# GREATEST COMMON DIVISOR

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1 DEFINITION: GREATEST COMMON DIVISOR The greatest common divisor (gcd) of a and b, denoted by (a, b), is the largest common divisor of integers a and b. THEOREM: If a and b are nonzero integers, then their gcd is a linear combination of a and b, that is there exist integer numbers s and t such that sa + tb = (a, b). Proof: Let d be the least positive integer that is a linear combination of a and b. We write where s and t are integers. We first show that d a. By the Division Algorithm we have From this and (1) it follows that d = sa + tb, (1) a = dq + r, where 0 r < d. r = a dq = a q(sa + tb) = a qsa qtb = (1 qs)a + ( qt)b. This shows that r is a linear combination of a and b. Since 0 r < d, and d is the least positive linear combination of a and b, we conclude that r = 0, and hence d a. In a similar manner, we can show that d b. We have shown that d is a common divisor of a and b. We now show that d is the greatest common divisor of a and b. Assume to the contrary that (a, b) = d and d > d. Since d a, d b, and d = sa + tb, it follows that d d, therefore d d. We obtain a contradiction. So, d is the greatest common divisor of a and b and this concludes the proof. EUCLID S LEMMA THEOREM (Euclid s Lemma): If p is a prime and p ab, then p a or p b. More generally, if a prime p divides a product a 1 a 2... a n, then it must divide at least one of the factors a i. Proof: Assume that p a. We must show that p b. By the theorem above, there are integers s and t with sp + ta = (p, a). Since p is prime and p a, we have (p, a) = 1, and so Multiplying both sides by b, we get sp + ta = 1. spb + tab = b. (2) Since p ab and p spb, it follows that p (spb + tab). This and (2) give p b. This completes the proof of the first part of the theorem. The second part (generalization) easily follows by induction on n 2. 1

2 COROLLARY: If p is a prime and p a 2, then p a. Proof: Put a = b in Euclid s Lemma. THEOREM: Let p be a prime. Then p is irrational. Proof: Assume to the contrary that p is rational, that is p = a, where a and b are integers b and b 0. Moreover, let a and b have no common divisor > 1. Then p = a2 b 2 pb 2 = a 2. (3) Since pb 2 is divisible by p, it follows that a 2 is divisible by p. Then a is also divisible by p by the Corollary above. This means that there exists q Z such that a = pq. Substituting this into (3), we get pb 2 = (pq) 2 b 2 = pq 2. Since pq 2 is divisible by p, it follows that b 2 is divisible by p. Then b is also divisible by p by the Corollary above. This is a contradiction. FUNDAMENTAL THEOREM OF ARITHMETIC THEOREM (Fundamental Theorem of Arithmetic): Assume that an integer a 2 has factorizations a = p 1... p m and a = q 1... q n, where the p s and q s are primes. Then n = m and the q s may be reindexed so that q i = p i for all i. Proof: We prove by induction on l, the larger of m and n, i. e. l = max(m, n). Step 1. If l = 1, then the given equation in a = p 1 = q 1, and the result is obvious. Step 2. Suppose the theorem holds for some l = k 1. Step 3. We prove it for l = k + 1. Let where a = p 1... p m = q 1... q n, (4) max(m, n) = k + 1. (5) From (4) it follows that p m q 1... q n, therefore by Euclid s Lemma there is some q i such that p m q i. But q i, being a prime, has no positive divisors other than 1, therefore p m = q i. Reindexing, we may assume that q n = p m. Canceling, we have p 1... p m 1 = q 1... q n 1. Moreover, max(m 1, n 1) = k by (5). Therefore by step 2 q s may be reindexed so that q i = p i for all i; plus, m 1 = n 1, hence m = n. COROLLARY: If a 2 is an integer, then there are unique distinct primes p i and unique integers e i > 0 such that a = p e p en n. Proof: Just collect like terms in a prime factorization. EXAMPLE: 120 = PROBLEM: Prove that log 3 5 is irrational. 2

3 EUCLIDEAN ALGORITHM THEOREM (Euclidean Algorithm): Let a and b be positive integers. Then there is an algorithm that finds (a, b). LEMMA: If a, b, q, r are integers and a = bq + r, then (a, b) = (b, r). Proof: We have (a, b) = (bq + r, b) = (b, r). Proof of the Theorem: The idea is to keep repeating the division algorithm. We have: therefore a = bq 1 + r 1, (a, b) = (b, r 1 ) b = r 1 q 2 + r 2, (b, r 1 ) = (r 1, r 2 ) r 1 = r 2 q 3 + r 3, (r 1, r 2 ) = (r 2, r 3 ) r 2 = r 3 q 4 + r 4, (r 2, r 3 ) = (r 3, r 4 )... r n 2 = r n 1 q n + r n, (r n 2, r n 1 ) = (r n 1, r n ) r n 1 = r n q n+1, (r n 1, r n ) = r n, (a, b) = (b, r 1 ) = (r 1, r 2 ) = (r 2, r 3 ) = (r 3, r 4 ) =... = (r n 2, r n 1 ) = (r n 1, r n ) = r n. EXAMPLE: Find (252, 198). By the Euclidean Algorithm we have 252 = = = = 18 2 therefore (252, 198) = 18. PROBLEM: Find (35, 55) and (326, 78). THEOREM: Let a = p e p en n and b = p f p fn n (a, b) = p min(e 1,f 1 ) 1... p min(en,fn) n. be positive integers. Then EXAMPLE: Since 720 = and 2100 = , we have: (720, 2100) = = 60. 3

4 Let d be the least positive integer that is a linear combination of a and b. We write d = sa + tb, ( ) where s and t are integers. We first show that d a. By the Division Algorithm we have a = dq + r, where 0 r < d. From this and ( ) it follows that r = a dq = a q(sa + tb) = a qsa qtb = (1 qs)a + ( qt)b. This shows that r is a linear combination of a and b. Since 0 r < d, and d is the least positive linear combination of a and b, we conclude that r = 0, and hence d a. In a similar manner, we can show that d b.

5 We have shown that d is a common divisor of a and b. We now show that d is the greatest common divisor of a and b. Assume to the contrary that (a, b) = d and d > d. Since d a, d b, and d = sa + tb, it follows that d d, therefore d d. We obtain a contradiction. So, d is the greatest common divisor of a and b and this concludes the proof.

6 THEOREM (Euclid s Lemma): If p is a prime and p ab, then p a or p b. More generally, if a prime p divides a product a 1 a 2... a n, then it must divide at least one of the factors a i. Proof: Assume that p a. We must show that p b. By the theorem above, there are integers s and t with sp + ta = (p, a). Since p is prime and p a, we have (p, a) = 1, and so sp + ta = 1. Multiplying both sides by b, we get spb + tab = b. ( ) Since p ab and p spb, it follows that p (spb + tab). This and ( ) give p b. This completes the proof of the first part of the theorem. The second part (generalization) easily follows by induction on n 2.

7 THEOREM: Let p be a prime. p is irrational. Then Proof: Assume to the contrary that p is rational, that is p = a, where a and b b are integers and b 0. Moreover, let a and b have no common divisor > 1. Then p = a2 b 2 pb 2 = a 2. ( ) Since pb 2 is divisible by p, it follows that a 2 is divisible by p. Then a is also divisible by p by the Corollary. This means that there exists q Z such that a = pq. Substituting this into ( ), we get pb 2 = (pq) 2 b 2 = pq 2. Since pq 2 is divisible by p, it follows that b 2 is divisible by p. Then b is also divisible by p by the Corollary. This is a contradiction.

8 Step 1. If l = 1, then the given equation in a = p 1 = q 1, and the result is obvious. Step 2. Suppose the theorem holds for some l = k 1. Step 3. We prove it for l = k + 1. Let where a = p 1... p m = q 1... q n, max(m, n) = k + 1. ( ) ( ) From ( ) it follows that p m q 1... q n, therefore by Euclid s Lemma there is some q i such that p m q i. But q i, being a prime, has no positive divisors other than 1, therefore p m = q i. Reindexing, we may assume that q n = p m. Canceling, we have p 1... p m 1 = q 1... q n 1. Moreover, max(m 1, n 1) = k by ( ). Therefore by step 2 q s may be reindexed so that q i = p i for all i; plus, m 1 = n 1, hence m = n.

9 LEMMA: If a, b, q, r are integers and a = bq + r, then (a, b) = (b, r). Proof: We have (a, b) = (bq + r, b) = (b, r). Proof (Euclidean Algorithm): We have: a = bq 1 + r 1, (a, b) = (b, r 1 ) b = r 1 q 2 + r 2, (b, r 1 ) = (r 1, r 2 ) r 1 = r 2 q 3 + r 3, (r 1, r 2 ) = (r 2, r 3 ) r 2 = r 3 q 4 + r 4, (r 2, r 3 ) = (r 3, r 4 )... r n 2 = r n 1 q n + r n, (r n 2, r n 1 ) = (r n 1, r n ) r n 1 = r n q n+1, (r n 1, r n ) = r n, therefore (a, b) = (b, r 1 ) = (r 1, r 2 ) = (r 2, r 3 ) = (r 3, r 4 ) =... = (r n 2, r n 1 ) = (r n 1, r n ) = r n.

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