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MAST20026
In this exam paper, R is the set of real numbers, R+ is the set of positive real numbers, Q is the set of rational numbers, Z is the set of integers, and N is the set of positive integers. Question 1 (7 marks) Let P be the statement P : (q _ ⇠ r) =) ((⇠ q) ^ (⇠ r)) where q, r are primitive statements. (a) Construct a truth table for the statement P . (b) If P is true, what can you conclude about q? What can you conclude about r? Explain briefly. Question 2 (10 marks) (a) (i) Express the following statement using only mathematical notation: Any integer which is not a perfect square is either a multiple of 2 or a multiple of 3. (Note: A number is a perfect square if it is the square of an integer.) (ii) Is the statement in (i) true? Explain briefly, with mathematical justification. (b) Consider the statement 8x, y 2 R x < y =) 9z 2 R x < z < y (i) Give an equivalent statement using only mathematical notation but without using a conditional. (ii) Negate the statement, and simplify by eliminating negation symbols ‘⇠’ as much as possible. Question 3 (6 marks) Prove that the product of any irrational number and any non-zero rational number is irrational. Page 2 of 5 pages MAST20026 Semester 1, 2017 Question 4 (6 marks) Listed below are axioms of the real numbers. A1. 8x, y 2 R, x+ y 2 R A2. 8x, y, w, z 2 R (x = w^y = z) =) x+y = w+z A3. 8x, y 2 R, x+ y = y + x A4. 8x, y, z 2 R, (x+ y) + z = x+ (y + z) A5. 9y 2 R, 8x 2 R, x+ y = x A6. 8x 2 R, 9y 2 R, x+ y = 0 O1. 8x, y 2 R, exactly one of the following is true: x < y, y < x, x = y. O2. 8x, y, z 2 R, if x < y and y < z then x < z M1. 8x, y 2 R, xy 2 R M2. 8x, y, w, z 2 R (x = w ^ y = z) =) xy = wz M3. 8x, y 2 R, xy = yx M4. 8x, y, z 2 R, (xy)z = x(yz) M5. 9y 2 R(8x 2 R, xy = x) M6. 8x 2 R \ {0}, (9y 2 R, xy = 1) D1. 8x, y, z 2 R, x(y + z) = xy + xz AO. 8x, y, z 2 R, x < y =) x+ z < y + z MO. 8x, y, z 2 R, (x < y ^ 0 < z) =) xz < yz Using only these axioms (and rules of logic), prove that 8x, y, z 2 R ⇣ zx = zy ^ z 6= 0 =) x = y⌘ Question 5 (16 marks) (a) For a set A ✓ R, state the definition of (i) ‘A is bounded above’ (ii) ‘A is bounded’ (b) Recall that a set A ✓ Q is a cut if it satisfies properties (I)-(III) below: (I) A 6= ; and A 6= Q (II) for all x, y 2 Q, if x 2 A and y < x then y 2 A (III) if x 2 A then there exists some y 2 A with y > x. (i) Prove that every cut is bounded above. (ii) Prove that every cut is unbounded. (c) What can you conclude about the supremum and infimum of a cut? Question 6 (12 marks) (a) State the ✏-M definition of lim n!1 fn = L, where (fn) is a sequence and L 2 R. (b) Let fn = n 6 3n2 + 1 . Prove using the definition that lim n!1 fn = 0. (c) Is the sequence (fn) from part (b) a Cauchy sequence? Explain briefly. A complete proof is not required. (d) Give an example of each of the following, or explain why they do not exist. (i) A bounded, non-monotonic sequence (ii) A divergent, bounded, monotonic sequence (iii) An unbounded convergent sequence If you are giving an example, it is sucient to just write down your example; you do not need to prove that it satisfies the given properties. If you claim that no such example exists, then quote a relevant fact from this subject to support your claim. Page 3 of 5 pages MAST20026 Semester 1, 2017 Question 7 (14 marks) Let f : R! R be given by f(x) = ( x2 x 2 Q x2 x /2 Q . (a) At which point(s) x 2 R is f continuous? Prove that f is continuous at these points using the ✏- definition. (b) Prove that f is not continuous at all other points. Question 8 (6 marks) Use the Intermediate Value Theorem to prove that the equation ex + x = 4 has a real solution. Question 9 (6 marks) Recall that • U(f, P ) and L(f, P ) denote the upper and lower Riemann sums, respectively, of f over the partition P ; • U(f) = inf{U(f, P ) : P 2 eP} and L(f) = sup{L(f, P ) : P 2 eP} where f : [a, b] ! R is a function, P is a partition of [a, b], and eP is the set of all partitions of [a, b]. (a) Explain briefly why U(f) and L(f) exist for any bounded function f : [a, b]! R. (b) Let f : [0, 1] ! R be given by f(x) = x2. Does L(f) = U(f)? If so, what is its value? Justify your answer briefly. Question 10 (8 marks) Use appropriate tests to determine: (a) if the series 1X n=1 2n + 1 n22n + n converges. (b) if the integral Z 1 1 e2x 21 dx exists as an improper Riemann integral. Page 4 of 5 pages MAST20026 Semester 1, 2017 Question 11 (18 marks) Let f : R+ ! R be given by f(x) = log(x). (a) Prove by induction that f (n)(x) = (1)n1(n 1)! xn for all n 2 N. Note: f (n)(x) denotes the n’th derivative of f . You may use the usual di↵erentiation rules for ddx(log x) and d dx(x ↵) without further proof. (b) Recall that the Taylor series of a function g about a point x = a is 1X n=0 g(n)(a) n! (x a)n. Verify that the Taylor series of f(x) = log(x) about x = 1 is 1X n=1 (1)n1 n (x 1)n. (c) Determine the interval of convergence of the Taylor series for f(x). Justify your conclu- sions with appropriate convergence tests or facts from this subject. (d) At which point(s) x 2 R does the Taylor series converge absolutely? At which point(s) does it converge conditionally? Justify your answer. (e) Could the function f be represented as a Fourier series? Briefly justify your answer. End of Exam—Total Available Marks = 109