facilities smoothing tax rates across states.\n", |
|
1778 | 1781 | } |
1779 | 1782 | ], |
1780 | 1783 | "metadata": { |
1781 | | - "date": 1618444998.7623057, |
| 1784 | + "date": 1618874888.5793111, |
1782 | 1785 | "filename": "amss.rst", |
1783 | 1786 | "kernelspec": { |
1784 | 1787 | "display_name": "Python", |
|
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