Logic. Facts (with proofs) CHAPTER 1. Definitions

Transcription

1 CHAPTER 1 Logic Definitions D1. Statements (propositions), compound statements. D2. Truth values for compound statements p q, p q, p q, p q. Truth tables. D3. Converse and contrapositive. D4. Tautologies and contradictions. Logical equivalence. D5. Quantifiers and. Facts (with proofs) F1. An implication and its contrapositive are logically equivalent statements. F2. (p q) (p q) (q p) F3. De Morgan s Laws. F4. Distributive Laws for conjunction and disjunction. F5. The negation of a statement involving one quantifier: ( xp (x)) x( P (x)) and ( xp (x)) x( P (x)). Negation of the statements involving several quantifiers. 1

2 2 1. LOGIC Easier Problems #1.1 Let p and q and r be the propositions: p = m/3 is an integer q = n/2 is an integer r = (nm)/6 is an integer Write the following statements using p, q, r and the symbols,,, and. (a) If m/3 is an integer and n/2 is an integer, then (nm)/6 is an integer. (b) The converse of your statement from part (a) above. (c) The contrapositive of your statement from part (a) above. (d) Is the statement in part (b) above true or false? Give your reasons. #1.2 Give truth tables for the following statements. Determine which are tautologies, which are contradictions and which are neither. (a) [(p p) q]. (b) (q p) p. (c) (p q) (p q). (d) (p q) r. (e) [(p q) (q r)] (p r). #1.3 Suppose that the truth values of the statements p and r are true and the truth values of the statements q and s are false. Find the truth values of the following compound statements. (a) ( ( p q) r) s. (b) (p q) (r s). #1.4 Prove De Morgan s Laws: (a) (p q) p q. (b) (p q) p q. #1.5 Prove that (p q) is logically equivalent to (p q) (q p). #1.6 Let p and q be compound statements. If p is a contradiction, what can be said about the truth value of statement p q? If p is a tautology, what can be said about the truth value of statement p q? If q is a tautology, what can be said about the truth value of the statement p q? #1.7 Find the truth values of the following statements. Then construct their negations. (a) There exists an integer x such that (x 2 x 1)(x + 1) = 0. (b) For all real numbers x and y, x 2 + y 3 0. #1.8 Find the truth values of the following statements. Assume that all letters represent real numbers. (a) For all real x, (x 2 x 1)(x + 1) = 0 implies x 2 x 1 = 0. (b) For all real numbers x, (x 2 +1)(x 3 x 10) = 0 implies x 3 x 10 = 0. (c) For all real numbers x, x 2 (x + 1) 2 > 1 implies x(x + 1) > 1. (d) For all real numbers x, 5x + 1 < 9 implies x < 16.

3 1. LOGIC 3 Medium #1.9 Is the sentence This statement is false a proposition? #1.10 Determine how many rows a truth table of a compound statement would have if that involved 2,3,4 distinct simple statements, respectively. As examples, consider compound statements p q, p q r, (p q) ( (r t))? You do not have to construct the complete tables to answer this question. Do you see any pattern in your answers? What is the answer if a compound statement is constructed out of n simple statements? #1.11 Let P (x) and Q(x) be two predicates over reals (i.e., x is a real number). Show that ( x such that P (x)) ( x such that Q(x)) and x such that (P (x)) Q(x)) are not necessarily logically equivalent. Hint: It is sufficient to find examples of such P (x) and Q(x). #1.12 Let P (x, y) be the predicate x = 2y + 1, where x and y are integers. Find the truth values of the following statement and explain your answer. (a) x y : P (x, y). (b) x y : P (x, y). (c) x y : P (x, y). (d) x y : P (x, y). #1.13 Determine whether the following statements are true or false. Assume that x, y are reals. Prove your answers. (a) x y ( x2 +y 2 2 xy). (b) x y ( x y + y x < 2). (c) x > 0 y > 0 ( x y + y x < 2). #1.14 Find the truth values of the following statements. (a) For all real numbers x, x 2 7x 12. (b) For all real numbers x and y, x 2 3xy + y 2 0. (c) For all real numbers x and y, x 2 + xy + y 2 0. #1.15 Let a, b, c be real numbers. Is the following statement correct? Prove your answer. a b c (a + b + c = 0 a 3 + b 3 + c 3 = 3abc). #1.16 Suppose a, b, c are integers. Are the following statements true? Prove your answers. (a) a b c (b 2 + c 2 = a 2 ). (b) a 0 b 0 c 0 (b 2 + c 2 = a 2 ). #1.17 In the following statement all letters represent real numbers: M N x (x > N x 2 5x > M). Construct the negation of this statement. Decide whether the statement or its negation is correct, and prove the correct statement.

4 4 1. LOGIC Harder #1.18 Is the following statement true? For every integer n, there exist integers x and y such that x 2 y 2 = 2n + 1. Prove your answer. #1.19 Prove that for all real numbers x, y, z, x 2 + y 2 + z 2 xy + xz + yz. Moreover, prove that the equality sign is achieved if and only if x = y = z. #1.20 Prove that for all real numbers x, y, z, 1 x + 1 y + 1 z = 1 x + y + z implies 1 x y z 5 = 1 (x + y + z) 5. #1.21 In the following statement all letters represent real numbers: M N x (x > N x sin x > M). Construct the negation of this statement. Decide whether the statement or its negation is correct, and prove the correct statement.

5 1. LOGIC 5 Some Answers and Hints Note that most of the comments below are hints. More details are needed when you explain/proof your claims. Easier 1.1 (1d) False. 1.3 (3a) False ; (3b) False 1.4 Use truth tables. 1.5 Use truth tables. 1.6 True; can be either True of False; True 1.7 (7a) True; (7b) False. 1.8 (8a) False; (8b) True; (8c) True; (8d) True. Medium 1.9 No. If it is true, then it is false. If it is false, so it is true. This is an example of how a self-reference may create a problem ; 8; 16; 2 n (12a) True; (12b) False; (12c) False; (12d) False (13a) True; (13b) True; (13c) False (14a) False. Find a counterexample. (14b) False. Find a counterexample. (14c) True. Complete the square Rewrite the first condition as a = b c (16a) True. Remember that 0 is an integer. (16b) False. Find a counterexample The negation can be constructed this way: Harder M N x [(x > N x 2 5x > M)] M N x [ (x > N) (x 2 5x > M)] M N x (x > N x 2 5x M)] The original statement is correct, and, in our opinion, it is easier to prove Yes. If you have difficulty in finding a proof, let n be, e.g., 0, ±1, ±2, ±3, ±4, ±5. In each of these cases find x and y as close to each other as possible. Try to observe a pattern. Then generalize for an arbitrary integer n Use Problem 13a. (Make sure you know how to prove the statement you are using) The condition 1 x + 1 y + 1 z = 1 x+y+z imposes very strong restrictions on x, y, z. Try to understand what are they Remember that sin x takes value 0 for all x = πk, k Z.

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7 CHAPTER 2 Set Theory Definitions Facts D1. What do the notations N, Z, Q, R, C stand for? D2. The empty set, the power set P (A) of a set A, the Cartesian product A B of two sets A and B, the Cartesian n-th power A n of a set A. D3. Subsets, equality of sets, cardinality of a finite sets, operations on sets (union, intersection, difference, symmetric difference). D4. Universal set, complement of a set. F1. Important Laws: (a) De Morgan s Laws: (A B) c = A c B c and (A B) c = A c B c (with proofs). (b) Distributive Laws: A (B C) = (A B) (A C) and A (B C) = (A B) (A C) (with proofs). F2. If A = m, B = n, then A B = mn. F3. If A = n, then P (A) = 2 n (with proof). 7

8 8 2. SET THEORY Easy Problems #2.1 Let A = {1, 2, 3, {1}, {1, 3}}. Which of the following statements are true or false. Explain your answers. (a) 1 A; 1 A; {1} A; {1} A. (b) 2 A; 2 A; {2} A; {2} A. (c) {1, 2} A; {1, 2} A. (d) {1, 3} A; {1, 3} A. (e) {1, 2, 3} A; {1, 2, 3} A. (f) A = 5. #2.2 Let X 1 = {MAT H210}, X 2 = {M, A, T, H}, X 3 = {210}, X 4 = {MAT H}, and X 5 = {2, 1, 0}. Find X i X j for i, j {1, 2, 3, 4, 5}. #2.3 Suppose that the set X is a subset of Y. Find (a) X Y (b) X Y (c) X \ Y #2.4 Find the power set of the following sets: (a) A = {, { }}. (b) B = {x, y, z}. #2.5 Determine if the following are elements of the set A B C where A = {0, 1, 2}, B = {1, 2}, and C = {0, 1}. (a) { }. (b) {1}. (c) (0, 2, 1). (d) (2, 2). (e) {(1, 1, 1)}. #2.6 Given A = m, B = n and C = k determine the lower and upper bounds for (A B) C. #2.7 List all elements of the set {x R : 2x 2 x 7 = 0}. #2.8 Is the statement true or false? Explain. (a) m (0, 1] x (0, 1] (m x). (b) x (0, 1] y (0, 1] (x < y). (c) x (0, 1] y (0, 1] (y < x). (d) M (0, 1] x (0, 1] (x M). #2.9 Is the statement true or false? Explain. (a) x Z y Z z Z (x < y x < z < y) (b) x Q y Q z Q (x < y x < z < y) #2.10 Given intervals of real numbers A = [1, 10], B = (10, 20], C = (5, 20). Then, using set notation, A \ C can be expressed as {x R : 1 x 5}. Express the following using set notation. (a) A \ B. (b) B \ C. (c) A B. (d) C \ B.

9 2. SET THEORY 9 Medium #2.11 Prove that the empty set,, is a subset of every set. #2.12 Let A = {a R : x R (x 2 + 6x a)}. Describe A. #2.13 Let C = {c R : x R (x 2 + 6x c)}. Describe C. #2.14 Let B = {b R : x R (2x 2 bx + 3 = 0)}. Describe B. #2.15 Determine which of the following statements are true in the case of three arbitrary sets P, Q, and R. (a) If P is an element of Q and if Q is a subset of R, then P is an element of R. (b) If P is an element of Q and if Q is a subset of R, then P is also a subset of R. (c) If P is a subset of Q and Q is an element of R, then P is an element of R. (d) If P is a subset of Q and Q is an element of R, then P is a subset of R. #2.16 Does A \ B = C imply A = B C? Prove your answer. #2.17 Does A = B C imply A \ B = C? Prove your answer. #2.18 Let A. Does A B = A C imply B = C? Prove your answer. What if A =? #2.19 Prove the following assertions involving three arbitrary sets A, B, and C. (a) (A \ B) \ C = A \ (B C). (b) (A \ B) \ C = (A \ B) \ (B \ C). #2.20 Prove (A \ B) (B \ A) = (A B) \ (A B) for every two sets A and B. #2.21 Prove that P (A) P (B) is equal to P (A B) for every two two sets A and B. What can we say if the intersection operation,, is replaced by the union operation,, in the above? #2.22 If the symmetric difference of sets A and B is equal to the symmetric difference of the sets A and C, is it necessary that B = C? Explain your answer. #2.23 Let A i = {1, 2, 3,..., i} for i = 1, 2, 3,.... Find n n and A i. i=1 A i #2.24 Let O be a point of a plane α. For each positive real number r, let i=1 C(O, r) = {P α : OP = r}. What geometrical figure C(O, r) is? Describe C(O, r). r R,r>0 #2.25 Let A = {1, 2, 3,..., 30}. For each integer i 2, define Describe the set X i = {ik : k N and k 2}. A \ 30 i=2 X i.

10 10 2. SET THEORY #2.26 Prove the following assertions involving three arbitrary sets A, B, and C. (a) A (B C) = (A B) (A C). (b) (A B) C = (A C) (B C). #2.27 A barber in an army was given an order to shave those and only those men who do not shave themselves. Should he shave himself? (This famous paradox shows that not every property can be used to define a set.)

11 2. SET THEORY 11 Some Answers and Hints to Set Theory Section Note that most of the comments below are hints. More details are needed when you explain/proof your claims (a) T ; F ; T; T. (b) T; F; F; T. (c) F; T. (d) T; T. (e) F; T. (f) T 2.2. E.g., X 3 X 5 =, X 2 X 4 =, X 2 X 2 = X 2. There are 15 different cases to consider (b) X 2.5. (c) Yes The lower bound is 0, the upper is mn. Think what C can be (a) F (c) T 2.9. (b) T (c) (d) (5, 10] a 9, or, equivalently, (, 9]. Hint: Complete the square in x 2 + 6x, or find the y-coordinate of the vertex of the parabola y = x 2 + 6x C = b 2 6 or b (a) T (c) F No. Give a counterexample No. Give a counterexample Can use either the method presented in the text, or the one presented in class Hint: there are 11 numbers in the set.

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13 CHAPTER 3 Relations Definitions Facts D1. Relation from a set A to a set B. Relation on a set A. D2. Domain and range of a relation. D3. Reflexive, symmetric, antisymmetric and transitive relations. Equivalence relation. Equivalence classes of an equivalence relation. Partition of a set. D4. Functions. One-to-one (injective), onto (surjective), bijections (same as one-to-one correspondence). Composition of functions. Inverse relation. Inverse function. D5. Cardinality of a set. Countable sets. Continuum. F1. Equivalence classes of an equivalence relation on a set A partition A. F2. Let f : A B be a function. The inverse relation f 1 from B to A is function if and only if f is a bijection. (with proof) F3. Union and Cartesian product of two countable sets are countable. Q is countable set. (with proof) F4. Sets (0, 1)) and R are not countable. (with proof) F5. For every set A, set P(A) has greater cardinality. (with proof) Important Examples E1. Examples of equivalence relations and the corresponding partitions of the sets (equality of sets, equality of numbers, congruence of plane figures, parallelism of lines, two integers having the same remainder when divided by m. E2. Relation which have exactly two of the properties of being reflexive, symmetric, or transitive, but not the third. E3. Examples of functions which are (i) neither one-to-one nor onto; (ii) oneto-one but not onto; (iii) onto but not one-to-one; (iv) bijections. 13

14 14 3. RELATIONS Easy Problems #3.1 Let R be the relation on N N (where N is the set of natural numbers) by ((a, b), (c, d)) R if a c and b d. Determine whether R is reflexive, symmetric, and/or transitive. Give reasons. #3.2 Let α be a relation on N N defined by the rule: ((a, b), (c, d)) α if ad = bc. Prove α is an equivalence relation. #3.3 Which of these collections of subsets are partitions of the set of integers? Answer yes or no to each, and provide a reason. (a) the set of negative integers and the set of positive integers. (b) the set of integers divisible by 3, the set of the integers leaving a remainder 1 when divided by 3, and the set of integers leaving a remainder of 2 when divided by 3. (c) the set of integers not divisible by 3, the set of even integers, and the set of integers that leave a remainder of 3 when divided by 6. #3.4 Find the range for the following functions defined over the integers (i.e., each function is from N to N). Assume that numbers are represented in (the usual) decimal system. Explain your answers. (a) the function that assigns each integer its last digit. (b) the function that assigns each integer the number of digits in it. #3.5 Give example of two sets A and B and a relation f from A to B such that (a) f is onto, but not one-to-one; (b) f is one-to-one, but not onto; (c) domain of f = A, but f is not a function; (d) range of f = B, but f is not a function. #3.6 Prove that the function y = f(x) = 3x + 5 from Z to Z is not a bijection, but a function given by the same formula from R to R is a bijection. #3.7 What can be said about the cardinalities of two finite nonempty sets A and B (a) if there exists a function f from A to B which is one-to-one? (b) if there exists a function f from A to B which is onto? (c) if there exists a function f from A to B which is bijective? (d) if there exists a function f from A to B? Medium #3.8 Let S = {x 1, x 2, x 3, x 4 }. Let P (S) be the power set of the set S. Define the equivalence relation R on P (S) by (S 1, S 2 ) R if and only if S 1 = S 2. Prove that R is an equivalence relation on P (S). How many equivalence classes are there? Generalize to sets S = {x 1,..., x n }. How many elements does each class have for n = 4? #3.9 Let S be a set and suppose that x S. Define the function f : P (S) P (S {x}) by f(a) = A {x} for all A P (S). Is this function f one-toone? Is this function f onto? Explain your answers. (P (S) denotes the power set of S.) #3.10 Is there a bijection between the set of all positive integers N and the set of all squares of positive integers {1, 4, 9, 16,... }? Prove your answer.

15 3. RELATIONS 15 #3.11 Show by an example that it is possible to have a bijective function between a set A and its proper subset B. (Hint: A must be an infinite set.) #3.12 Let f : A B and g : B C be two one-to-one functions. Prove that their composition g f is a one-to-one function from A to C. #3.13 Let f : A B and g : B C be two functions, and let their composition g f : A C be one-to-one. Does it imply that both f and g are one-to-one? Prove your answer. #3.14 Let f : A B and g : B C be two one-to-one functions, and let their composition g f : A C be onto. Does it imply that both f and g are onto? Prove your answer. Harder #3.15 Is there a bijection between the set of points of the whole real line and an open segment (0, 1)? #3.16 Find a bijection between (0, 1) and (0, 1]. #3.17 A circular disc in a plane is a union of a circle and its interior. Prove that every infinite set of circular discs in plane such that no two discs share a common point is countable.

16 16 3. RELATIONS Some Answers and Hints to Relations Section Note that most of the comments below are hints. More details are needed when you explain/proof your claims Reflexive and transitive. Not symmetric It may remind the equality of two fractions a/b = c/d, but do not use it in your arguments (a) No; (b) Yes (c) No (a) R(f) = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9} One can use small sets A and B for all these examples (a) A B. (d) They can be arbitrary positive integers For n = 4 there are five equivalence classes. In general, i.e., for an arbitrary positive integer n, there are n + 1 equivalence classes. For n = 4 there are two equivalence classes with 1 element in each class, two equivalence classes with 4 elements in each class, and one equivalence class consisting of 6 elements Yes. There are several ways to present such bijection explicitly A part of your argument may use the idea similar to the one in The infinite hotel discussion held in class First try to explain that in any real interval (a, b), a < b, one can always find a rational number.

17 CHAPTER 4 Mathematical Induction Definitions Facts D1. Define induction or inductive reasoning in a broad sense. D2. Define deduction or deductive reasoning in a broad sense. D3. What is the base of induction, what is the induction hypothesis? F1. State the Theorem of Mathematical Induction (both versions). Important Examples E1. Give examples of mathematical statements of the form For all n n 0, P (n), which are correct for many initial values of n but false in general. E2. Give examples of mathematical statements of the form For all n n 0, P (n), where the implication P (k) P (k + 1) is correct for all k n 0, but which are generally false due to the fact that P (n 0 ) is false. 17

18 18 4. MATHEMATICAL INDUCTION Problems Prove the following statements by using Mathematical Induction. Easier #4.1 n j=1 j2 = (n 1) 2 + n 2 = n(n+1)(2n+1) 6. # n 2 +2n = 3 4 2n+3 2n 2 +6n+4. # n(n + 1)(n + 2) = n(n+1)(n+2)(n+3) 4. # x + x x n = xn+1 1 x 1, x 1. #4.5 n #4.6 n i=1 i=1 (i + 1)2i = n2 n+1. n(n+1)(4n 1) (2i 1)(2i) = 3 # ( 1) n 1 n 2 = ( 1) n 1 n(n+1) 2 # ! + 2 2! + 3 3! n n! = (n + 1)! 1. (By definition, 1! = 1, and n! = n (read n factorial ). #4.9 Investigate for which positive integers n, n! > n2 n, and prove your result. #4.10 Investigate for which positive integers n, 3 n > 10n 2, and then prove your result #4.11 Prove that for n 2, (1 + 1/3) n > 1 + n/3 #4.12 Let x 1, x 2,..., x n be n real numbers, n 2. Prove that x 1 + x x n x 1 + x x n. #4.13 Prove that for all n N, 5 (n 5 n). Medium # x x x x n 1+x 2n = 1 x 1 + 2n+1 1 x 2n+1 x = 1. sin n+1 2 x sin x 2 #4.15 sin x + sin 2x + sin 3x sin nx = is an integer. #4.16 cos x + cos 2x + cos 3x cos nx = k is an integer. #4.17 Let x 1, x 2,..., x n be n real numbers, n 2. Prove that sin nx 2, x 2πk, where k (n+1) cos nx n cos(n+1)x 1, x 2πk, 4 sin 2 x 2 sin(x 1 + x x n ) sin x 1 + sin x sin x n. #4.18 (Bernulli s Inequality) Prove that for any fixed real number x, 1 < x 0, and every integer n 2, (1 + x) n > 1 + nx. This inequality is useful to provide a simple rough bound on the values of exponential functions. For example, what does it imply about the values of or ?

19 4. MATHEMATICAL INDUCTION 19 #4.19 Prove that for n 2, 1 n n n > 1/2. This inequality leads to an easy proof of the famous fact that the sum 1/1 + 1/2 + 1/ /n can exceed any fixed number provided n being sufficiently large. #4.20 Prove that for every positive integer n, n n. #4.21 Prove that for every positive integer n, > 2 n n #4.22 Prove that for n 2, 1 2 n < n 1 1 < 2n 3n + 1 #4.23 Prove that for every fixed a and b, a, b 0, and every integer n 2, ( ) n a + b an + b n, 2 2 where the equality is attained if and only if a = b. #4.24 Let a 0 = 0, a 1 = 3 and a n = 3 2 (a n 1 + 3a n n ) for all n 2. Prove that for all n 0, a n = n3 n. #4.25 Consider a sequence {a n } n 1, where a 1 = 2 and a n+1 = 2a n + 5 for n 1. (i) Prove that a n+1 > a n for all n 1. (ii) Prove that a n < 4 for all n 1. #4.26 Consider a sequence {b n } n 1, where b 1 = 3 and b n+1 = b n + 2 for n 1. (i) Prove that b n+1 < b n for all n 1. (ii) Prove that b n > 2 for all n 1. #4.27 Prove by induction that for all n N, 133 (11 n n+1 ). #4.28 Prove that the sum of the cubes of three consecutive positive integers is divisible by 9. #4.29 Prove that for every integer n 1, there exist positive integers a and b such that finding the gcd(a, b) by using the Euclidean algorithm takes exactly n steps. #4.30 3n + 1 coins are on a table, n 1. Two people play the following game: in turn, they take either one or two coins from the table. The player who gets the last coin looses the game. Prove that the second player has a winning strategy, i.e., the second player can play the game in such a way that he/she will always be a winner. (Of course, the decisions the second player makes along the game may depend on how the first player has been playing.) #4.31 Consider the sequence of Fibonacci numbers defined as: F 1 = 1, F 2 = 1, and F n = F n 1 + F n 2 for all integers n 3. Prove that (i) (F n+1 ) 2 F n F n+2 = ( 1) n for all n 1. (ii) F 2 + F 4 + F F 2n = F 2n+1 1

20 20 4. MATHEMATICAL INDUCTION #4.32 Prove that every integer amount of n 18 dollars can be paid by using 4 or 7 dollar bills only. #4.33 Show that for every positive integer n, (2+ 3) n = A+B 3, where A and B are integers. Having done this, show that and (2 3) n = A B 3, where A, B are the same as in the first equality. #4.34 Prove that the greatest number of regions that n 0 lines can divide the plane is n(n 1) 1 + n +. 2 #4.35 Prove that the greatest number of regions that n 1 circles can divide the plane is n 2 n + 2. #4.36 Consider a map formed on a plane by a finite number of lines, where the countries are the regions formed by the lines. Prove that the countries can be colored with just two colors such that any two countries that share a common border (a segment) have different colors. #4.37 Given an equal number of 0 s and 1 s distributed around the circle, show that it is possible to start at some number and proceed around the circle to the original starting position in such a way that, at any point during the cycle, one has seen at least as many 0 s as 1 s. #4.38 Let p n denote the n th positive prime. Thus p 1 = 2, p 2 = 3, p 3 = 5, p 4 = 7,.... Prove that for n 5, p n > 2n. Harder #4.40 Consider a tournament which starts with n 1 teams. In the first round, all teams are divided into pairs if n is even, and the winner in each pair passes to the next round (no ties). If n is odd, then one random (lucky) team passes to the next round without playing. The second round proceeds similarly. At the end, only one team is left the winner. Find a simple formula for the total number of games played in the tournament. #4.41 Consider the sequence of Fibonacci numbers defined as: F 1 = 1, F 2 = 1, and F n = F n 1 + F n 2 for all integers n 3. Prove that every positive for every n 1, F n = 1 5 (α n β n ), where α = , and β = #4.42 Prove that every fraction p/q, p and q are positive integers, p < q, can be written in the form p q = , n 1 n 2 n 3 n k where n 1, n 2, n 3,..., n k are positive integers satisfying n 1 < n 2 <... < n k. This way of representing fractions played an important role in Egyptian mathematics.

21 4. MATHEMATICAL INDUCTION 21 #4.43 Prove that the greatest number of regions that n 0 planes can divide the space is n(n 1) n(n 1)(n 2) 1 + n #4.44 Given an equal arm balance capable of determining only the relative weights of two quantities and 3 n 1 coins, n 1, all of equal weight except possibly one which is lighter. Show that it is possible to determine whether there is a light coin and identify it in at most n weighings. #4.45 (Little Fermat s Theorem) Prove that for a given positive prime number p and all integers a 0, p (a p a). #4.46 Prove the Binomial Formula:! nx (a + b) n n = a n i b i, i i=0 where the binomial coefficients `n i are defined by: `n 0 = 1, and! n n (n 1)... (n i + 1) = i k for 1 i n.

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23 CHAPTER 5 Number Theory Definitions Facts D1. When do we say that an integer b divides an integer a? What is a divisor, and what is a multiple of an integer? D2. When do we say that an integer b divides an integer a with remainder r? D3. Congruence of two integers modulo integer n D4. The greatest common divisor. Linear combination. Relatively prime integers. D5. Prime integers, composite integers. F1. The Well-Ordering Principle F2. For all integers x, y, a, b, c, c 0, if c a and c b, then c (xa + yb) (with proof). F3. Division with Remainder Theorem. Proof of the uniqueness part. F4. Be able to prove the following properties of congruences. For any modulus m N, and all integers a, b, c, d, x, n, n 2, (a) a a (mod m) (reflexive property) (b) a b (mod m) b a (mod m) (symmetric property) (c) If a b (mod m) and b c (mod m), then a c (mod m) (transitive property) (d) a b (mod m) m (a b) a = mt + b for some t (e) If a b (mod m) and c d (mod m), then a + c b + d (mod m) and a c b d (mod m) (f) If a b (mod m), and c d (mod m), then ac bd (mod m). In particular, ac bc (mod m). (g) If a b (mod m), then a n b n (mod m) (h) If a r (mod m) and 0 r < m, then r is the remainder of the division of a by m. F5. Be able to prove the following. Let N = a n 1... a 1 a 0 be an n digit positive integer, where a 0 is the number of units, a 1 be the number of tens, and so on. Then (a) N a 0 + a a n 1 (mod 3) (b) N a 0 + a a n 1 (mod 9) (c) N a 0 a ( 1) n 1 a n 1 (mod 11) (d) N a 1 a 0 (mod 4), where a 1 a 0 is the number formed by two last digits of N 23

24 24 5. NUMBER THEORY (e) N a 2 a 1 a 0 (mod 8), where a 2 a 1 a 0 is the number formed by three last digits of N F6. Prove that the Euclidean algorithm gives the gcd(a, b). F7. Prove that if d = gcd(a, b), then there exist integers u and v such that d = ua + vb. F8. Prove that (a) if c ab and gcd(a, c) = 1, then c b (b) if gcd(a, c) = gcd(b, c) = 1, then gcd(ab, c) = 1 (c) if a c, b c and gcd(a, b) = 1, then ab c (d) gcd(a, b) = d gcd(a/d, b/d) = 1 (e) if gcd(c, n) = 1, then a b (mod n) ac bc (mod n), i.e., both sides of a congruence can be multiplied or divided by an integer relatively prime to n. F9. Statement and proof of the Prime Factorization Theorem. F11. Prove that for every positive integer n, there exist n consecutive composite integers. F12. Prove the following criterion for verifying whether the integer is prime: If n 2 and no prime number p n divides n, then n is prime.

25 CHAPTER 6 Combinatorics Definitions Facts D1. Power set P(A) of a set A. D2. k-permutations of a set A. Permutations of a set A. k-subsets (same as k-combinations) of a set A. D3. Binomial coefficient ( n k). F1. If A i is finite for all i = 1, 2,..., k, then A 1 A 2... A k = A 1 A 2... A k. F2. Let A = n. The number of all subsets of A is 2 n (with proof). F3. The number of all binary relations from A to B, A = m, B = n, is 2 mn (with proof). F4. Inclusion-Exclusion Principle. F5. Let A = m and B = n. (1) The number of all functions from A to B is n m (with proof). (2) The number of all one-to-one functions from A to B is n (n 1)... (n m + 1) (with proof). (3) The number of all k-permutations of A is m (m 1)... (m k + 1) = m! (m k)!, and the number of permutations of A is m! (with proof). (4) The number of all k-subsets of A (same as k-combinations of A) is m! (m k)!k! (with proof). (5) P(B) = n k=0 ( n k) = 2 n (with proof). (6) Binomial formula: (a + b) n = n k=0 ( n k) a n k b k (with proof). 25

26 26 6. COMBINATORICS Easy Problems #6.1 How many groups of 12 people which contain 4 men, 5 women and 3 children can be chosen from all people in an apartment building, if 22 men, 20 women and 28 children live there? #6.2 A four-digit number is a number which is represented in decimal system by four digits with the first digit not equal to zero. How many four-digit numbers can be made out of digits 0, 1, 2, 3, 8, 9 if (a) the digits in a number can repeat; (b) no digit can repeat; (c) the number is odd and digits can repeat; (d) the number is odd and digits can not repeat. #6.3 Twenty points are taken on the circumference of a circle. Any two of them are endpoints of a chord. How many chords are there? #6.4 How many integers in the set {1, 2,..., 1000} are (a) divisible by 5? divisible by 3? (b) divisible by 15? (c) divisible by 5 or by 3? (d) divisible by neither 3 nor 5? (e) divisible by only one of the numbers 3 or 5? #6.5 What is the coefficient at x 5 y 28 in the expansion of (2x 3y 2 ) 19? #6.6 Prove that for n N, n i=0 ( n i) 4 i = 5 n. Medium #6.7 Given 2n points on a plane. One wants to draw n segments which join pairs of these points such that no two segments share a common vertex. In how many ways this can be done? #6.8 (a) Expand 0 = (1 1) n by using the Binomial Theorem. (b) Prove that for every n 1, ( n 0 ) + ( n 2 ) + ( n 4 ) + = ( n 1 ) + ( ) n + 3 ( ) n + = 2 n 1. 5 (c) Show that there exist as many subsets of an n-element set which contain an odd number of elements as there are those which contain an even number of elements. #6.9 What is the term with the largest coefficient in the expansion of (2x + 3y) 100? #6.10 Given sets A and B, A = s 1, B = t 1. (a) How many relations from A to B are there? (b) How many relations from A to B have their domain consisting of exactly one element of A? (c) How many functions from A to B are there? (d) How many 1-to-1 functions from A to B are there? (e) How many functions from A to B are bijections?

27 6. COMBINATORICS 27 (f) How many functions from A to B have their range consisting of exactly one element of B (constant functions)? (g) For how many relations φ from A to B the following is true: a A b B (aφb)? (h) For how many relations φ from A to B the following is true: a A b B (aφb)? (i) For how many relations φ from A to B the following is true: a A b B (aφb)? #6.11 Each of 7 radio operators from city A wants to establish a connection with every one of 5 radio operators from city B. (a) How many distinct connections between these two groups can be established? (b) In how many connections between the groups every person from A is connected to (i) exactly one person from B; (ii) at least one person from B? (iii) at most one person from B? #6.12 Let A and B be two different people in a group of twelve. Suppose these twelve people form a line to the cashier s office. In how many of these lines (a) A stands immediately in front of B? (b) A and B stand next to each other? (c) There are exactly three people between A and B? (d) A stands somewhere before B? #6.13 A four-digit number is a number which is represented in decimal system by four digits with the first digit not equal to zero. How many four-digit numbers can be made out of digits 0, 1, 2, 3, 8, 9 if the number if even and digits do not repeat. #6.14 Let n = pqr, where p, q, r are distinct positive prime numbers. How many integers from {1, 2,..., n} are relatively prime with n? #6.15 How many sequences of length seven made out of letters A, C, G, U are there with the property that a sequence coincides with itself if the order of the letters is reversed? #6.16 How many distinct words can be made by rearranging letters in the word MATHEMATICS? In the word MISSISSIPPI? #6.17 How many diagonals does a polygon with n 3 vertices have? (A diagonal is the segment joining two vertices of the polygon and which is not a side of the polygon.) #6.18 How many graphs with a given set of n vertices are there? #6.19 How many graphs with a given set of n vertices and m edges are there? #6.20 Let N = p e1 1 pe pe k k, where all p i are distinct primes, and all e i are nonnegative integers. How many positive distinct divisors does N have? #6.21 In how many ways 12 identical coins can be placed in 4 distinct wallets such that no wallet is empty?

28 28 6. COMBINATORICS #6.22 How many solutions (x 1, x 2,..., x 6 ) (ordered 6-tuple) does the equation x 1 + x x 6 = 50 have if (a) all x i s are nonnegative integers? (b) all x i s are positive integers? (c) all x i s are positive integers and each is at least 4? (a) A man has 10 distinct candies and he puts them in two distinct bags such that each bag contains 5 candies. In how many ways can he do it? (b) A person has 10 distinct candies and he puts them in two identical bags such that no bag is empty. In how many ways can he do it? (c) A person has 10 identical candies and he puts them in two identical bags such that no bag is empty. In how many ways can he do it? #6.23 A woman has 6 friends. Each evening, for 5 days, she invites 3 of them so that the same group is never invited twice. How many ways are there to do this? (Assume that the order in which groups are invited matters.) #6.24 In how many ways 14 distinct books can be placed on 5 distinct shelves if the order of books on a shelf matters? HARDER #6.25 In how many 0-1 sequences of length 15 no two 1 s are next to each other? #6.26 A four-digit number is a number which is represented in decimal system by four digits with the first digit not equal to zero. What is the sum of all four-digit numbers which can be made out of digits 0, 1, 2, 3, 8, 9 such that no digit repeats? #6.27 Fifteen points are taken on the circumference of a circle. Through any two of them a chord is drawn. If no three of the chords intersect at a point inside the circle, how many points of intersections of these chords are there (including points themselves)? #6.28 Suppose that all streets in a city form a rectangular grid with 5 (k) horizontal streets and 8 (n) vertical streets. In how many ways one can walk from the South-West corner to the North-East corner if the only directions one can walk are North and East? #6.29 An international committee consists of 9 (or n) members. Committee materials are stored in a safe. How many lock should the safe have, and how many keys should be made for these locks, and how these keys have to be distributed among the committee members such that the safe can be open if and only if at least 6 (or k) members of the committee are present? #6.30 What is the greatest number of regions that n planes can divide the space?