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ECON6003/6703 Mathematical Methods for Economics Problem Set 2 (Solutions)
发布时间:2022-05-20
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Problem Set 2 (Solutions)
ECON6003/6703 Mathematical Methods for Economics
1 Limits of Sequences of Real numbers
Exercise 1. Use the definition of the limit of a sequence directly to show that limn!1 an = 2, where
an = That is, explicitly exhibit an integer N"
Solution. Consider
6 n ¡ 1
such that if n N" , jan ¡ 2j < ".
6n ¡ 1
3n+ 2
= ¡3n(1)2(4) =
:
Now pick any " > 0. Whenver n is such that 5/(3 n + 2) < " or n > , then
jan ¡ 2j < ". Now choose N" to be the smallest positive greater than
.
,
jan ¡ 2j < " 8n N" :
Exercise 2. Use the Sandwich Theorem
to show that limn!1 an =0 where
an =pn2 + 2 ¡ n
Hint: You may verify an 0 for all n. Next, find a sequence cn by creating a suit- able upper bound for each an , so that an
cn and limn!1cn = 0. Then apply the
Sandwich Theorem
.
Solution. We will first show that 0an
.
an = pn2 + 2 ¡ n
rn2 + 2 +
¡ n
= p(n+1/n)2 ¡ n
= (n+1/n) ¡ n=1/n:
Also, note that an > pn2 ¡ n = 0. Thus, we have shown that 0
an
. By the
Exercise 3. Fix positive integers k and ` and let
an =nk /(1+n`)
Calculate limn!1 an for different cases when k >`, k =` and k <`.
Solution. First consider xn = np where p is an integer. I leave it to you to con- vince yourself that if p > 0, then xn ! 1 and xn ! 0 if p < 0. When p = 0, np = 1 and hence xn ! 1.
First write
nk
1 + n`
nk
=
=
where p=(k ¡ `) and q = ¡`.
Since q < 0, the denominator nq + 1 ! 1. So the limit of an is the limit of the numerator divided by 1. For the limit of the numerator, we have three cases.
Case 1: k =`. Now, p=0 and hence limn!1np = 1. Therefore limn!1an = 1 Case 2: k <`. Now, p < 0 and hence limn!1np =0. Therefore limn!1an =0. Case 3: k >`. Now, p > 0 and hence limn!1np = 1. Therefore limn!1an = 1.
Exercise 4. In Economic models, typically the sequences under consideration are generated endogenously for example you may choose the capital stock in period one, and the production as well as the consumption determines the capital stock in the next period and so one. In other words, the next element of the sequence is defined
recursively
using the previous elements. In this problem, the sequence is defineed recursively and you are asked to find its limiting properties.
Assume x1 2 (0; 1). For n
1, let xn+1 =1 ¡ p1 ¡ xn . Show that xn ! 0. Hint: Use induction to show that xn is a monotonically decreasing sequence.
Solution. By assumption 0 < x1 < 1. Now let us make the hypothesis that we
have shown that 0 < xn < 1 for an arbitrarily chosen integer n. Then 0 < (1 ¡
than the number itself. Therefore,
Thus, it follows from (1) that 0 < xn+1 < 1. By induction this is true for all n.
Note that (1) also tells us that xn is a monotone decreasing sequence bounded below. It must therefore converge, i.e. xn ! L for some L. How do we know L = 0? We take limits across the identity that defines xn :
lim xn+1 = lim (1 ¡ p1 ¡ xn )
or L = (1 ¡ p1 ¡ L):
or (1 ¡ L) = p(1 ¡ L): (2)
There are two solutions to the above equation, L = 0 or L = 1 in general. How-
ever, if L = 1 it taken to be the solution, we get a contradiction
since 1 ¡ xn >
L=0.
Exercise 5. Let an ! a. Consider the infinite set
S = fa ; a1 ; :::; an ; :::; g:
Is this set closed? Is it also compact?
Solution. Assume that a is finite. (If a = 1, the set S does not make sense). Take any infinite sequence (xn) such that xn 2 S for all n. I will leave it to you to to show that xn ! a since an ! a. Since the limit a 2 S, S is closed. It is also bounded, since an converges and every convergent sequence is bounded. Hence, S is also bounded. Therefore, it is compact.
Exercise 6. Let xn be an infinite sequence in S Rn such that limn!1 _xn = x. Show that if S is closed, then x 2 S.
Solution. Assume, by way of contradiction, that x S even though xn 2 S for all S and S is closed. That is x 2 Sc , and Sc is an open set. Therefore, x must be an interior point of Sc , i.e. there exists an " > 0 such that B" (x)
Sc , i.e. every y such that d(x ; y) < " must lie in Sc . Therefore, we cannot find infinitely many ele- ments of xn that are of distance less than " from x, since by assumption all xn 2 S. That contradicts that fact that limn!1xn = x.
Exercise 7. Let S R be closed and bounded set, i.e. compact and c = supremum(S). Show that c 2 S.
Solution. By the definition of a supremum, for each integer n 1, there exists
xn 2 S such that c ¡
< xn
c. Applying the
Sandwich Theorem
, we conclude
2 Continuity
Exercise 8. Consider the function f(x) = x/jxj if x 0 and f(0) = A. Examine the continuity of f at 0. Is there are value of A such that f can be made contin- uous?
Solution. Note that jxj = x whenever x > 0 and jxj = ¡x when x < 0. Therefore
8 1 if x > 0
f(x) = : ¡A
Take any sequence xn ! 0 such that xn < 0. Then f(xn) = ¡ 1 and hence f(xn) !
limn!1f(xn)=limn!1f(yn).
Note 1. For the following problems, it is useful to note that for every real number x0 , it is possible to find two sequences an and bn such that an ! x0 and bn ! x0 but for every n, an is a rational number while bn is an irrational number.
Exercise 9. In each case, give examples of two functions f and g such that nei- ther f is continuous nor g is continuous but a). f + g is continuous and b). f g is continuous.
Solution. For part (a), take f and g as in Question 12. Let h = f + g , that is for any x, h(x) = f(x) + g(x). If x is rational, then h(x) = 1 + 2 = 3 and otherwise h(x) = 2 + 1 = 3. Since h is constant, it is continuous. However, neither f nor g is continuous on its own.
For part (b), h(x) = f(x) g(x). If x is rational, then h(x) = 1
2 = 2 and if x is not rational, then h(x) = 2
1 = 2.
Exercise 10. Consider the two functions:
f(x) =
g(x) =
1. Is f a continuous function? What about g?
2. Given a function g: A ¡! Y let B = Range(g). Suppose we aer given another
With f and g as defined above, show that h = f g is continuous even though neither f nor g is continuous.
Solution.
Part (1) Neither f nor g is continuous. To see that f is not continuous anywhere, pick any point x0 and consider a sequence of rational numbers xn such that xn ! x0 . For this sequence, f(xn) = 1 and hence f(xn) ! 1. Next take a sequence of irrational numbers yn ! x0 and note that f(yn) ! 2. Similarly g .
Part (2). h(x) = f(g(x)). Note that g(x) = 1 or g(x) = 2, in either case y = g(x) is
a rational number. Therefore, h(x) = f(1) or h(x) = f(2) and either case h(x) = 1. So h is continuous, even though neither f nor g is continuous.
Exercise 11. Study the statement of the Intermediate Value Theorem carefully. Given counter-examples to the theorem when in each case exactly one of the assumptions does not hold.
Solution. We require that the function is continuous, two points in the domain where the function has different signs, and the entire interval between those two points is in the domain.
Case 1. Discontinuous function on an interval. Take f : [ ¡ 1; 1] ¡¡ R to be f(x) =
[0.1; 1] and f(x) = x. (Check this is continuous, even though the middle
bit is missing.)
Case 3. Continuous, and interval but function never alternates in sign. f : [0.5;
1] ¡! R where f(x) = x.