About This Document. MTH299 - Examples Weeks 1-6; updated on January 5, 2018

Transcription

1 About This Document This is the examples document for MTH 299. Basically it is a loosely organized universe of questions (examples) that we think are interesting, helpful, useful for practice, and serve as a preview of what may appear on homework and exams. Here is how we expect you to interact with this document: You will have a paper copy of this document with you during all class meetings. Each day, your instructor will choose a list of examples from this document that will be the focus of that day s practice activities. This list is dynamic and is designed to address the mixture of needs presented by: new topics, your understanding of old topics, your past homework performance, your upcoming homework assignment. You should always record each day s suggested list of examples. Those lists are suggestive that your instructor thinks those examples are particularly interesting. You will work examples in class, with your group-mates. You will go home and examine the solutions for these examples (and any other ones about which you are curious). The solutions are posted on your class D2L page. You will look for examples that are similar to homework questions in order to practice techniques that will help you do your homework. You can generally use the solutions to the examples as a guide for how you will write your solutions. I say generally because not all of the solutions in the document have been brought up to the our current presentation standards. When in doubt, ask your instructor. (Note, solutions contain many sentences and phrases intended as extra explanation to the students who are reading them and learning from them. You will obviously modify your homework solutions so that this extra explanation is not there.) MSU 1

2 Week 1 (things related to: sets, set operations, statements, conditional statements) Example 1.1. Let S = {1, {2, 3}, 4}. Indicate whether each statement is true or false. (a) S = 4 (b) {1} S (c) {2, 3} S (d) {1, 4} S (e) 2 S. (f) S = {1, 4, {2, 3}} (g) S Example 1.2. Write each of the following sets by listing its elements within braces. Find their cardinalities. (a) A = {n Z : 4 < n 4} (b) B = {n Z : n 2 < 5} (c) C = {n N : n 3 < 100} (d) D = {x R : x 2 x = 0} (e) E = {x R : x = 0} Example 1.3. Find the cardinalities of the following sets. (a) {, { }, {{ }}} (b) {n Z 2014 n 2014} (c) {n N n has exactly 3 digits} For example, 7 has one digit and 107 has three digits. Example 1.4. Compute the cardinality of the set, E, where E is defined as E = {x R : sin(x) = 1/2 and x 5} Example 1.5. List all the proper subsets for each of the following sets: (a) {0, 1, 2} (b) {0, {1, 2}} (c) {1, {2, {3}}} MSU 2

3 Example 1.6. Suppose A = {0, 2, 4, 6, 8}, B = {1, 3, 5, 7} and C = {2, 8, 4}. Find: (a) A B (b) A \ C (c) B \ A (d) B C (e) C \ B Example 1.7. Let U = {1, 2, 3,..., 14, 15} and E c = U \ E. Let A = {1, 5, 9, 13} and B = {3, 9, 13}. Determine the following: (a) A B (b) A B (c) A \ B (d) B \ A (e) A c (f) A B c. Example 1.8. Let U = {1, 2, 3, 4, 5} and E c = U \ E. Let A = {1, 2}, B = {2, 3} and C = {1, 3}. Determine the following: (a) (A B) \ (B C) (b) A c (c) (B C) c (d) A B. Example 1.9. Describe each set below in the form (a) D = {3, 5, 7, 9,... } { 1 (b) E = 2, 2 3, 3 } 4,... (c) F = {2, 4, 8, 16,... } {x X : x satisfies property P } Example Find the union of of {2k k N} and {2k + 1 k N}. Prove your answer. Recall if E = A B, you need to show E A B and A B E. Example Define E = {z Z z = 2k for some k N}. Show that E N. Example Prove that X = Y, where X = {x N : x 2 < 30} and Y = {1, 2, 3, 4, 5}. Example Prove that {x Z : x is divisible by 18} {x Z : x is divisible by 6}. MSU 3

4 Example Prove that {9 n : n Z} {3 n : n Z}, but {9 n : n Z} {3 n : n Z}. Example Prove that {9 n : n Q} = {3 n : n Q}. Example Define the sets and A = {n Z : n = 2k, for some k Z} B = {m Z : m = 2 i, for some i N}. Give a careful explanation of why each of the following two statements are true: (i) B A and (ii) A B. (Note that this means that B is a proper subset of A, see p. 7.) Example Find the intersection of A = {2k k N} and B = {3k k N}. Example Suppose A = {π, e, 0} and B = {0, 1}. Write out the indicated sets by listing their elements. (a) A B (b) B A (c) A A (d) B B (e) A (f) A B B Example For A = { 1, 0, 1} and B = {x, y}, determine A B. Example Give a brief justification for the theorem: Theorem: If A = n and B = m then A B = nm Example Let A = {(x, y) x 2 + y 2 1} and B = {(x, y) (x 1) 2 + y 2 1} inside of R 2 (a) Sketch the regions A and B. (Both are closed discs: circles together with the points they enclose.) (b) Sketch the region (A B) c (c) Sketch the region A c B c (d) Formulate a conjecture which explains your observations in the previous two steps. Example For A = {a R : a 1} and B = {b R : b = 1}, give a geometric description of the points in the xy-plane belonging to (A B) (B A). Example Which of the following sentences are statements? For those that are, indicate the truth value. (i) The integer 123 is prime. (ii) The integer 0 is even. (iii) Is 5 2 = 10?? MSU 4

5 (iv) x 2 4 = 0. (v) Multiply 5x + 2 by 3. (vi) 5x + 3 is an odd integer. Example Which of the following statements are true? Give an explanation for each false statement. (a) (b) { } (c) {1, 3} = {3, 1} (d) = { } (e) { } (f) 1 {1}. Example Imagine that there are exactly three coins on a table, as below. Which of these statements are true? (a) There are four coins on the table. (b) There are two coins on the table. (c) There are three coins on the table. (d) There is a coin on the table. Example Decide whether or not the following are statements. In the case of a statement, say if it is true or false. (i) Every real number is an even integer. (ii) If x and y are real numbers and 5x = 5y, then x = y. (iii) The integer x is a multiple of 7. (iv) The sets Z and N are infinite. (v) The derivative of any polynomial of degree 5 is a polynomial of degree 6. Example Consider the sets A, B, C and D below. Which of the following statements are true? Give an explanation for each false statement. A = {1, 4, 7, 10, 13, 16,... } C = {x Z : x is prime and x 2} B = {x Z : x is odd} D = {1, 2, 3, 5, 8, 13, 21, 34, 55,... } (a) 25 A (b) 33 D (c) 22 / A D (d) C B (e) B D (f) 53 / C. MSU 5

6 Example Decide if the following statements are true or false. Explain. (a) R 2 R 3 (b) {(x, y) R 2 : x 1 = 0} {(x, y) R 2 : x 2 x = 0} Example For a conditional statement P (A) : A {1, 2, 3} over the domain S = P({1, 2, 4}) determine: (a) all A S for which P (A) is true. (b) all A S for which P (A) is false. (c) all A S for which A {1, 2, 3} =. Example Write a conditional statement P (n) over the domain S = {3, 5, 7, 9} such that P (n) is true for half of the elements of S and false for the other half. Example Let the domain of a conditional statement be R and the conditional statement to be P (x) : x(x 1) = 6 (a) For what values of x is P (x) a true statement? (b) For what values of x is P (x) a false statement? Example Let the domain of a conditional statement be Z and the conditional statement to be P (x) : 3x 2 > 4. (a) For what values of x is P (x) a true statement? (b) For what values of x is P (x) a false statement? Example For A = {x R : x 1 2} and B = {y R : y 4 2}, give a geometric description of the points in the xy-plane belonging to A B. Example Define the sets and E := { 4, 3, 2, 1, N} F := N {0} {Z}. (i) Solve the equation 2x 3 + 4x 2 = 0 for x E. (ii) Solve the equation 2x 3 + 4x 2 = 0 for x F. (iii) Now, express F in a more elementary form, such as (just for example!) {A, {0, 1}, a, b, c,... }, and then go back and do (ii) over again. MSU 6

7 Example List the elements of the set S = {(x, y) Z Z : x + y = 3}. Plot the corresponding points in the Euclidean xy-plane. Example Sketch the set X = {(x, y) R 2 drawing, shade in the set X c. : 1 x 2 + y 2 4} on the plane R 2. On a separate Example List the elements of each set below and then sketch a Venn diagram to illustrate how they are related. For this problem, U = {1, 2, 3,..., 10}. A = {n U n is even }, B = {n U n is the square of an integer}, C = {n U n 2 3n + 2 = 0} Shade each of the following sets: (i) U \ (A B) (ii) (U \ A) (U \ B) (iii) U \ (A B) (iv) (U \ A) (U \ B). MSU 7

8 Week 2 (things related to: operations on statements, truth tables, quantifiers) Example 2.1. Express each statement as one of the forms P Q, P Q, or P. Be sure to also state exactly what statements P and Q stand for. (i) The number 8 is both even and a power of 2. (ii) The number x equals zero, but the number y does not. (iii) x A \ B. (iv) x y. (v) y x. (vi) A {X P(N) : X c < }. Example 2.2. Complete the following truth table P Q P Q T T T F F T F F Example 2.3. Let P : 15 is odd. and Q : 21 is prime. State each of the following in words, and determine whether they are true or false. (a) P Q (b) P Q (c) ( P ) Q (d) P ( Q) Example 2.4. Let S = {1, 2,..., 6} and let be conditional statements over the domain P(S). P (A) : A {2, 4, 6} =. and Q(A) : A. (a) Determine all A P(S) for which P (A) Q(A) is true. (b) Determine all A P(S) for which P (A) ( Q(A)) is true. (c) Determine all A P(S) for which ( P (A)) ( Q(A)) is true. Example 2.5. For the sets A = {1, 2,..., 10} and B = {2, 4, 6, 9, 12, 25}, consider the statements P : A B. Q : A \ B = 6. Determine which of the following statements are true. (a) P Q (b) P ( Q) (c) P Q (d) ( P ) Q (e) ( P ) ( Q). Example 2.6. Complete the following truth table MSU 8

9 Example 2.7. Write a truth table for the following: (i) (P P ) Q, (ii) ( P Q). P Q Q P ( Q) T T T F F T F F Example 2.8. Decide whether the following pairs of statements are logically equivalent. (i) P Q and ( P Q) (ii) P (Q R) and (P Q) R Example 2.9. Use truth tables to show that the following statements are logically equivalent. (i) P (Q R) (P Q) (P R) (ii) (P Q R) ( P ) ( Q) ( R) (iii) (P Q R) ( P ) ( Q) ( R) (iv) (P Q) ( P ) ( Q) Example Let the sets A and B be given as A = {3, 5, 8} and B = {3, 6, 10}. Show that the following quantified statement is indeed true: b B, a A, a b < 0. Example Write the following as English sentences. Say whether they are true or false. (a) x R, x 2 > 0 (b) x R, n N, x n 0 (c) a R, x R, ax = x Example Rewrite each statement using,, as appropriate and determine if the statement is true or false. (a) There exists a positive number x such that x 2 = 5. (b) For every positive number M, there is a positive number N such that N < 1/M. (c) No positive number x satisfies the equation f(x) = 5. Example Rewrite the following using and. For each statement, determine if it is true or false. MSU 9

10 (a) There exists a real number r such that r = cos(r). (b) For all n N, either n or n + 1 is even. (c) For all x R, x 2 is nonnegative. (d) There exists a natural number n satisfying 2n 2 = n 3. (e) If x is a real number with 1 < x < 0, then x < x 3. (f) If a and ɛ are real numbers with ɛ > 0, then there exists a natural number n such that a n < ɛ. (g) If f is continuous and f(0) < 0 < f(1), then there exists a real number c (0, 1) such that f(c) = 0. (Look up the intermediate value theorem to try to figure out this one!) Example Decide whether the following are true or false. Provide a proof to justify your answer. (a) x Z, y Z, x + y = 1. (b) y Z, x Z, x + y = 1. (c) x Z, y Z, xy = x. (d) y Z, x Z, xy = x. Example Determine the truth value of each of the following statements, and explain/justify your answer. (a) x R, x 2 x = 0. (b) x R, x 2 = x. Example Decide whether the following are true or false. Explain your answers. (a) p P 2, x R, p(x) < 0. (b) x R, p P 2, p(x) < 0. (c) x Z, y Z, (x 2 = y). (d) y Z, x Z, (x 2 = y). Example Decide whether the following are true or false. Explain your answers. (i) n N such that n 2 n. (j) n Z such that n 2 < n. MSU 10

11 Example Determine if the following statement is true or false, and justify your answer. q Q, x R, e qx Q. Example Determine if the following statement is true or false, and justify your answer. q Q, x R \ {0}, e qx Q. Example Determine if the following statement is true or false, and justify your answer. (a, b) { (x, y) R 2 : x < y }, q P 3, q(a) = q(b) = 0. Example Determine if the following statement is true or false, and justify your answer. (a, b) { (x, y) R 2 : x 2 + y 2 = 1 }, θ [0, π], (a, b) (cos θ, sin θ). Example Determine if the following statement is true or false, and justify your answer. p P 3, q P 5, q = p. Example Determine if the following statement is true or false, and justify your answer. [ 2 ] p P 3, ( x R, (p (x) 0)) and p(s) ds 0. (Note to reader: you could interpret this as: p P 3, 1 [ p constant and 2 1 ] p(s) ds 0.) Example Determine if the following statement is true or false, and justify your answer. ( 2 ) p P 3, deg(p) = 3 and p(s) ds = 0. Example Assume that you know for any real numbers, a, b, c R that if a 0 and b c, then it holds that ab ac. Carefully justify the statement: for any real number, x, it holds that x 2 0. Example Assume that you know that x < y. Carefully justify the statement that x < x + y 2 Example Show that there exists a positive even integer m such that for every positive integer n, 1 m 1 n < y. MSU 11

12 Week 3, (things related to: implications, negation, contradiction) Example 3.1. Consider the statements P : 17 is even. and Q : 19 is prime. Write each of the following statements in words and indicate whether it is true or false. (a) P (b) P Q (c) P Q (d) P Q Example 3.2. Without changing their meanings, convert each of the following sentences into a sentence having the form If P, then Q. (a) The quadratic equation ax 2 + bx + c = 0 has real roots provided that b 2 4ac 0. (b) A function is rational if it is a polynomial. (c) An integer is divisible by 8 only if it is divisible by 4. (d) Whenever a circle, C, has a circumference of 4π, it has an area of 4π. (e) The integer n 3 is even only if n is even. Example 3.3. Let P : 18 is odd, and Q : 25 is even. State P Q in words. Is P Q true or false? Example 3.4. Write a truth table for the logical statements: (i) P (Q R) (ii) (P P ) Q (iii) (P Q) Example 3.5. Use truth tables to show that the following statements are logically equivalent. (i) P Q ( P ) Q (ii) P Q (P Q) (Q Q) Example 3.6. Use truth tables to determine the following questions. (i) Is the statement ( P ) (P Q) logically equivalent to the statement (Q P )? (ii) Is the statement P Q logically equivalent to the statement ( P ) Q? Example 3.7. Let A = {x Z : 6 x}, B = {x Z : 2 x}, C = {x Z : 3 x}. Prove the following statement. x A ( y B and z C such that x = yz) Example 3.8. Prove that if x < 4 and y > 2, then the distance from (x, y) to (1, 2) is at least 6. Note that the distance between two points (x 1, y 1 ) and (x 2, y 2 ) is given by d = (x 1 x 2 ) 2 + (y 1 y 2 ) 2 MSU 12

13 Example 3.9. Prove that if an isosceles triangle has sides of length x, y, and z, where x = y and z = 2xy, then it is a right triangle using The Law of Cosines. Example Prove that if x 2 1, then x 2 7x > 10. Example Show that A B is logically equivalent to not(a and not B) This is the idea behind a proof by contradiction. (a) Using truth tables (b) Using other rules of logic we have developed in this course. Example Fill in the blank with necessary, sufficient or necessary and sufficient. (a) The number x > 1 is for x 2 > 1. (b) The number x N is for x 0. (c) The number x > 1 is for x 2 > 1. Example Consider the implication If m 2 > 0, then m > 0. Is it true or false? Write down its converse and determine its truth value. Example Prove the statement: If n Z is even, and m Z is odd, then n + m is odd. Example Prove that: (a) If A B then A B = B. (b) If A B = B then A B. Example Let P, Q, and R be statements. Prove that (P = Q) = R is equivalent to (P ( Q)) R. Example Prove that if A B, B C, and C A, then A = B and B = C. Example Prove that if x, y R, then 3 4 x y2 xy. Example If n is an integer, show that n 2 + n 3 is an even number. Example Consider the statements P : 2 is rational. and Q : 22/7 is rational. Write each of the following statements in words and indicate whether it is true or false. (a) P Q (b) Q P (c) ( P ) ( Q) (d) ( Q) ( P ). Example In each of the following, two conditional statements P (x) and Q(x) over a domain S are given. Determine the truth values of P (x) Q(x) for each x S. (a) P (x) : x = 4; Q(x) : x = 4; S = { 4, 3, 1, 4, 5}. MSU 13

14 (b) P (x) : x 2 = 16; Q(x) : x = 4; S = { 6, 4, 0, 3, 4, 8}. (c) P (x) : x > 3; Q(x) : 4x 1 > 12; S = {0, 2, 3, 4, 6}. Example Let S = {1, 2, 3, 4}. Consider the following conditional statements over the domain S: n(n 1) P (n) : is even. 2 Q(n) : 2 n 2 ( 2) n 2 is even. R(n) : 5 n n is prime. Determine the distinct elements a, b, c, d in S such that (i) P (a) = Q(a) is false; (ii) Q(b) = P (b) is true; (iii) P (c) R(c) is true; (iv) Q(d) R(d) is false; Example For statements P and Q, construct a truth table for (P Q) ( P ). Example Show that if n N, then 1 + ( 1) n (2n 1) is a multiple of 4. Next, show the converse, i.e. that if k Z is a multiple of 4, then k = 1 + ( 1) m (2m 1) for some m N. Example Define the Euclidean norm of x = (x 1, x 2,..., x n ) R n by x = x x 2 n. Prove that x = 0 if and only if (x 1, x 2,..., x n ) = (0, 0,..., 0). Example Decide whether the following pairs of statements are logically equivalent. (i) (P Q) and P Q (ii) (P Q) R and ((P Q) R) (iii) ( P ) (P Q) and (Q P ) (iv) P (Q Q) and ( P ) (Q Q) Example Suppose the statement ((P and Q) or R) = (R or S) is false. Find the truth values of P, Q, R and S. (This can be done without a truth table.) Example State the negation of each of the following statements. (a) 2 is a rational number. (b) 0 is not a negative number. (c) 111 is a prime number. Example State the negation of each of the following statements. (a) The real number r is at most 2. (b) The absolute value of the real number a is less than 3. (c) Two angles of the triangle are 45. MSU 14

15 (d) The area of the circle is at least 9π. (e) Two sides of the triangle have the same length. (f) The point P in the plane lies outside of the circle C. Example State the negations of the following quantified statements: (a) For every rational number r, the number 1/r is rational. (b) There exists a rational number r such that r 2 = 2. Example Negate the following implications: (a) If you are good I will buy you ice cream. (b) In order to earn a 4.0 in MTH 299, it is sufficient that one scores 95% on all exams and assignments. Example Negate the following statements and determine which is true, the original or the negation. Justify your answers. (a) y Z, x Z, x + y = 1. (b) x Z, y Z, xy = x. Example Negate the following. State whether each original statement is true or false. If the original is a true statement, prove it. If the original is false, prove that the negated statement is true (this is one of the great uses of negation... of the original statement and its negation, exactly one of them is always true). (a) n Z, m Z, m n = 1. (b) x Q, y Q, x y = y. Example Of the following two statements, one is true and one is false. Determine which one is which, prove why the true one is true, and why the false one is false. (Note, to prove a statement is false, it is equivalent to prove that its negation is true.) (i) x R, y R, such that y 2 3x + 5. (ii) y R, x R, such that y 2 3x + 5. Example Of the following two statements, one is true and one is false. Determine which one is which, prove why the true one is true, and why the false one is false. (Note, to prove a statement is false, it is equivalent to prove that its negation is true.) (i) c R, p {f P 2 : deg(f) 1}, cxp (x) = p(x) (ii) c R, p P 2, xp (x) + c = p(x) MSU 15

16 Example Use the method of proof by contradiction to prove the following statement: If A and B are sets, then A (B \ A) =. Example Let n be an integer. Show that n 2 is even if and only if n is even. Example Recall the definition: Definition(s): A real number x is rational if x = a for some a, b Z where a and b have no common b factors and b 0. Also, x is irrational if it is not rational. Use contradiction to prove that: The number 2 is irrational. Example Prove that 6 is an irrational number. Further, show that there are infinitely many positive integers n such that n is irrational. Example Prove the following: (a) (Instructor) There are infinitely many prime numbers. (b) There are no rational number solutions to the equation x 3 + x + 1 = 0. Example Prove that there is no largest even integer. Example Suppose a circle has center (2, 4). (a) Prove that ( 1, 5) and (5, 1) are not both on the circle. (b) Prove that if the radius is less than 5, the circle does not intersect the line y = x 6. (c) Prove that if (0, 3) is not inside the circle, then (3, 1) is not inside the circle. Example Let n be an integer. Show that n 2 is odd if and only if n is odd. Example 3.44 (Prove using Contradiction). Suppose a Z. Prove that if a 2 is even, then a is even. Example Suppose a, b, c Z. If a 2 + b 2 = c 2, then a or b is even. Example Prove that If a, b Z, then a 2 4b 2. Example Use the method of proof by contradiction to prove the following: (Hint: Consider (sin x cos x) 2.) x [π/2, π], sin x cos x 1. Example Prove: For every real number x [0, π/2], we have sin x + cos x 1. Example Suppose a, b R. If a is rational and ab is irrational, then b is irrational. Example Prove that if p is prime and p > 2, then p is odd. MSU 16

17 Week 4 (things related to contrapositive, induction, and divisibility) Example 4.1. Find the contrapositive of the following statements. (a) If Jane has grandchildren, then she has children. (b) If x = 1, then x is a solution to x 2 3x + 2 = 0. (c) If x is a solution to x 2 3x + 2 = 0, then x = 1 or x = 2. Example 4.2. Write down the contrapositive of the following: (a) If n is an integer, then 2n is an even integer. (b) You can work here only if you have a college degree. (c) The car will not run whenever you are out of gas. (d) Continuity is a necessary condition for differentiability. Example 4.3. Consider the conditional statements over the domain N. P (x) : x is prime Q(x) : x is odd (a) Determine the set S whose elements make the statement P Q false. Explain your answer. (b) Write down in words the contrapositive of P Q and determine the set T for which the contrapositive is a true statement. (c) Is there any relation between the sets S and T? Explain your answer. Example 4.4. Write down the contrapositive of the implication If 7m is an odd number, then m is an odd number. Then prove the original implication is true by showing that the contrapositive implication is true. Example 4.5. Assume that x Z. Prove that if x 2 6x + 5 is even, then x is odd. (Hint: Use the fact that an implication and its contrapositive are logically equivalent. You may want to write the desired implication as A = B so that you do the contrapositive correctly.) Example 4.6. Suppose x, y {z R : z 0}. Prove if xy > 100 then x > 10 or y > 10. Example 4.7. Let x be a positive real number. Prove that if x 2 x > 1, then x > 2 by (a) a direct proof, (b) a proof by contrapositive and MSU 17

18 (c) a proof by contradiction. Example 4.8. Give a proof for the claim: If x and y are odd integers, then xy is odd using: (a) A direct proof. (b) Proof by Contrapositive. (c) Proof by Contradiction. Example 4.9. Prove by induction that for all n N, Example Prove the following: If n N, then n i=1 i 2 = n (n + 1)(2n + 1) ! + 2 3! + 3 4! + + n (n + 1)! = 1 1 (n + 1)!. Example If n is a non-negative integer, use mathematical induction to show that 5 (n 5 n). Example Prove the following using mathematical induction: For every integer n N, n 3 = n2 (n + 1) 2. 4 Example Prove the following statement For all n N, n i i! = (n + 1)! 1. i=1 Example Prove that for each n N, 9 divides 4 3n 1. If you do not know what divides means, you can look it up in the text it is in the index. Example Assume that f : R R is defined via the assignment rule f(x) = e 2x. We will use the notation that f (n) (x) is the n-th derivative of f. Prove, using induction that for all n N, f (n) (x) = 2 n e 2x. You are allowed to assume the standard differentiation rules for g(x) = e x and h(x) = cx (for a constant, c), as well as the chain rule. Example Prove that 5 (3 3n n+1 ) for every positive integer n. Example Use mathematical induction to prove the following: For any integer n 0, it follows that 3 (n 3 + 5n + 6). MSU 18

19 Week 5 (things about functions, injective, surjective) Example 5.1. There are four different functions f : {a, b} {0, 1}. List them all. Example 5.2. For each of the following, determine the largest set A R, such that f : A R defines a function. Next, determine the range, f(a) := {y R : f(x) = y, for some x A}. (i) f(x) = 1 + x 2, (ii) f(x) = 1 1 x, (iii) f(x) = 3x 1, (iv) f(x) = x 3 8, (v) f(x) = x. x 3 Example 5.3. Suppose A = {a, b, c, d, e, f, g}, B = {1, 2, 3, 4, 5, 6} and the assignment rule for f is given by f(a) = 2, f(b) = 3, f(c) = 4, f(d) = 5. State the domain and a choice of co-domain of f so that the assignment rule, f, given, defines a function. As a followup, determine the range of f (range is defined in the supplementary document). (Note to student, there are infinitely many correct choices for the co-domain so that the resulting triple gives a function, but there is exactly one correct choice for the range of this function.) Example 5.4. Suppose A = { 101, 10, 5, 0, 1, 2, 3, 4, 5, 6, 10}, B = {1, 2, 3, 4, 5, 6} and the assignment rule for f is defined by the table: x f(x) First, evaluate f(2) and f( 5). Then, state the domain and a choice of co-domain of f so that the assignment rule, f, given, defines a function. As a followup, determine the range of f (range is defined in the supplementary document). (Note to student, there are infinitely many correct choices for the co-domain so that the resulting triple gives a function, but there is exactly one correct choice for the range of this function.) Example 5.5. Find the largest possible domain, X R, so that the formula, defines a function f : X R. Justify your answer. f(x) = 3x + 1 5x 2 MSU 19

20 Example 5.6. Does the formula, f(x) = 3x + 1 5x 2 define a function f : Z Q? Justify your answer. Example 5.7. Attempt to define an assignment for a real number, x R, as { x if x is even 2 f(x) = x+1 if x is odd. 2 Does this assignment make sense for x R? Justify your answer. Does this assignment define a function f : R R? Justify your answer. Example 5.8. Let f : R Z be defined via the assignment f(x) = x := max{z Z : z x}. (Note, we call f the integer floor function.) (i) evaluate f(0.1), f(4.7), f(π), f( e). (ii) does this assignment define a function? Justify your answer. Example 5.9. Let A = {x R x > 0}. The function f : A B (with B R) is defined by the assignment rule f(x) = x 2 4x + 5. What is the largest codomain i.e. choice of the set, B so that f is surjective? Carefully justify your answer. Example Define the function, f : P 3 R via the operation f(p) := 1 0 p(x)dx. Is f injective and or surjective from P 3 to R? Justify your answer. Example The following statements give certain properties of functions. You are to do two things: (a) rewrite the defining conditions using quantifiers and logical connectives, as appropriate (b) write the negation of part (a) using the same symbolism. Example: A function f is odd if for every x, f( x) = f(x). (a) defining condition: x, f( x) = f(x). (b) negation: x such that f( x) f(x). It is not necessary that you understand precisely what each term means. MSU 20

21 (i) A function f is strictly decreasing if for every x and y, if x < y, then f(x) > f(y). (ii) A function f : A B is injective if for every x and y in A, if f(x) = f(y), then x = y. (iii) A function f : A B is surjective if for every y in B there exists an x in A such that f(x) = y. Example The following statements give certain properties of functions. You are to do two things: (a) rewrite the defining conditions using quantifiers and logical connectives, as appropriate (b) write the negation of part (a) using the same symbolism. Example: A function f is odd if for every x, f( x) = f(x). (a) defining condition: x, f( x) = f(x). (b) negation: x such that f( x) f(x). It is not necessary that you understand precisely what each term means. (i) A function f is even if for every x, f( x) = f(x). (ii) A function f is periodic if there exists a k > 0 such that for every x, f(x + k) = f(x). (iii) A function f is increasing if for every x and y, if x y, then f(x) f(y). Example Which of the following best identifies f : R R as a constant function, where x and y are real numbers. (a) x, y, f(x) = y. (b) x, y, f(x) = y. (c) y, x, f(x) = y. (d) y, x, f(x) = y. Example Define the set G = {1, 2, 3} and the function f : {1, 2, 3} Z be given by f(n) = n 2. Define the following sets: A = {n Z : m G with f(m) < n} B = {n Z : m G with f(m) > n} C = {n Z : m G, f(m) < n} D = {n Z : m G, f(m) > n} Give a basic description of the sets A, B, C, and D, just using subsets of the integers combined with inequalities, e.g. for some fictitious set, E, we have E = {x Z : x 7029}. MSU 21

22 Example Carefully justify both of these two claims under the assumption that x and y are real numbers. (i) max{x, x} = x. (ii) max{x, y} = 1 ( x y + x + y) 2 Note, it may be useful to recall that x = { x if x 0 x if x < 0. Example Let A = {1, 2, 3, 4} and B = {a, b, c}. Give an example of a function f : A B that is neither injective nor surjective. Example Let A = {1, 2, 3, 4, 5} and B = {1, 2, 3, 4}. (a) Give an example of a function f : A B such that y B x A, f(x) = y. (b) Give an example of a function g : A B such that y B x A, g(x) = y. (c) Give an example of a function h : B A such that x, y B, h(x) = h(y) = x = y. Example Negate each statement in Question 5.17 and give an example of a function f, g, h satisfying the corresponding negation, if possible. If no example can be provided explain why. Example A function f : Z Z Z is defined as f((m, n)) = 2n 4m. Verify whether this function is injective and whether it is surjective. Example Show that the function f : R R given by f(x) = x is injective. Example Prove or disprove that the function f : R R defined by f(x) = x 3 x is injective. Hint: A graph can help, but a graph is not a proof. Example Let the function f : [ 1, 1] [0, 1] be defined by f(x) = x. Is this function injective? How about if its domain is [0, 1], instead of [ 1, 1]? Example Consider the cosine function cos : R R. Decide whether this function is injective and whether it is surjective. What if it had been defined as cos : R [ 1, 1]? Example Let A = {x R x 2} and the function f : A R be defined by f(x) = 4x x 2. Prove that the function f is injective. Example A function f : Z Z is defined as f(n) = 2n + 1. Verify whether this function is injective and whether it is surjective. Example Prove or disprove that the function f is surjective MSU 22

23 (i) f : R { R defined by x f(x) = if x 0 x + 10 if x < 0, (ii) f : Z N {0} defined by f(x) = x. (iii) f : R 2 R 2 defined by f(x, y) = (x + y, x y). (iv) Let A = {x R 0 x 5} and B = {x R 2 x 8}. Let f : A B be defined by f(x) = x + 2. Example Consider the following function: f : Z { 1, 1} { 1 n is odd. f(n) = 1 n is even. a) Show that f is surjective. (just give a brief justification) b) Show that f is not injective. (just give a brief justification) Example For k R, consider the function f : R R defined by { kx + 5 x < 0 f(x) = x x 0. For what values of k is f a bijection? Example Let f : Z Z be the assignment defined by the relationship { n if n is even 2 f(n) = n+1 if n is odd. 2 (a) Does this description of f define a function? Justify your answer. (b) Is f injective? Justify your answer. (c) Is f surjective? Justify your answer. Example Define the operation f(p) := d dx p. Does f define a function from P 4 to P 4? Justify your answer. Is f an injective function from P 4 to P 4? Justify your answer. Is f a surjective function from P 4 to P 4? Justify your answer. (P n is the collection of polynomials or order at most n, see the supplementary document.) MSU 23

24 Example Let Int be the set of intervals, and let Tri be a set that contains a class of triangles, Int = {[a, b] a, b R, a < b} Tri = {( ABC) ABC is the unique triangle with vertices A = (a, 0), B = (b, 0), C = (x, y) and with a < b, y > 0}. We can think of Int as the set of closed intervals in R and Tri as the triangles in R 2 that have one of their sides as an interval in Int (sitting on the x-axis) and the third vertex somewhere above the x-axis. Now, we define a function from Int to Tri as h : Int Tri using the assignment rule input = I = [a, b], output = h(i) = the unique equilateral triangle whose base is I with its 3rd vertex above the x-axis. (i) Evaluate h([1, 5]). (ii) Is h an injective function? (iii) Is h a surjective function? (Note, it may be useful for you to recognize that because of the equilateral shape of h(i), you can write an explicit formula for the three vertices of h(i), in terms of a and b when I = [a, b]). Example Let the alphabet set be A = { a, b, c,... z }, and for n N, define, A n as the set of words or more formally, strings consisting of at most n letters from the alphabet set A. Formally, this could be A n = {string = θ 1 θ m m n and θ i A for i = 1,..., m}. Notice that both eleven and xjastr are strings of length 6 in A 6, so here we don t care if our strings are valid words in the English dictionary. Consider the function s : A 6 A 6 that takes a string of length at most 6 to the string with all of the same letters sorted alphabetically. For example, s( number ) = bemnru, and s( eleven ) = eeelnv. Determine whether s is injective and whether it is surjective. If s is not surjective, how can we redefine the codomain of s to make it surjective? Example Define the set D P(N), as D = {A N : A contains a finite number of elements}, and the set Even = {x Z : x = 2k for some k Z}. MSU 24

25 Define a function, c : D N {0}, via the assignment c(a) = A Even. (Recall that the cardinality, E, is defined in the text, and the power set, P(N), is defined in the supplementary material.) Answer the questions about the function c. (a) Let E = {1, 3, 4, 6, 17, 20, 21}. Evaluate c(e). (b) Let B = {x N : 100 < x < 141 and x is divisible by 7}. Evaluate c(b). (c) Let H = {x N : x is divisible by 20}. Can you evaluate c(h)? (d) Prove that c is not an injective function. (e) Prove that c is a surjective function. Example Consider the function G : P 4 P 2 given by taking the second derivative, that is, for a polynomial f P 4, we have G(f) = d2 f dx 2 (a) Prove that G is not injective. (b) Prove that G is surjective. (c) Let X = {p P 4 : 1+a 2 x 2 +a 3 x 3 +a 4 x 4 }, and define H, similarly to G as H : X P 2, via H(f) = d2 f dx 2. Prove that H is injective. Example Let the function, R : P 2 P 4, be defined via the assignment rule [R(p)](x) := p(x) 2. (a) For q(x) = x 2 + 3x, evaluate the output, R(q). (b) Is R an injective function? Justify your answer. (c) Is R a surjective function? Justify your answer. Example Give an example of a function G : P 2 P 1 such that when the input is p(x) = x 2, the output is [G(p)](x) = 10x Example Given an example of a function g : R R that is injective but not surjective. Justify your answer. Example Give an example of p : R R such that p is a polynomial of degree 3 that is surjective but not injective. You don t need to justify your answer. MSU 25

26 Example Define the function T : P 2 P 3 via the assignment [T (f)](x) := x 0 f(s)ds, for all x R. That is to say, the input to T is a polynomial f P 2 and the output from T is the new polynomial made by antidifferenting f with a zero constant of integration. (a) Evaluate T (f) for each of the three instances of f as: f(s) = 1, f(s) = s, and f(s) = s 2. (note, since T (f) is a polynomial, you must show which polynomial it is by plugging in a real variable, x. So, really, you must show what is [T (f)](x) for x R.) (b) Prove that T is an injective function. (c) Prove that T is not a surjective function. NOTE! The fundamental theorem of calculus may be incredibly helpful to you here. Also note, you probably do not have to bother to write f(x) = a 0 + a 1 x + a 2 x 2, etc... you can just leave your functions as p, q, f, etc... Example Let the function, R : P 3 R, be defined via the assignment rule R(p) := (i) is R an injective function? Justify your answer. (ii) is R a surjective function? Justify your answer. p(4) p(0). 4 (iii) Describe the function, R, in terms of the graph of p; of the following list of phrases, include the correct and relevant ones in your description: derivative, secant line, antiderivative, integral, slope, area under the curve, gravitational singularity. MSU 26

27 Week 6 (things related to bijections and inverse functions) Example 6.1. Let f : N N be the function defined via the assignment f(n) = (i) Prove that f is not an injective function. (ii) Prove that f is a surjective function. { n n is even 2 3n + 1 n is odd (iii) Does there exist a function g : N N that satisfies g(f(x)) = x for all x N? (iv) Does there exist a function h : N N that satisfies f(h(y)) = y for all y N? Example 6.2. The function f : R R defined as f(x) = πx e is bijective. Find its inverse. Example 6.3. Prove that the function f : R \ {2} R \ {5} defined by f(x) = 5x+1 is bijective, and find x 2 the assignment rule for f 1 (you don t have to prove why the assignment rule is what it is). Example 6.4. Check that the function f : Z Z defined by f(n) = 6 n is bijective. Then find the assignment rule for f 1. Example 6.5. Let A = R \ {1} and define f : A A by f(x) = (i) Prove that f is bijective. (ii) Determine f 1. x x 1 for all x A. Example 6.6. Let F : (, ln(6) 4) ( 3, 3) be the function defined via F (x) = e x+4 3 (a) Show that F is a bijection. (b) Find the assignment rule for F 1 and state the domain and range of F 1. Example 6.7. Consider the function f : R N N R defined as f(x, n) = (n, 3xn). Check that this is bijective; find its inverse function. Carefully justify that your answer does indeed yield the inverse function. Example 6.8. The function f : R 2 R 2 is defined by (a) Show that f is a bijection. f(x, y) = (2x 3y, x + 1) (b) Find the assignment rule for the inverse, f 1, of f. Example 6.9. Suppose f : A B and g : X Y are bijective functions. h : A X B Y by h(a, x) = (f(a), g(x)). Prove that h is bijective. Define a new function MSU 27

28 Example (i) Prove the following: If f : N N and f is strictly increasing, then f is injective. (ii) Give an example of a function, f : Z Z such that f is strictly increasing, but f is not bijective. { x if x < 0 Example Consider the functionf : Z Z by the rule f(x) = 1 x 2 if x 0 Determine and justify if f is injective and/or surjective. If f is bijective find an inverse function. Example Consider the function f : R B given by the assignment rule { x + 13 if x < 0 f(x) = 14 + x 2 if x 0. Find the largest set B R such that f has an inverse. Find the assignment rule for the inverse function (you do not need to justify that it is the inverse). Example Suppose X is the set of all continuous functions from R to R. Define a function M : X X via the assignment M(f)(x) := 1 f(x 1), for all x R Show that: (a) M is a bijection. (b) Find M 1. Example Define the function T : P 2 R P 3 via the assignment [T (f, a)](x) := x 0 f(s)ds + a, for all x R. That is to say, the input to T is the pair (f, a) P 2 R, of a polynomial and a real number. The output from T is the new function made by anti-differentiating f with a zero constant of integration, and then adding the constant a to it. (a) Evaluate T (f, 6) when f is the polynomial given by f(x) = 2x 2 x. (b) Prove that T is an injective function. (c) Prove that T is a surjective function. (Note, if p P 3 is given, and you claim to have found some pair (q, a) such that T (q, a) = p please don t forget to check that indeed [T (q, a)](x) = p(x) for all x and that q in indeed an element of P 2!) (d) Find the assignment rule for the function, T 1. NOTE! The fundamental theorem of calculus may be incredibly helpful to you here. Also note, you probably do not have to bother to write f(x) = a 0 + a 1 x + a 2 x 2, etc... you can just leave your functions as p, q, f, etc... MSU 28

29 Example Let P n be the set of real polynomials of order at most n. For each of the following functions, determine if: (i) the function is surjective, (ii) the function is injective, and (iii) if the function is not injective, and if its domain is X and its codomain is Y, can you give an example of a set A X such that the function is injective from A Y? (a) Given a polynomial p P 3, such that p(x) = a 0 + a 1 x + a 2 x 2 + a 3 x 3 define the function D : P 3 P 3 with the assignment rule [D(p)](x) = p (x) + a 0 x 3 Note that D is a function that takes a real polynomial p as an input and returns another real polynomial, D(p). (b) Given a polynomial p P 3, define the function I : P 3 R with the assignment rule I(p) = 1 0 p(x) dx Note that I is a function that takes a real polynomial p as an input and returns a real number. (c) Given a polynomial p P 3, define the function S : P 3 P 6 with the assignment rule S(p) = p 2 Note that S is a function that takes a real polynomial p as an input and returns another real polynomial, S(p). When we plug in a real variable x into the polynomial S(p), we get that. (d) Given a polynomial p P 3, such that [S(p)](x) = [p(x)] 2 p(x) = a 0 + a 1 x + a 2 x 2 + a 3 x 3 define the function C : P 3 R 4 with the assignment rule [C(p)](x) = (a 0, a 1, a 2, a 3 ) Note that C is a function that takes a real polynomial p as an input and returns a real 4-vector. MSU 29