课程名称︰数量方法入门 课程性质︰经研所必修 课程教师︰朱玉琦 开课学院：社科院 开课系所︰经研所 考试日期（年月日）︰110.08.20 考试时限（分钟）：180 试题 : 部分数学式用latex语法撰写。 Preample: You have 3 hours to answer the following questions. The total number of points is 110. You need to answer each questions in English. If you have stated a the- orem in lecture notes (including homework), you may use it without proving it unless I explicitly ask you to, but you need to describe what the theorem is a- nd why you can apply that theorem. For example, if there are some assumptions for that theorem to be applicable, you need to show that those assumptions are met in your problem. 1. (20 points) Consider a sequence {v_n} in \R^m. Show that \norm{v_n} converg- es to 0 if and only if v_n converges to 0. 2. (14 points) Consider a sequence {x_n}_{n=1}^\infty in \R^m converges to x. (a) (8 points) Show that every subsequence of {x_n}_{n=1}^\infty converges. (b) (6 points) Given your answer from (a), why does a subsequence of {x_n}_{n=1}^\infty cannot converge to a point different from x? 3. (6 points) Let {x_n} and {y_n} be two bounded sequences in \R^1. Show that given any n, \inf{x_n+y_n,x_{n+1}+y_{n+1},...} \geq \inf{x_n,x_{n+1},...}+\inf{y_n,y_{n+1}, ...}. 4. (8 points) Find the supremum and maximum for the following case: X = \Set{x\in\R | x=n/(2n+1), n=1,2,...}. 5. (8points) The functions F and G are defined as F(x)=2x+1,x>0, and G(x) = 1/x if x\geq 1 \\ = 0 if x<1. (a) Verify that R(F)\subseteq D(G). Find the domain and range of (GoF)(x) (Note: here I am asking the range that (GoF) maps“on to"). (b) Find the formula for (GoF)(x). 6. (8points) Consider a function f : S\toT, with A\subset S and B\subset T. (a) Define the set of f^{-1}(f(A)). (b) Define the set of f(f^{-1}(B)). 7. (8 points) Suppose that a function is f = {(1, 3), (2, 5), (3, 8), (4, 10), (5, 11), (6, 4), (7, 6), (8, 8), (9, 10), (10, 12)} (recall that a function is a set of ordered pairs), so this is a function from {1, 2, ...., 10} into \R^1. Let A={1,2,3} and B={6,8,10,12,15}. (a) Write down all the elements in f^{-1}(f(A)). (b) Write down all the elements in f(f^{-1}(B)). 8. (8 points) Suppose a and b are real numbers. Find a and b such that the fol- lowing vectors in \R^4 are linearly dependent. v_1 = [1 2 -2 4]^T, v_2 = [0 1 1 2]^T, v_3 = [2 -1 a b]^T. 9. (8 points) A = \bmatrix{1 2 \\ 3 2}. (a) (5 points) Find the eigenvalues and eigenvectors of A (b) (3 points) There exists a unique solution to solve Ax = b for every b. True or False? 10. (10 points) Discuss why the following proofs are incorrect. (a) Prove that if a sequence has only one limit point p, then this sequence co- nverges to p. Proof: Let us prove by contradiction. Suppose that the sequence xn does not converge to p. Then there exists an e>0 and for all N such that d(x_n,p)\geq e if n\geqN . Then we find an e that there are finite indices n for which d(x_n,p)<e. Ther- efore, p cannot be a limit point. (b) Suppose that x_n\to0. Show that \lim_{n\to\infty}x_n\sin\frac{1}{x_n}=0. Proof: By \lim_{n\to\infty}a_nb_n=\lim_{n\to\infty}a_n\lim_{n\to\infty}b_n, we have \lim_{n\to\infty}x_n\sin\frac{1}{x_n} = \lim_{n\to\infty}x_n\lim_{n\to\infty}\sin\frac{1}{x_n}. Because the range of \sin(1/x_n) is bounded in [-1,1], and x_n tends to zero as n is sufficiently large, the product of the two must eventually tend to zero 11. (12 points) Please state whether each of the following statements is true or false. If your answer is true, give a brief proof or explanation to the sta- tement. If your answer is false, you must provide a counter example, or explain why the statement is false. (a) If \sup A exists, then \sup A = \max A. (b) If \sup A does not exist, then \max A does not exist. (c) If \max A exists, then \sup A\in A. (d) \sup A has to be an element of A. --

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