• Home
  • Textbooks
  • Competitive Equilibrium: Theory and Applications
  • Exchange

Competitive Equilibrium: Theory and Applications

Bryan Ellickson

Chapter 1

Exchange - all with Video Answers

Educators


Chapter Questions

01:01

Problem 1

The most straightforward way to show that $S \subset T$ is to prove the following assertion:
"if $x \in S$, then $x \in T . "$
Any assertion of the form $A \Rightarrow B$ (read: " $A$ implies $B$ ") can be transformed into the logically equivalent assertion given by the contrapositive: $\neg B \Rightarrow \neg A$ (read: "not B implies not $\mathrm{A}$ ").
(a) Use the contrapositive of $(*)$ to obtain an equivalent assertion that can be used to show that $S \subset T$. Apply (*) and its contrapositive to prove that $\mathbf{Z} \subset \mathbf{R}$.
(b) The converse of an assertion $A \Rightarrow B$ is the assertion $B \Rightarrow A$. Use assertion (*) and its converse to illustrate that an assertion can be true but its converse false.

Raj Bala
Raj Bala
Numerade Educator
02:56

Problem 2

Prove that $\cap_{\alpha \in \mathbf{R}_{+}}[-\alpha, \alpha]=\{0\}$.

Tatiana Graham
Tatiana Graham
Numerade Educator
01:32

Problem 3

Which of the following functions $f: \mathbf{R} \rightarrow \mathbf{R}$ are injective? surjective? bijective?
(a) $f: x \mapsto 2+3 x$
(b) $f: x \mapsto 2$
(c) $f: x \mapsto x^3-x$
(d) $f: x \mapsto x /(1+|x|)$.
For each function that fails to be bijective, find a restriction of the domain and/or range to a subset of $\mathbf{R}$ which will make the function bijective. For each bijective function $f: X \rightarrow Y$ obtained (where $X$ and $Y$ denote the subsets of $\mathbf{R}$ which you specified in order to make the function bijective), describe the inverse of the function and verify that $f \circ f^{-1}=\mathrm{id}_Y$ and $f^{-1} \circ f=\mathrm{id}_X$.

Goutam Chand
Goutam Chand
Numerade Educator
04:01

Problem 4

Describe the span of each of the following subsets of $\mathbf{R}^3$ :
(a) $\{(0,1,1)\}$
(b) $\{(0,1,1),(1,0,1)\}$;
(c) $\{(0,1,1),(1,0,1),(1,1,0)\}$.

Himanshu Kushwaha
Himanshu Kushwaha
Numerade Educator
05:08

Problem 5

Draw the hyperplanes and closed halfspaces in $\mathbf{R}^2$ corresponding to each of the following linear functionals represented by the vectors $p \in \mathbf{R}^2$
(a) $p=(1,2)$
(b) $p=(1,-2)$;
(c) $p=(-1,-2)$.

Victor Salazar
Victor Salazar
Numerade Educator
01:19

Problem 6

Verify that the dual space of a vector space is itself a vector space.

Monica Miller
Monica Miller
Numerade Educator

Problem 7

Prove that if $M=M_1 \oplus M_2$ where $M_1$ and $M_2$ are subspaces of the vector space $L$, then each $x \in M$ can be uniquely expressed as a sum $x=m_1+m_2$ where $m_1 \in M_1$ and $m_2 \in M_2$.

Check back soon!

Problem 8

Suppose that $M_1$ is a subspace of codimension one of the vector space $L$. Let $z$ be any nonzero vector that does not belong to the subspace $M_1$.
(a) For each $x \in L$, there is a unique scalar $p(x)$ and a unique vector $m_1(x) \in M_1$ such that $x=m_1(x)+p(x) z$. Why?
(b) Show that the mapping $p: L \rightarrow \mathbf{R}$ defined in part (a) is linear with kernel $H(p, 0)=M_1$.

Check back soon!
07:44

Problem 9

Conversely, suppose that we define a hyperplane to be the kernel of a linear functional $p: L \rightarrow \mathbf{R}$ where $p \cdot z \neq 0$ for some $z \in L$. Let $M_1=\operatorname{ker} p$ and $M_2=\{x \in L \mid x=\lambda z, \lambda \in \mathbf{R}\}$. Prove that $L=M_1 \oplus M_2$, so that $M_1=\operatorname{ker} p$ is a subspace of codimension one.

Anthony Ramos
Anthony Ramos
Numerade Educator
View

Problem 10

Describe the consumption set of a consumer in a two-commodity exchange economy in which:
(a) the first commodity is perfectly divisible while the second can be consumed only in integer amounts; and
(b) the quantities of both commodities are nonnegative.

Victor Salazar
Victor Salazar
Numerade Educator
01:38

Problem 11

Letting $A=\{(1,1),(2,3)\}$ and $B=\{(0,0),(-1,-2)\}$, describe the sets $A+B, 2 A$, and $2 A+B$.

Vicki Stebbins
Vicki Stebbins
Numerade Educator

Problem 12

Give an example of a set $A \subset \mathbf{R}^2$ for which $2 A \subset A+A$ but $2 A \neq$ $A+A$.

Check back soon!
01:19

Problem 13

Develop an alternative proof of Theorem 1.17 along the lines of the alternative proof given for Theorem 1.12.

Nick Johnson
Nick Johnson
Numerade Educator

Problem 15

Consider a pure exchange economy with two consumers and two commodities. Each consumer has consumption set $X_i=\mathbf{R}_{+}^2$, and endowments are given by
$$
w_i= \begin{cases}(1,3) & \text { for } i=1 \\ ,(3,1) & \text { for } i=2 .\end{cases}
$$
Utility functions are of "Leontief type" (i.e., indifference sets are L-shaped), described by utility functions of the form:
$$
u_i\left(x_i\right)=\min \left\{x_{i 1} / \alpha_i, x_{i 2} / \beta_i\right\}
$$
where $\alpha_i$ and $\beta_i$ are positive constants. For each of the following two cases, determine the set of Pareto optima, Walrasian, and core allocations for the economy. Illustrate each of the sets in an Edgeworth box and in a net trade diagram.
$$
\left(\alpha_i, \beta_i\right)= \begin{cases}(2,1) & \text { if } i=1 \\ (1,2) & \text { if } i=2 .\end{cases}
$$
$$
\left(\alpha_i, \beta_i\right)=(2,1) \text { for } i=1,2 .
$$
1.15 Repeat the preceding exercise for a two-person exchange economy in which endowments are given by
$$
w_i= \begin{cases}(0,4) & \text { if } i=1, \\ (4,0) & \text { if } i=2\end{cases}
$$
and preferences are represented by the utility function
$$
u_i\left(x_i\right)=x_{i 1}+2 x_{i 2} \text { for } i=1,2 .
$$

Check back soon!

Problem 16


A pure exchange economy contains two consumers, both with preferences represented by a utility function of the form:
$$
u_i\left(x_i\right)=x_{i 1}\left(4-x_{i 2}\right)
$$
defined over the consumption set $[0,5] \times[0,3] \subset \mathbf{R}_{+}^2$. Thus, the first commodity is a "good" and the second a "bad." Endowments are given by
$$
w_i= \begin{cases}(4,3) & \text { if } i=1 \\ (1,0) & \text { if } i=2 .\end{cases}
$$
(a) Show that consumer demand functions are given by
$$
\phi_i(p)=\left(\frac{p \cdot w_i-4 p_2}{2 p_1}, \frac{p \cdot w_i+4 p_2}{2 p_2}\right)
$$
(b) Show that a feasible allocation $x$ is Pareto optimal iff $x_{11}+x_{12}=$ 4 where $x_1=\left(x_{11}, x_{12}\right)$ is the commodity bundle allocated to consumer one.
(c) Determine the set of core allocations for this economy and illustrate in an Edgeworth box.
(d) Show that the Walrasian equilibrium price functional for this economy is $p=(1,-1)$ where prices have been normalized by choosing the first commodity as numéraire.
(e) Calculate the Walrasian equilibrium for this economy and indicate this allocation as a point in the Edgeworth box.
(f) Sketch the relevant hyperplane, Walrasian allocation, and preferred net trade sets in the net trade diagram.
(g) What happens to the core and to the Walrasian equilibrium if the first consumer has the right to dump all of her endowment of the second commodity onto the second consumer (i.e., she is a polluter with the right to pollute without compensating the other consumer)?
(h) What happens to the core and to the Walrasian equilibrium if property rights regarding the second commodity are not assigned (i.e., whether the first consumer can dump her garbage onto the second consumer's lawn without compensating him depends on political or other considerations not specified by the model)?

Check back soon!

Problem 17

Consider a two-person exchange economy in which endowments are given by
$$
w_i= \begin{cases}(1,3) & \text { if } i=1 \\ (3,1) & \text { if } i=2\end{cases}
$$
and the preferences of both consumers by the vector ordering preference relation: i.e., for any two commodity bundles $x^{\prime}=$ $\left(\xi_1^{\prime}, \xi_2^{\prime}\right)$ and $x=\left(\xi_1, \xi_2\right)$, we have $x^{\prime} \succ x$ iff $\xi_1^{\prime}>\xi_1$ and $\xi_2^{\prime}>\xi_2$. Illustrate in an Edgeworth box and in a net trade diagram the allocations which are Walrasian and those which are in the core.

Check back soon!

Problem 18

The two consumers of an exchange economy have preferences represented by a utility function of the form
$$
u_i\left(x_i\right)=\max \left\{\min \left\{x_{i 1}, 2 x_{i 2}\right\}, \min \left\{2 x_{i 1}, x_{i 2}\right\}\right\},
$$
which yield stair step indifference contours as illustrated below. Endowments are given by $w_1=w_2=(1.5,1.5)$.
ILLUSTRATION CANT COPY
(a) Illustrate the budget set, demand set, and strict preference set
$P_i\left(x_i\right)$ for $x_i \in \phi_i(p)$ for either one of the consumers when $p=$ $(.5, .5)$; when $p=(.25, .75)$.
(b) In an Edgeworth box, indicate the set of (i) individually rational, (ii) Pareto optimal, and (iii) core allocations for this economy.
(c) In this economy, there are two Walrasian allocations, say $x$ and $y$. Determine the equilibrium price functional $p$ (letting $p_1+$ $p_2=1$ ), and describe the allocations $x$ and $y$. (Give the answer numerically.) Calculate the corresponding net trade allocations, and depict the Walrasian equilibria in a net trade diagram. In this sketch, show as well the strict preference sets corresponding to either allocation.
(d) Now suppose that there is a third consumer with the same consumption sets, preferences, and endowment as the other two. Using the net trade diagram, show that there is no Walrasian equilibrium for the economy.
(e) If we add a fourth consumer of the same type, do Walrasian equilibria exist for the economy?
(f) Combining your answers to parts (d) and (e), what is your conjecture about the existence or nonexistence of Walrasian equilibria if we continue to add consumers of the same type to this economy?

Check back soon!
03:43

Problem 19

By writing down the analogs of equations (1.1) and (1.2) for a replica of the two-person Cobb-Douglas economy with $r$ consumers of each type, verify that the Walrasian equilibrium prices and allocation do not change as $r$ increases.

Jennifer Stoner
Jennifer Stoner
Numerade Educator
04:46

Problem 20

Following along the lines of the argument given in Section 1.4.2, show that in the sequence of Cobb-Douglas replica economies the allocation
$$
x_i= \begin{cases}(2.2,2.2) & \text { for } i=1 \\ (1.8,1.8) & \text { for } i=2\end{cases}
$$
can eventually be improved upon by some coalition. (Hint: consider a coalition with $r-1$ members of type 1 and $r$ of type 2.)

James Kiss
James Kiss
Numerade Educator

Problem 21

Verify the assertions made for cases 1 and 2 in the contingent commodity model of Section 1.5.1.

Check back soon!
00:25

Problem 22

It hardly seems possible that the points labeled $x$ and $w$ in Figure 1.13 lie on the hyperplane defined by $p=(1 / 4,1 / 4,1 / 4,1 / 4)$, but they do. Explain.

Ashley High
Ashley High
Numerade Educator
03:30

Problem 23

Where do the proofs of Theorems 1.12 and 1.17 go wrong when applied to the infinite dimensional overlapping generations model?

Jay Patel
Jay Patel
Numerade Educator