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* Fix emphasis vs definitions in linear_algebra.md
- Changed all first-use technical terms from italic to bold
- Complies with QuantEcon style guide: bold for definitions, italic for emphasis
- Related to issue #721
* Fix emphasis vs definitions in linear_models.md
- Changed technical terms from italic to bold on first definition
- Fixed: probability distributions, companion matrix, vector autoregression,
deterministic/indeterministic seasonal, linear time trend, moving average,
martingale with drift, unconditional mean/variance-covariance matrix,
ensemble, cross-sectional average, autocovariance function, stationary distribution,
covariance stationary, ergodicity, Markov property, conditional covariance matrix,
discrete Lyapunov
- Related to issue #721
* Fix emphasis vs definitions in lln_clt.md
- Changed Kolmogorov's strong law from italic to bold (named theorem)
- Changed variance-covariance matrix from italic to bold (definition)
- Related to issue #721
* Fix emphasis vs definitions in markov_asset.md
- Changed Gordon formula from italic to bold (named formula)
- Changed Lucas tree model terms (tree, fruit, shares, dividend) to bold
- Changed infinite horizon, call option, strike price to bold (definitions)
- Note: kept 'exercises' and 'not to exercise' as italic (emphasis of choice)
- Related to issue #721
* Fix emphasis vs definitions in markov_perf.md
- Changed 'Markov perfect equilibrium' from italic to bold in formal definition
- Related to issue #721
* Fix emphasis vs definitions in mccall_model.md
- Changed 'values' from italic to bold when introducing value functions concept
- Note: 'Bellman equation', 'policy', 'reservation wage' already use bold
- Related to issue #721
* Fix emphasis vs definitions in mle.md
- Changed 'parametric class' from italic to bold (technical concept)
- Changed 'Poisson regression' from italic to bold (named model)
- Changed 'cumulative normal distribution' from italic to bold (technical term)
- Note: 'maximum likelihood estimates' already uses bold
- Related to issue #721
* Fix emphasis vs definitions in ols.md
- Changed 'exogenous' from italic to bold (key econometric term)
- Changed 'marginal effect' from italic to bold (technical definition)
- Changed 'the sum of squared residuals' from italic to bold (OLS definition)
- Note: 'omitted variable bias', 'multivariate regression model', 'endogeneity',
'two-stage least squares', 'instrument' already use bold
- Related to issue #721
* Fix emphasis vs definitions in rational_expectations.md
Changes per #721:
- rational expectations equilibrium (first introduction)
- perceived law of motion
- actual law of motion
- belief function
- Euler equation
- transversality condition
- recursive competitive equilibrium
- planning problem
All terms changed from italic to bold as they are definitions per style guide.
* Fix emphasis vs definitions in re_with_feedback.md
Changes per #721:
- backward shift (operator definition)
- lag (operator definition)
- forward shift (operator definition)
- explosive solution
All terms changed from italic to bold as they are definitions per style guide.
* Fix emphasis vs definitions in samuelson.md
Changes per #721:
- second-order linear difference equation
- national output identity
- consumption function
- accelerator
- accelerator coefficient
- aggregate demand
- aggregate supply
- business cycles
- stochastic linear difference equation
- marginal propensity to consume
- steady state
- random
- stochastic
- shocks
- disturbances
- second-order scalar linear stochastic difference equation
- characteristic polynomial
- zeros
- roots
All terms changed from italic to bold as they are definitions per style guide.
* Fix emphasis vs definitions in sir_model.md
Changes per #721:
- transmission rate
- infection rate
- recovery rate
- effective reproduction number
All terms changed from italic to bold as they are definitions per style guide.
* Fix emphasis vs definitions in uncertainty_traps.md
Changes per #721:
- propagation mechanism
Term changed from italic to bold as it is a definition per style guide.
* Fix emphasis vs definitions in von_neumann_model.md
Changes per #721:
- activities
- goods
- input matrix
- output matrix
- intensity
- goods used in production
- total outputs
- productive
- cost
- revenue
- costs
- revenues
- irreducibility
All terms changed from italic to bold as they are definitions per style guide.
* Fix emphasis vs definitions in ak_aiyagari.md
Changes per #721:
- Lifecycle patterns
- Within-cohort heterogeneity
- Cross-cohort interactions
All terms changed from italic to bold as they are definitions per style guide.
* Fix emphasis vs definitions in ak2.md
Changes per #721:
- numeraire
Term changed from italic to bold as it is a definition per style guide.
* Fix emphasis vs definitions in cake eating lectures
Changes per #721:
- exogenous (cake_eating_egm.md)
- adapted (cake_eating_stochastic.md)
- state (cake_eating_stochastic.md)
- control (cake_eating_stochastic.md)
- topologically conjugate (cake_eating_time_iter.md)
All terms changed from italic to bold as they are definitions per style guide.
* Fix emphasis vs definitions in career and cass_koopmans_1
Changes per #721:
- career (career.md)
- job (career.md)
- aggregation theory (cass_koopmans_1.md)
All terms changed from italic to bold as they are definitions per style guide.
* Fix emphasis vs definitions in likelihood_bayes.md
Changes per #721:
- recursion
- multiplicative decomposition
Terms changed from italic to bold as they are definitions per style guide.
* Fix emphasis vs definitions in morris_learn.md
Changes per #721:
- prior distributions
- posterior distributions
- speculative behavior
- ex dividend
- Short sales are prohibited
- Harsanyi Common Priors Doctrine
All terms changed from italic to bold as they are definitions per style guide.
* Fix emphasis vs definitions in odu and opt_transport
Changes per #721:
- reservation wage (odu.md)
- reservation wage functional equation (odu.md)
- matrix (opt_transport.md)
- vector (opt_transport.md)
All terms changed from italic to bold as they are definitions per style guide.
* Fix emphasis vs definitions in kalman and ifp_advanced
Changes per #721:
- prior (kalman.md)
- filtering distribution (kalman.md)
- predictive (kalman.md)
- Kalman gain (kalman.md)
- predictive distribution (kalman.md)
- savings (ifp_advanced.md)
All terms changed from italic to bold as they are definitions per style guide.
* Fix emphasis vs definitions in cass_fiscal.md
Changes per #721:
- Household (changed from italic to bold - also fixed typo 'Frim' to 'Firm')
- Firm
Terms changed from italic to bold as they are economic agents being defined per style guide.
* Fix emphasis vs definitions in exchangeable.md
Changes per #721:
- conditionally (as part of 'conditionally independently and identically distributed')
Term changed from italic to bold as it is a definition per style guide.
* Revert incorrect emphasis changes back to italic
Per review feedback on #721:
- ifp_advanced.md: 'savings' - emphasis on grid type, not definition
- exchangeable.md: 'conditionally' - alternate description, not definition
- cass_koopmans_1.md: 'aggregation theory' - referencing theory, not defining it
These should remain italic for emphasis, not bold for definitions.
* Revert incorrect emphasis-to-bold changes (batch 2)
Reverted changes in 3 files where emphasis/contrast was incorrectly changed to bold:
- opt_transport.md: matrix/vector (contrast between types)
- cake_eating_egm.md: exogenous (contrast with endogenous)
- ak_aiyagari.md: section headers (organizational emphasis)
These are not formal definitions, so should remain italic per style guide.
* Revert checked emphasis comments back to italic (batch 3)
Based on PR review feedback with checked [x] emphasis comments, reverted
the following terms from bold back to italic (emphasis, not definitions):
- linear_models.md: ergodicity (concept emphasis)
- markov_asset.md: tree, fruit, shares, dividend (metaphorical emphasis)
- mccall_model.md: values (concept emphasis)
- mle.md: parametric class (emphasis not definition)
- morris_learn.md: prior/posterior distributions, speculative behavior,
ex dividend, Short sales, Harsanyi Common Priors Doctrine (emphasis)
- ols.md: exogenous, marginal effect (emphasis not definitions)
- rational_expectations.md: rational expectations equilibrium (intro emphasis),
perceived/actual law of motion (intro emphasis, formal definitions come later)
- samuelson.md: second-order linear difference equation, national output identity,
consumption function, accelerator, accelerator coefficient, aggregate demand/supply,
random, stochastic, shocks, disturbances (emphasis not definitions)
These are emphasis on concepts or references, not formal definitions.
* Fix typos introduced during formatting changes
Fixed the following typos:
- linear_algebra.md: removed extra '.*.' after 'square' and 'symmetric'
- linear_algebra.md: removed extra '.l.' after 'diagonal'
- sir_model.md: removed extra 'd)' after 'infected)'
- von_neumann_model.md: removed extra '.).' after 'consumed)'
- von_neumann_model.md: removed extra '****' after 'outputs'
- von_neumann_model.md: removed extra 'es' from 'activitieses' → 'activities'
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@@ -147,8 +147,8 @@ $$ (eq:gov_budget)
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Given a budget-feasible government policy $\{g_t\}_{t=0}^\infty$ and $\{\tau_{ct}, \tau_{kt}, \tau_{nt}, \tau_{ht}\}_{t=0}^\infty$ subject to {eq}`eq:gov_budget`,
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- *Household* chooses $\{c_t\}_{t=0}^\infty$, $\{n_t\}_{t=0}^\infty$, and $\{k_{t+1}\}_{t=0}^\infty$ to maximize utility{eq}`eq:utility` subject to budget constraint{eq}`eq:house_budget`, and
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- *Frim* chooses sequences of capital $\{k_t\}_{t=0}^\infty$ and $\{n_t\}_{t=0}^\infty$ to maximize profits
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- **Household** chooses $\{c_t\}_{t=0}^\infty$, $\{n_t\}_{t=0}^\infty$, and $\{k_{t+1}\}_{t=0}^\infty$ to maximize utility{eq}`eq:utility` subject to budget constraint{eq}`eq:house_budget`, and
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- **Firm** chooses sequences of capital $\{k_t\}_{t=0}^\infty$ and $\{n_t\}_{t=0}^\infty$ to maximize profits
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@@ -85,7 +85,7 @@ One way to summarize our knowledge is a point prediction $\hat x$
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* Then it is better to summarize our initial beliefs with a bivariate probability density $p$
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* $\int_E p(x)dx$ indicates the probability that we attach to the missile being in region $E$.
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The density $p$ is called our *prior* for the random variable $x$.
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The density $p$ is called our **prior** for the random variable $x$.
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To keep things tractable in our example, we assume that our prior is Gaussian.
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@@ -317,7 +317,7 @@ We have obtained probabilities for the current location of the state (missile) g
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This is called "filtering" rather than forecasting because we are filtering
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out noise rather than looking into the future.
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* $p(x \,|\, y) = N(\hat x^F, \Sigma^F)$ is called the *filtering distribution*
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* $p(x \,|\, y) = N(\hat x^F, \Sigma^F)$ is called the **filtering distribution**
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But now let's suppose that we are given another task: to predict the location of the missile after one unit of time (whatever that may be) has elapsed.
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@@ -331,7 +331,7 @@ Let's suppose that we have one, and that it's linear and Gaussian. In particular
Our aim is to combine this law of motion and our current distribution $p(x \,|\, y) = N(\hat x^F, \Sigma^F)$ to come up with a new *predictive* distribution for the location in one unit of time.
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Our aim is to combine this law of motion and our current distribution $p(x \,|\, y) = N(\hat x^F, \Sigma^F)$ to come up with a new **predictive** distribution for the location in one unit of time.
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In view of {eq}`kl_xdynam`, all we have to do is introduce a random vector $x^F \sim N(\hat x^F, \Sigma^F)$ and work out the distribution of $A x^F + w$ where $w$ is independent of $x^F$ and has distribution $N(0, Q)$.
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@@ -356,7 +356,7 @@ $$
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$$
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The matrix $A \Sigma G' (G \Sigma G' + R)^{-1}$ is often written as
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$K_{\Sigma}$ and called the *Kalman gain*.
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$K_{\Sigma}$ and called the **Kalman gain**.
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* The subscript $\Sigma$ has been added to remind us that $K_{\Sigma}$ depends on $\Sigma$, but not $y$ or $\hat x$.
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@@ -373,7 +373,7 @@ Our updated prediction is the density $N(\hat x_{new}, \Sigma_{new})$ where
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\end{aligned}
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```
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* The density $p_{new}(x) = N(\hat x_{new}, \Sigma_{new})$ is called the *predictive distribution*
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* The density $p_{new}(x) = N(\hat x_{new}, \Sigma_{new})$ is called the **predictive distribution**
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The predictive distribution is the new density shown in the following figure, where
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@@ -85,7 +85,7 @@ from scipy.linalg import inv, solve, det, eig
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```{index} single: Linear Algebra; Vectors
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```
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A *vector* of length $n$ is just a sequence (or array, or tuple) of $n$ numbers, which we write as $x = (x_1, \ldots, x_n)$ or $x = [x_1, \ldots, x_n]$.
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A **vector** of length $n$ is just a sequence (or array, or tuple) of $n$ numbers, which we write as $x = (x_1, \ldots, x_n)$ or $x = [x_1, \ldots, x_n]$.
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We will write these sequences either horizontally or vertically as we please.
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```{index} single: Vectors; Norm
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```
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The *inner product* of vectors $x,y \in \mathbb R ^n$ is defined as
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The **inner product** of vectors $x,y \in \mathbb R ^n$ is defined as
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$$
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x' y := \sum_{i=1}^n x_i y_i
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$$
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Two vectors are called *orthogonal* if their inner product is zero.
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Two vectors are called **orthogonal** if their inner product is zero.
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The *norm* of a vector $x$ represents its "length" (i.e., its distance from the zero vector) and is defined as
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The **norm** of a vector $x$ represents its "length" (i.e., its distance from the zero vector) and is defined as
@@ -273,7 +273,7 @@ np.linalg.norm(x) # Norm of x, take three
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Given a set of vectors $A := \{a_1, \ldots, a_k\}$ in $\mathbb R ^n$, it's natural to think about the new vectors we can create by performing linear operations.
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New vectors created in this manner are called *linear combinations* of $A$.
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New vectors created in this manner are called **linear combinations** of $A$.
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In particular, $y \in \mathbb R ^n$ is a linear combination of $A := \{a_1, \ldots, a_k\}$ if
\text{ for some scalars } \beta_1, \ldots, \beta_k
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$$
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In this context, the values $\beta_1, \ldots, \beta_k$ are called the *coefficients* of the linear combination.
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In this context, the values $\beta_1, \ldots, \beta_k$ are called the **coefficients** of the linear combination.
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The set of linear combinations of $A$ is called the *span* of $A$.
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The set of linear combinations of $A$ is called the **span** of $A$.
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The next figure shows the span of $A = \{a_1, a_2\}$ in $\mathbb R ^3$.
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@@ -349,7 +349,7 @@ plt.show()
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If $A$ contains only one vector $a_1 \in \mathbb R ^2$, then its
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span is just the scalar multiples of $a_1$, which is the unique line passing through both $a_1$ and the origin.
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If $A = \{e_1, e_2, e_3\}$ consists of the *canonical basis vectors* of $\mathbb R ^3$, that is
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If $A = \{e_1, e_2, e_3\}$ consists of the **canonical basis vectors** of $\mathbb R ^3$, that is
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$$
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e_1 :=
@@ -399,8 +399,8 @@ The condition we need for a set of vectors to have a large span is what's called
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In particular, a collection of vectors $A := \{a_1, \ldots, a_k\}$ in $\mathbb R ^n$ is said to be
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**linearly dependent* if some strict subset of $A$ has the same span as $A$.
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**linearly independent* if it is not linearly dependent.
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***linearly dependent** if some strict subset of $A$ has the same span as $A$.
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***linearly independent** if it is not linearly dependent.
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Put differently, a set of vectors is linearly independent if no vector is redundant to the span and linearly dependent otherwise.
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@@ -469,19 +469,19 @@ Often, the numbers in the matrix represent coefficients in a system of linear eq
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For obvious reasons, the matrix $A$ is also called a vector if either $n = 1$ or $k = 1$.
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In the former case, $A$ is called a *row vector*, while in the latter it is called a *column vector*.
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In the former case, $A$ is called a **row vector**, while in the latter it is called a **column vector**.
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If $n = k$, then $A$ is called *square*.
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If $n = k$, then $A$ is called **square**.
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The matrix formed by replacing $a_{ij}$ by $a_{ji}$ for every $i$ and $j$ is called the *transpose* of $A$ and denoted $A'$ or $A^{\top}$.
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The matrix formed by replacing $a_{ij}$ by $a_{ji}$ for every $i$ and $j$ is called the **transpose** of $A$ and denoted $A'$ or $A^{\top}$.
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If $A = A'$, then $A$ is called *symmetric*.
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If $A = A'$, then $A$ is called **symmetric**.
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For a square matrix $A$, the $i$ elements of the form $a_{ii}$ for $i=1,\ldots,n$ are called the *principal diagonal*.
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For a square matrix $A$, the $i$ elements of the form $a_{ii}$ for $i=1,\ldots,n$ are called the **principal diagonal**.
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$A$ is called *diagonal* if the only nonzero entries are on the principal diagonal.
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$A$ is called **diagonal** if the only nonzero entries are on the principal diagonal.
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If, in addition to being diagonal, each element along the principal diagonal is equal to 1, then $A$ is called the *identity matrix* and denoted by $I$.
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If, in addition to being diagonal, each element along the principal diagonal is equal to 1, then $A$ is called the **identity matrix** and denoted by $I$.
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### Matrix Operations
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@@ -641,9 +641,9 @@ See [here](https://python-programming.quantecon.org/numpy.html#matrix-multiplica
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Each $n \times k$ matrix $A$ can be identified with a function $f(x) = Ax$ that maps $x \in \mathbb R ^k$ into $y = Ax \in \mathbb R ^n$.
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These kinds of functions have a special property: they are *linear*.
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These kinds of functions have a special property: they are **linear**.
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A function $f \colon \mathbb R ^k \to \mathbb R ^n$ is called *linear* if, for all $x, y \in \mathbb R ^k$ and all scalars $\alpha, \beta$, we have
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A function $f \colon \mathbb R ^k \to \mathbb R ^n$ is called **linear** if, for all $x, y \in \mathbb R ^k$ and all scalars $\alpha, \beta$, we have
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$$
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f(\alpha x + \beta y) = \alpha f(x) + \beta f(y)
@@ -773,7 +773,7 @@ In particular, the following are equivalent
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1. The columns of $A$ are linearly independent.
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1. For any $y \in \mathbb R ^n$, the equation $y = Ax$ has a unique solution.
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The property of having linearly independent columns is sometimes expressed as having *full column rank*.
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The property of having linearly independent columns is sometimes expressed as having **full column rank**.
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#### Inverse Matrices
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A similar expression is available in the matrix case.
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In particular, if square matrix $A$ has full column rank, then it possesses a multiplicative
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*inverse matrix* $A^{-1}$, with the property that $A A^{-1} = A^{-1} A = I$.
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**inverse matrix** $A^{-1}$, with the property that $A A^{-1} = A^{-1} A = I$.
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As a consequence, if we pre-multiply both sides of $y = Ax$ by $A^{-1}$, we get $x = A^{-1} y$.
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```
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Another quick comment about square matrices is that to every such matrix we
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assign a unique number called the *determinant* of the matrix --- you can find
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assign a unique number called the **determinant** of the matrix --- you can find
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the expression for it [here](https://en.wikipedia.org/wiki/Determinant).
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If the determinant of $A$ is not zero, then we say that $A$ is
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*nonsingular*.
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**nonsingular**.
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Perhaps the most important fact about determinants is that $A$ is nonsingular if and only if $A$ is of full column rank.
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A v = \lambda v
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$$
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then we say that $\lambda$ is an *eigenvalue* of $A$, and
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$v$ is an *eigenvector*.
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then we say that $\lambda$ is an **eigenvalue** of $A$, and
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$v$ is an **eigenvector**.
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Thus, an eigenvector of $A$ is a vector such that when the map $f(x) = Ax$ is applied, $v$ is merely scaled.
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### Generalized Eigenvalues
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It is sometimes useful to consider the *generalized eigenvalue problem*, which, for given
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It is sometimes useful to consider the **generalized eigenvalue problem**, which, for given
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matrices $A$ and $B$, seeks generalized eigenvalues
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$\lambda$ and eigenvectors $v$ such that
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$$
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The norms on the right-hand side are ordinary vector norms, while the norm on
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the left-hand side is a *matrix norm* --- in this case, the so-called
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*spectral norm*.
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the left-hand side is a **matrix norm** --- in this case, the so-called
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**spectral norm**.
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For example, for a square matrix $S$, the condition $\| S \| < 1$ means that $S$ is *contractive*, in the sense that it pulls all vectors towards the origin [^cfn].
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For example, for a square matrix $S$, the condition $\| S \| < 1$ means that $S$ is **contractive**, in the sense that it pulls all vectors towards the origin [^cfn].
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