# MATH 2112/CSCI 2112, Discrete Structures I Winter 2007 Toby Kenney Homework Sheet 5 Hints & Model Solutions

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1 MATH 11/CSCI 11, Discrete Structures I Winter 007 Toby Kenney Homework Sheet 5 Hints & Model Solutions Sheet 4 5 Define the repeat of a positive integer as the number obtained by writing it twice in a row (in decimal). For example, the repeat of 364 is Find a positive integer n such that the repeat of n is eual to m for some integer m. [Hint: the repeat of n is always a multiple of n. You may find the following divisibility test useful: a number is divisible by 11 if and only if the sum of its odd digits minus the sum of its even digits is divisible by 11. For example, 154 is divisible by 11 since the sum of its odd digits is 1+5=6 and the sum of its even digits is +4=6, so their difference is 0, which is divisible by 11. You won t find the answer by trial and error.] The repeat of a number n with k digits is (10 k + 1)n. If this is a suare, then every prime factor of 10 k + 1 must divide it, and must therefore also divide its suare root. Thus, the suare of every prime factor of 10 k + 1 must divide (10 k + 1)n, so either its suare will divide 10 k + 1, or it must divide n. Since n < 10 k + 1, if 10 k + 1 is not divisible by the suare of any prime number, then (10 k + 1)n cannot be a suare. Therefore, we have to look for a positive integer k such that 10 k + 1 is divisible by the suare of some prime. To save time in our search, we can note that if k is even, then 10 k + 1 = Therefore, if one of the numbers of the form is divisible by 11, then 11 will divide the corresponding 10 k + 1. There is a useful test for divisibility by 11: add up the odd digits and subtract the even ones; if the answer is divisible by 11, then the original number was. Using this test, we see that if there are 5 9 s in the number, then it will be divisible by 11. Indeed, = , so the number is a perfect suare. However, this is not the repeat of a number, because it has a leading has too few digits. To remedy this problem, we multiply by a suare number to increase the number of digits by. The suare numbers which do this are 16, 5, 36, 49, 64, 81 and 100. These give us = (1) = () = (3) = (4) 1

2 = (5) = (6) = (7) These are the only solutions less than Consider the integers whose last digit (in decimal) is 1. The product of any two such integers is another such integer. Any such integer can therefore be factored as a product of integers of this type that cannot be written as non-trivial products of other integers of this type. For example, 7, 11 = , and 11, 1, and 31 cannot be expressed as products of integers whose last digit is 1. Can every integer of this type be written in a uniue way as such a product? Give a proof or a counterexample. This factorisation is not uniue consider 4641 = 1 1 = The easy way to find this counterexample is to note that by uniue prime factorisation for positive integers, the numbers must be composed from the same primes multiplied together in two different ways. For example, we could form the product of the 4 primes p 1, p, p 3, and p 4 as either (p 1 p )(p 3 p 4 ) or (p 1 p 4 )(p 3 p ). We want all the bracketed products to be congruent to 1 modulo 10. Therefore, p p (p 1 p 4 ) = (p p 1 )p 4 p 4 (mod 10), and similarly p 1 p 3 (mod 10). This will work if p 1 p 3 3 (mod 10) and p p 4 7 (mod 10); the smallest collection of primes of this form is 3,7,13,17. Alternatively, we could have p 1 p p 3 p 4 9 (mod 10), for example = = = Sheet 5 1 Use Euclid s algorithm to find the greatest common divisor of the following pairs of numbers. Write down all the steps involved. (a) 13,456 and 654, = = = = =

3 7 = = = 4 3 So the greatest common divisor of 13,456 and 654,31 is 3. (b) 1,111,111 and 1,11, = = = = = = + 1 = 1 So the greatest common divisor of 1,111,111 and 1,11,1 is 1. Find integers a and b such that 13579a + 468b = = = = = = So working backwards: 1 = 10 9 = 10 ( ) = = 13 (139 19) 19 = = ( ) = = 47 ( ) = So a = 47, b = 1359 works. 3

4 3 (a) Show that any number congruent to 3 modulo 4 is divisible by a prime number congruent to 3 modulo 4.[You may assume that the product of any collection of integers that are all congruent to 1 modulo 4 is also congruent to 1 modulo 4.] Proof. Suppose for contradiction that the result is not true. Let n be congruent to 3 modulo 4, but not divisible by any prime number congruent to 3 modulo 4. n is odd, so it is not divisible by. Therefore, all its prime factors are odd, so they must be congruent to 1 modulo 4. Their product is therefore also congruent to 1 modulo 4. This contradicts the fact that n was congruent to 3 modulo 4. Therefore, by contradiction, n must have a prime factor congruent to 3 modulo 4. (b) Prove that there are infinitely many prime numbers congruent to 3 modulo 4. Hint: This is similar to the proof that there are infinitely many primes. We start by supposing that there are only finitely many prime numbers congruent to 3 modulo 4, and let them be p 1, p,..., p k. Now we take their product: N = p 1 p p k. Now divide into two cases: N 1 (mod 4) and N 3 (mod 4). In either case, we can add something to N to get a number congruent to 3 modulo 4 that isn t divisible by any of the primes that are congruent to 3 modulo 4. This will contradict (a). 4 Are the following numbers rational or irrational? Give proofs: (a) 6 This is irrational. Proof. Suppose 6 is rational. Let p, Z such that p = 6, with 0. Now let p = p (p,) and = (p,), so that (p, ) = 1 and p = 6. Thus p = 6, so p is even. Therefore, p is even. Thus p = k for some integer k. Therefore, 4k = (k) = 6, so k = 3. Therefore, is even, so is even, and thus p and have common factor. This contradicts (p, ) = 1, so our initial assumption that 6 is rational must be false. Therefore, 6 is irrational. (b) + 3 [Hint: What is ( + 3 )?] This is irrational: 4

5 Proof. Suppose that + 3 is rational. Then ( + 3) = = must also be rational. Now = 6 is the difference of two rational numbers, and therefore rational, but it is not by (a), so by contradiction, + 3 must be irrational. 5 Show that the difference between a rational number and an irrational number is irrational. Proof. Let a be a rational number and let b be an irrational numbers. Let c = a b. We want to show that c is irrational. Suppose that c is rational. Then a c is the difference between two rational numbers, and therefore, rational. However, a c = b which is irrational. This is a contradiction, so our assumption that c is rational must be impossible. Therefore, c must be irrational. ( ) 6 Observe that = ( ) = =. Prove that there are two irrational numbers α and β such that α β is rational. Hint: This is a division into cases proof: Either is rational or it is irrational. From either of these cases, we can find α and β. This is an example of a non-constructive proof of an existential result. 7 (Bonus Question) Prove that if a positive integer n is not a suare, then n is irrational. Proof. Let the uniue prime factorisation of n be n = p e1 1 pe pe k k. Now if e 1, e,..., e k are all even, then n would be a suare. (If e i = f i for i = 1,..., k, then n is the suare of p f1 1 pf pf k k.) Thus, at least one of e 1, e,..., e k must be odd. Let e i be odd. Now suppose n is rational. Let n = p for p, Z with 0. Let p = p (p,) and = (p,). Then p = n, so p = n, so p i p. Therefore by uniue prime factorisation, p i p (see Sheet 4 Question 3). Therefore, since (p, ) = 1, p i does not divide. Now p ei i p, so we must have p ei+1 i p, since if the highest power of p i dividing p were less than p e i +1 i then it would be at most p ei 1 i, so the highest power of p i dividing p would be at most p ei 1 i, and we know that p ei i p. Therefore, p ei+1 i p, and so p ei+1 i p. Let p = p ei+1 i k, then we have p i k =, so p i, and thus, p i This contradicts the fact that (p, ) = 1, so our assumption that n was rational must be false. Therefore, if n is not a suare, then n is irrational. 8 Find 0 n < 310 satisfying: 5

6 n 7 (mod 11) (8) n 10 (mod 14) (9) n 11 (mod 15) (10) The easy way to do this is to notice that n = 4 satisfies these congruences, so the uniue solution with 0 n < 310 (11,14, and 15 are pairwise coprime) must be congruent to -4 modulo 310, so it must be = 306. A more generally applicable way is the way used in the proof of the Chinese Remainder Theorem: Observe that 14 3 (mod 11) and (mod 11), so 10 3 (4 14) (mod 11), so n 4 (mod 11 14) is the uniue solution to the first two congruences. Now (mod 154), so we need to solve the congruences n 150 (mod 154) (11) n 11 (mod 15) (1) Now (mod 15), and (mod 15), so (4 154) 11 (mod 15), Also, 11 4 = (mod 15), so n = 306 (mod 310) is the uniue solution. 6

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