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1 Problem 1. Prove that a b (mod c) if and only if a and b give the same remainders upon division by c. Solution: Let r a, r b be the remainders of a, b upon division by c respectively. Thus a r a (mod c) and b r b (mod c). It follows that a b (mod c) iff r a r b (mod c), i.e. iff c (r a r b ). Note that 0 r a, r b < c, so r a r b < c. Thus the only way for c (r a r b ) to hold is to have r a = r b. Problem 2. a) Find the remainder upon division of 2 85 by 341. b) Find smallest a > 0 such that 2 a 1 (mod 341). Thus Solution: a) Use succesive squarings. We have 85 = (mod 341), (mod 341), (mod 341), (mod 341), 2 24 ( 85) 2 64 (mod 341), (mod 341), = 16 (mod 341), 2 85 = = ( 85) ( 340) (mod 341) b) Note that from a) we have (mod 341). Since 4 85 = 340, we have 2 10 = ( 85) 4 = (mod 341). Since 2 8 < 341 and 2 9 = (mod 341), a = 10 is the smallest positive integer such that 2 a 1 (mod 341). Problem 3. For positive integers a, b define [a, b] = ab/ gcd(a, b). a) Prove that a/gcd(a, b) and b/ gcd(a, b) are relatively prime. b) Prove that if a c and b c then [a, b] c. c) Conlcude that [a, b] is the smallest positive integer divisible by both a and b (we call it the least common multiple of a and b). Solution: a) If d > 0 is a common divisor of a/gcd(a, b) and b/ gcd(a, b) then d gcd(a, b) divides both a and b and hence d gcd(a, b) gcd(a, b). It follows that d 1, i.e. d = 1. In other words, a/gcd(a, b) and b/ gcd(a, b) do not have any positive common divisors different from 1, i.e. they are relatively prime. 1

2 a b) Note that a c implies that gcd(a,b) c gcd(a,b). Similarly, b gcd(a,b) c gcd(a,b). Since by a) the numbers a/gcd(a, b) and b/ gcd(a, b) are relatively prime, we conclude that their product ab c also divides c/gcd(a, b). In other words. It follows that [a, b] = ab gcd(a,b) 2 gcd(a,b) gcd(a,b) c. c) Clearly [a, b] is divisible by both a and b. On the other hand, any positive integer divisible by both a and b is according to b) also divisible by [a, b], hence it can not be smaller than [a, b]. It means that [a, b] is the lest common multiple of a and b. Problem 4. Let F n = 2 2n + 1, for n = 0, 1, 2,... a) Prove that F 0 F 1 F 2... F n = F n+1 2 for every n. b) Prove that gcd(f n, F m ) = 1 for n m. c) Use b) to prove that the set of primes is infinite. Solution: a) The easiest proof seems to be by induction on n. Since F 0 = 3 = 5 2 = F 1 2, the result holds for n = 0. Suppose that it holds for some n 0, i.e. F 0 F 1 F 2... F n = F n+1 2 = 2 2n+1 1. Multiplying both sides by F n+1 = 2 2n we get F 0 F 1 F 2... F n F n+1 = (2 2n+1 1)(2 2n+1 + 1) = 2 2n+2 1 = F n+2 2. so the result holds for n + 1. By induction, it holds for every n 0. b) Suppose that m < n and d is the greatest common divisor of F m and F n. Clearly d divides F 0 F 1 F 2... F n 1 (since F m is one of the factors) and therefore it divides the difference F n F 0 F 1 F 2... F n 1, which is 2 by a). Thus d 2, i.e. d = 1 or d = 2. Hovewer d = 2 is not possible, since the numbers F k are all odd. Hence d = 1, i.e. gcd(f n, F m ) = 1. c) Each number F n has at least one prime divisor. Choose any one of them and call it p n. Since gcd(f n, F m ) = 1, we have p n p m if n m. Thus p 1, p 2,... is an infinite list of pairwise different primes. Problem 5. Let a be a number written (in base 10) as where 0 a i < 10. a = a 0 + a a a n 10 n, (i) Prove that a a 0 (mod 2). In particular, 2 a iff 2 a 0. (ii) Prove that a a 0 + 2a 1 (mod 4). In particular, 4 a iff 4 a 0 + 2a 1. (iii) Prove that a a 0 + 2a 1 + 4a 2 (mod 8). In particular, 8 a iff 8 a 0 + 2a 1 + 4a 2. (iv) Prove that a a 0 (mod 5). In particular, 5 a iff 5 a 0. (v) Prove that a a 0 +a a n (mod 9). In particular, 9 a iff 9 a 0 +a a n. 2

3 (vi) Prove that a a 0 +a a n (mod 3). In particular, 3 a iff 3 a 0 +a a n. (vii) Prove that a a 0 a 1 +a 2... (mod 11). In particular, 11 a iff 11 a 0 a 1 +a Solution: Let a = a 0 + a a a n 10 n. Since 10 k 0 (mod 2) for k > 0 we see that Thus 2 a iff 2 a 0. Similarly, 10 k 0 (mod 5) for k > 0 so a a 0 (mod 2). a a 0 (mod 5). Thus 5 a iff a 0 = 0 or a 0 = 5 (recall that the numbers a i are the digits of a, i.e. they all are in the set {0, 1, 2, 3, 4, 5, 6, 7, 8, 9}). Now 10 2 (mod 4) and 10 k 0 (mod 4) for k 2 so a a 0 + a 1 10 a 0 + 2a 1 (mod 4). It follows that 4 a iff 4 a 0 + 2a 1. Similarly, we have 10 2 (mod 8), (mod 8) and 10 k 0 (mod 8) for k 3. Thus a a 0 + a a a 0 + 2a 1 + 4a 2 (mod 8). In particular, 8 a iff 8 a 0 + 2a 1 + 4a 2. Note now that 10 1 (mod 3) and 10 1 (mod 9). Thus 10 k 1 (mod 3) and 10 k 1 (mod 9) for every k 0. It follows that and a a 1 + a a n (mod 3) a a 1 + a a n (mod 9). In particular, 3 a iff 3 a 1 + a a n and 9 a iff 9 a 1 + a a n. Since 10 1 (mod 11), we have 10 k 1 (mod 11) for k even and 10 k 1 (mod 11) for k odd. Consequently, Thus 11 a iff 11 a 0 a 1 + a 2 a a a 0 a 1 + a 2 a (mod 11). Problem 6. Compute λ, µ Z such that 89λ + 55µ = 1 and find all solutions x Z to 89x 7 (mod 55). Solution: We perform Euclidean algorithm to 89 and 55: 89 = , 55 = , 3

4 34 = , 21 = , 13 = 8 + 5, 8 = 5 + 3, 5 = 3 + 2, 3 = 2 + 1, 2 = Thus 1 = 3 2 = 3 (5 3) = = 2(8 5) 5 = = (13 8) = = = 5 (21 13) 3 13 = = (34 21) = = 13 (55 34) 8 34 = = (89 55) = Thus λ = 21, µ = 34 work. From 1 = we see that Mulyplying this congruence by 7 we get 89 ( 21) 1 (mod 55). 89 ( 21) 7 7 (mod 55). To simplyfy, note that ( 21) 7 = (mod 55), so (mod 55). Since (89, 55) = 1, all solutions are given by x = 18 + k 55, k Z. Problem 7. Solve the system of congruences: x 17 (mod 504), x 4 (mod 35), x 33 (mod 16). Solution: We are asked to solve the system x 17 (mod 504), x 4 (mod 35), x 33 (mod 16). Since the moduli are not pairwise relatively prime we can not apply Chinese remainder theorem to this system. Note that 504 = and 35 = 5 7. Now the congruence x 17 (mod 504) is equivalent to the three congruences x 17 1 (mod 8), x 17 1 (mod 9), x 17 3 (mod 7). Similarly, the congruence x 4 (mod 35) is equivalent to the system x 4 1 (mod 5), x 3 (mod 7). 4

5 Finally the congruence x 33 (mod 16) is the same as x 1 (mod 16). So our original system is equivalent to the system x 1 (mod 16), x 1 (mod 9), x 3 (mod 7), x 1 (mod 5). (note that we did not include x 1 (mod 8) since this congruence is a consequence of x 1 (mod 16) ). Now we may apply Chinese remainder theorem. We calculate that ( 59) (9 7 5) = 1, ( 4) = 1, ( 1) = 1, ( 403) (16 9 7) = 1. A solution to our system is therefore given by x = 3 (9 7 5) + ( 1) ( 4) ( 1) (16 9 7) = All solotions to this sustem are theorefore given by where k Z. x = k = k 5040, Problem 8. An old women goes to market and a horse steps on her basket and crushes her eggs. The rider offers to pay for the damages and asks her how many eggs she had brought. She does not remember the exact number, but when she had taken them out two at a time, there was one egg left at the end. The same thing happened when she picked them out three, four, five, and six at a time, but when she took them out seven at a time, no egg left at the end. What is the smallest number of eggs she could have had? Solution: The problem asks us to find smalest positive solution to the system of congruences x 1 (mod 2), x 1 (mod 3), x 1 (mod 4), x 1 (mod 5), x 1 (mod 6), x 0 (mod 7). The moduli are not pairwise relatively prime here. Note however that the congruences x 1 (mod 3), x 1 (mod 4), x 1 (mod 5), x 0 (mod 7) imply the remaining two congruences. So the latter systm is equivalent to the former. Now the moduli are pairwise relatively prime and we can apply Chinese remainder theorem ( 1) = 1, ( 26) = 1, ( 1) = 1, 5

6 A solution to our system is given by ( 17) = 1. x = ( 1) ( 1) = 119. The smallest positive solution is then = 301. Problem 9. a) Prove that any natural number n such that n 3 (mod 4) has a prime divisor p such that p 3 (mod 4) (hint: note that every odd prime number q either satisfies q 1 (mod 4) or q 3 (mod 4) ; what can you say about product of primes of the first type?) b) Prove that n! 1 3 (mod 4) for any n > 3. c) Prove that every prime divisor of n! 1 is bigger than n. d) Conclude that there are infinitely many primes q such that q 3 (mod 4). Solution: a) Note that for every odd integer m either m 1 (mod 4) or m 3 (mod 4). Also, iff a 1,..., a s are integers satisfying a i 1 (mod 4) for i = 1, 2,..., s then by multiplying these congruences we see that a 1 a 2...a s 1 (mod 4). Consider now a positive integer n 3 (mod 4). Note that n is odd, so all its prime divisors are odd. We know that n is a product of prime numbers. If each of these prime numbers were 1 (mod 4), then according to our remark above, also their product n would be 1 (mod 4), which is false. Thus n must have a prime divisor p 3 (mod 4). b) If n > 3 then n! = n is divisible by 4. Thus n! 0 (mod 4) and n! (mod 4). c) If p n then p divides n! and therefore does not divide n! 1. It follows that any divisor of n! 1 is larger than n. d) Let n > 3. By a) and b) there is a prime number q 3 (mod 4) which divides n! 1 and q > n by c). Thus there are arbitrarily large primes 3 (mod 4), so the set of such primes is infinite. Problem 10. Prove that every composite number n has a prime divisor not larger than n. Solution: Since n is composite, we may factor n as n = ab, where 1 < a b. Thus n = ab a 2 and n a. Any prime divisor of a is a prime divisor of n and it is n. Problem 11. The numbers n, n + 2, n + 4 are prime. What is n? Prove your answer. (Hint: consider these numbers modulo 3). Solution: Note that one of the congruences n 0 (mod 3), n 1 (mod 3), n 2 (mod 3) holds. If n 0 (mod 3) then 3 n. Since n is a prime, we must have n = 3 and then indeed n + 2 = 5, n + 4 = 7 are primes. If n 1 (mod 3) then n (mod 3), i.e. 3 n + 2. Since n + 2 is a prime, we must have 3 = n + 2, i.e. n = 1. Hovewer, n = 1 is not a prime, so this case is not possible. 6

7 If n 2 (mod 3) then n (mod 3), i.e. 3 n + 4. Again, since n + 4 is prime, we have n + 4 = 3, i.e. n = 1, which is not possible. Thus n = 3 is the only solution. Problem 12. Use Euler Theorem to find the remainder upon division of n by m, where a) n = , m = 13; b) n = , m = 23 c) n = , m = 20 d) n = , m = 14 Solution: a) Note that 29 3 (mod 13). Thus (mod 13). It suffices then to find remainder upon division of by 13. By Euler s theorem (or its special case Fermat s Little Theorem) we have (mod 13). Now 202 = Thus = = (3 12 ) (mod 13). Now 3 10 = 9 5 and 9 4 (mod 13). Thus 3 10 ( 4) 5 = ( 4) 16 2 ( 4) 3 2 = ( 4) 9 ( 4) 2 = 16 3 (mod 13). Thus the remainder is 3. Note: The solution can be simplified by observing that (mod 13). b) Note that 99 7 (mod 23), so (mod 23). By Euler s theorem, (mod 23). Now we want to find r such that = 22 k + r and 0 r < 22. Note that both and 22 are divisible by 11 and therefore so is r. Thus r = 0 or r = 11. Since r must be odd, we have r = 11 (alternatively, you can find r by performing division algorithm). It follows that = = ( 2) 12 = (mod 23). Thus the remainder is 22. c) Note that 29 9 = 3 2 (mod 20), so (mod 20). Now φ(20) = φ(4 5) = φ(4)φ(5) = 2 4 = 8, so (mod 20). Now 396 = Thus Thus the remainder is = 81 1 (mod 20). d) Note that φ(14) = φ(2 7) = φ(2)φ(7) = 6. Also, = 6k + 4 for some k. Thus Thus the remainder is = (mod 14). Problem 13. Prove that if n is relatively prime to 72 then n 12 1 (mod 72). 7

8 Solution: Note that 72 = 8 9. Since φ(8) = 4, we have n 4 1 (mod 8) for any n relatively prime to 8. It follows that n 12 1 (mod 8) for any n such that gcd(8, n) = 1. Similarly, φ(9) = 6 so n 6 1 (mod 9) for any n relatively prime to 9. It follows that if gcd(n, 9) = 1 then n 12 1 (mod 9). Suppose that gcd(n, 72) = 1. Then both gcd(n, 8) = 1 = gcd(n, 9). Thus n 12 1 (mod 8) and n 12 1 (mod 9). In other words, 8 n 12 1 and 9 n Since 8 and 9 are relatively prime, we have 8 9 = 72 n 12 1, i.e. n 12 1 (mod 72). Remark. Note that what we proved is stronger than what Euler s theorem implies for 72. In fact, φ(72) = φ(8)φ(9) = 4 6 = 24, so we only get n 24 1 (mod 72) from Euler s theorem for 72. Problem 14. Let p, q be distinct prime numbers. Prove that p q 1 + q p 1 1 (mod pq). Solution: Since p q are prime numbers, we have gcd(p, q) = 1. By Fermat s Little Theorem, p q 1 1 (mod q). Clearly q p 1 0 (mod q). Thus p q 1 + q p 1 1 (mod q). Exchanging the roles of p and q in the above argument, we prove that p q 1 + q p 1 1 (mod p). In other words, p q 1 + q p 1 1 is divisible by both p and q. Since p and q are relatively prime, we conclude that p q 1 + q p 1 1 is divisible by pq, i.e. p q 1 + q p 1 1 (mod pq). Problem 15. Let m, n be positive integers such that m n. Prove that φ(m) φ(n) and that φ(mn) = mφ(n) Solution: Since m n, we can number the prime divisors of n such that m = p a pas s and n = p b p bs s p b s+1 s+1...pbt t, where t s, 0 < a i b i for i = 1, 2,..., s and 0 < b i for i > s, and p 1,..., p t are pairwise distinct prime numbers. Now φ(m) = (p 1 1)p a (p s 1)p as 1 s and φ(n) = (p 1 1)p b (p s 1)ps bs 1 (p s+1 1)p bs (p t 1)p bt 1 t. It is clear now that φ(m) φ(n). Moreover, mn = p a 1+b ps as+bs p b s+1 s+1...pbt t φ(mn) = (p 1 1)p a 1+b (p s 1)ps as+bs 1 (p s+1 1)p bs (p t 1)p bt 1 t and = mφ(n). Second solution: Suppose that the result is false and let m n be a counterexample with smallest possible n. Clerly m > 1 (since the result holds trivially for m = 1). Let p be a prime divisor of m. Thus we can write m = p a m 1 and n = p b n 1 for some 0 < a b 8

9 and natural numbers n 1, m 1 not divisible by p. Since m 1 n = p b n 1 and gcd(p, m 1 ) = 1, we have m 1 n 1. Also φ(m) = φ(p a )φ(m 1 ) = (p 1)p a 1 φ(m 1 ), and φ(n) = φ(p b )φ(n 1 ) = (p 1)p b 1 φ(n 1 ) φ(mn) = φ(p a+b )φ(m 1 n 1 ) = (p 1)p a+b 1 φ(m 1 n 1 ). Since m 1 n 1 and n 1 < n, the result is true for m 1, n 1, i.e. φ(m 1 ) φ(n 1 ) and φ(m 1 n 1 ) = m 1 φ(n 1 ). But then and φ(m) = (p 1)p a 1 φ(m 1 ) (p 1)p b 1 φ(m 1 ) (p 1)p b 1 φ(n 1 ) = φ(n) φ(mn) = (p 1)p a+b 1 φ(m 1 n 1 ) = p a m 1 (p 1)p b φ(n 1 ) = mφ(n) so the result is true for m, n contrary to our assumption. The contradiction proves that no counterexample to our result exists. Problem 16. Compute φ(2592), φ(111111), φ(15!). Solution: We have 2592 = = = = Thus φ(2592) = φ(2 5 )φ(3 4 ) = = Clearly is divisible by 11,3 so = = Now 3367 is divisible by 7: 3367 = The next prime to consider is 13 and indeed 481 = Thus = and φ(111111) = φ(3)φ(7)φ(11)φ(13)φ(37) = = Finally 15! = , so φ(15!) = φ(2 11 )φ(3 6 )φ(5 3 )φ(7 2 )φ(11)φ(13) = = Problem 17. Prove that 561 is a composite number and a 561 a (mod 561) for every integer a. Solution: Let us first note the following corollary from Fermat s Little Theorem: Proposition 1. Let p be a prime number. For any integer n and any natural number k we have n k(p 1)+1 n (mod p). Indeed, if p a then both sides of the congruence are 0 (mod p) and if gcd(p, a) = 1 then n p 1 1 (mod p) and n k(p 1)+1 = n(n p 1 ) k n (mod p). We have 561 = = , so 561 is not a prime. Now 561 = = = By the proposition, n 561 n (mod 3), n 561 n (mod 11) and n 561 n (mod 17) for every integer n. Thus n 561 n is divisible by 3, 11, 17 and since these numbers are pairwise relatively prime, n 561 n, i.e. n 561 n (mod 561) for every integer n. 9

10 Problem 18. Let p be a prime number and let n be a positive integer such that p 4 n 3. Prove that p 2 n. Solution: Suppose that p k n (i.e. p k is the highest power of p which divides n). Then p 3k n 3 and therefore 3k 4. Since k is an integer, we have k 2 and therefore p 2 n. Second solution: Clearly p 4 n 3 implies that p n 3. Since p is prime we have p n, i.e. n = pm for some integer m. Now p 4 n 3 = p 3 m 3 implies that p m 3. Again, since p is a prime number, we get p m. It follows that m = pk for some integer k, so n = pm = p 2 k, i.e. p 2 n. 10

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