You signed in with another tab or window. Reload to refresh your session.You signed out in another tab or window. Reload to refresh your session.You switched accounts on another tab or window. Reload to refresh your session.Dismiss alert
In general, `np.linalg.solve` is more numerically stable than using
207
-
`np.linalg.inv` directly.
206
+
`np.linalg.inv` directly.
208
207
However, stability is not an issue for this small example. Moreover, we will
209
208
repeatedly use `A_inv` in what follows, so there is added value in computing
210
209
it directly.
@@ -366,9 +365,9 @@ $$
366
365
367
366
You can read about multivariate normal distributions in this lecture [Multivariate Normal Distribution](https://python.quantecon.org/multivariate_normal.html).
Notice that the covariance between $y_t$ and $y_{t-1}$ -- the elements on the superdiagonal -- are **not** identical.
506
505
507
-
This is is an indication that the time series respresented by our $y$ vector is not **stationary**.
506
+
This is is an indication that the time series respresented by our $y$ vector is not **stationary**.
508
507
509
508
To make it stationary, we'd have to alter our system so that our **initial conditions** $(y_1, y_0)$ are not fixed numbers but instead a jointly normally distributed random vector with a particular mean and covariance matrix.
510
509
@@ -530,7 +529,7 @@ There is a lot to be learned about the process by staring at the off diagonal el
530
529
531
530
## Moving Average Representation
532
531
533
-
Let's print out $A^{-1}$ and stare at its structure
532
+
Let's print out $A^{-1}$ and stare at its structure
534
533
535
534
* is it triangular or almost triangular or $\ldots$ ?
536
535
@@ -546,7 +545,7 @@ with np.printoptions(precision=3, suppress=True):
546
545
547
546
548
547
549
-
Evidently, $A^{-1}$ is a lower triangular matrix.
548
+
Evidently, $A^{-1}$ is a lower triangular matrix.
550
549
551
550
552
551
Let's print out the lower right hand corner of $A^{-1}$ and stare at it.
@@ -561,13 +560,13 @@ Notice how every row ends with the previous row's pre-diagonal entries.
561
560
562
561
563
562
564
-
565
563
566
-
Since $A^{-1}$ is lower triangular, each row represents $ y_t$ for a particular $t$ as the sum of
567
-
- a time-dependent function $A^{-1} b$ of the initial conditions incorporated in $b$, and
564
+
565
+
Since $A^{-1}$ is lower triangular, each row represents $ y_t$ for a particular $t$ as the sum of
566
+
- a time-dependent function $A^{-1} b$ of the initial conditions incorporated in $b$, and
568
567
- a weighted sum of current and past values of the IID shocks $\{u_t\}$
569
568
570
-
Thus, let $\tilde{A}=A^{-1}$.
569
+
Thus, let $\tilde{A}=A^{-1}$.
571
570
572
571
Evidently, for $t\geq0$,
573
572
@@ -577,7 +576,7 @@ $$
577
576
578
577
This is a **moving average** representation with time-varying coefficients.
579
578
580
-
Just as system {eq}`eq:eqma` constitutes a
579
+
Just as system {eq}`eq:eqma` constitutes a
581
580
**moving average** representation for $y$, system {eq}`eq:eqar` constitutes an **autoregressive** representation for $y$.
0 commit comments