# 3. Applications of Number Theory

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1 3. APPLICATIONS OF NUMBER THEORY Applications of Number Theory 3.1. Representation of Integers. Theorem Given an integer b > 1, every positive integer n can be expresses uniquely as n = a k b k + a k 1 b k a 1 b + a 0, where k 0, 0 a 0, a 1, a 2,..., a k < b, and are all integers. Definition Base b expansion of n is (a k a k 1 a 1 a 0 ) b if the a i are as described in Theorem Example Here are examples of common expansions other than the more familiar decimal expansion. Binary expansion is the base 2 expansion. Octal expansion is the base 8 expansion. Hexadecimal expansion is base 16 expansion. The symbols A through F are used to represent 10 through 15 in the expansion. Theorem asserts that each positive integer n can be expressed uniquely as a linear combination of powers of a fixed integer b > 1. The coefficients in the linear combination must be less than b and must be greater than or equal to zero. These coefficients are, by definition, the digits of the base b expansion of n, n = (a k a k 1... a 1 a 0 ) b Constructing Base b Expansion of n. Use the division algorithm to get the base b expansion of n: 1. n = bq 1 + a 0, 0 a 0 < b and q 1 < n. 2. q 1 = bq 2 + a 1, 0 a 1 < b and q 2 < q q 2 = bq 3 + a 2, 0 a 2 < b and q 3 < q etc. until q i = 0. Then n = (a k a k 1... a 1 a 0 ) b. Example Find the binary expansion of 42. Solution: We can use the division algorithm to get the a i s.

2 3. APPLICATIONS OF NUMBER THEORY = 2(21) = 2(10) = 2(5) = 2(2) = 2(1) = 2(0) + 1 This gives us 42 = (1)(2 5 ) + (0)(2 4 ) + (1)(2 3 ) + (0)(2 2 ) + (1)(2 1 ) + 0. Thus the binary expansion of 42 is (101010) 2. Example Find the hexadecimal expansion of 42. Solution: This time we use 16 for b. 42 = 16(2) = 16(0) + 2 So the hexadecimal expansion of 42 is (2A) 16 (recall we use A = 10 in hexadecimal notation). Example Find the decimal notation of the octal representation (1024) 8. (1024) 8 = 1(8 3 ) + 0(8 2 ) + 2(8 1 ) + 4 = Cancellation in Congruences. Theorem Suppose GCD(c, m) = 1 and ac bc(mod m). b(mod m). Then a Proof. Suppose GCD(c, m) = 1 and ac bc(mod m). (We may assume m > 1 so that c 0.) Then for some integer k. This implies ac bc = c(a b) = km c km. Since GCD(c, m) = 1, Lemma 2 from Module 5.2 asserts that if c km, then Write k = cd for some integer d, and substitute for k in the equation above: c k. c(a b) = km = (cd)m = c(dm).

3 3. APPLICATIONS OF NUMBER THEORY 165 Since the cancellation law holds for integers and c 0, we can cancel c to get Thus, a b(mod m). a b = dm. Theorem provides a criterion for being able to cancel when you have a congruence. Notice that in order to perform the cancellation, the modulus m and the factor to be cancelled must be relatively prime. Here is an example to illustrate why. Example (mod 12), but 3 1(mod 12). The reason cancellation fails is that 6 and 12 are not relatively prime. Example (mod 5). Here 6 and 5 are relatively prime and we can easily check that 3 8(mod 5). if 3.4. Inverses mod m. Definition An integer a is a (multiplicative) inverse to a modulo m aa 1(mod m). Example The inverse of 14 modulo 9 is 2, since (mod 9). There is no inverse to 6 modulo 9, however. In general, an inverse refers to something that undoes another thing leaving something that is an identity. With regular multiplication of real numbers, the inverse of x is 1 since x( 1) = x x 1. Inverses do not necessarily exist if we look only at integers. With regular addition of real numbers, the inverse of x is x since x+( x) = 0, With matrices and matrix multiplication, the inverse of a matrix, A, is a matrix A 1, such that AA 1 = A 1 A = I, where I is the identity matrix. Not all matrices have inverses. With functions and composition, the inverse of a function, f, is a function, f 1, such that (f f 1 )(x) = (f 1 f)(x) = x = identity(x). Not all functions have inverses. Not all integers, even nonzero integers, have inverses modulo m. Moreover, if an inverse does exist it is not unique. This last part is different from all the other ones mentioned before! We shall see below, however, that if an integer a has an inverse modulo m, then it has a unique inverse lying between 0 and m.

4 3.5. Linear Congruence. 3. APPLICATIONS OF NUMBER THEORY 166 Definition A linear congruence is a congruence of the form ax b(mod m), where a, b, and m are fixed integers and m > 0. One may solve for x by finding an inverse of a modulo m, if an inverse exists. Example Solve the linear congruence 2x 7(mod 15) for x. Solution: An inverse of 2 modulo 15 is 8. Thus (8 2)x 8(7)(mod 15) x 56(mod 15) x 11(mod 15) Solving a linear congruence, ax b ( mod m), is very similar to solving an ordinary linear equation ax = b. We can solve for x in the linear equation by multiplying through by the multiplicative inverse 1/a of a, provided a 0. In a similar manner, we can solve a linear congruence, ax b(mod m), provided a has a multiplicative inverse a modulo m. Then x a ax a b(mod m). To get a canonical choice for x, we would reduce a b modulo m. Caution. DO NOT express the solution to a linear congruence ax b(mod m) as x = b, as you would the solution to the linear equation ax = b. We have previously a cautioned against using fractional notation when doing integer arithmetic, but, in the world of integers modulo m, they are expressly forbidden Criterion for Invertibility mod m. Theorem Suppose a and m are integers and m > 1. Then a has an inverse modulo m if and only if GCD(a, m) = 1. Moreover, if GCD(a, m) = 1, then a has a unique inverse, a, with 0 a < m. Proof. GCD(a, m) = 1 if and only if there are integers s and t such that 1 = as + mt. This is true if and only if there is an integer s such that 1 as(mod m). By definition, this is true if and only if a has an inverse, namely s, modulo m.

5 3. APPLICATIONS OF NUMBER THEORY 167 Theorem provides us with the conditions required for inverses modulo m to exist: For a to have an inverse modulo m, a and m must be relatively prime. The proof of the moreover part is complete once you solve the following exercise. Exercise Prove that if ab 1(mod m) and b c(mod m), then ac 1(mod m) Example We can use the Euclidean Algorithm and the division algorithm to find the unique inverse of a modulo m. Example Find the inverse of 8 modulo Apply the Euclidean Algorithm. 35 = 4(8) = 2(3) = 1(2) = 2(1) Find the linear combination of 8 and 35 that equals 1, the GCD. 1 = 3 1(2) = [35 4(8)] 1[8 2(3)] = [35 4(8)] 1[8 2[35 4(8)]] = 3(35) 13(8) 3. This gives 13(8) 1(mod 35), so an inverse of 8 modulo 35 is To find the inverse between 0 and 35 use the division algorithm 13 = 1(35) The unique inverse of 8 modulo 35 between 0 and 35 is Check: 8 22 = 176 = (mod 35) 3.8. Fermat s Little Theorem. Theorem If p is a prime that does not divide the integer a, then a p 1 1(mod p). and a p a(mod p).

6 3. APPLICATIONS OF NUMBER THEORY 168 Example Find mod 11. Solution: Since 158 = 15(10) + 8, we have = (5 15 ) 10 (5 8 ) 5 8 (mod 11), by Fermat s little theorem, applied to a = 5 15 and p = 11. Now, 5 8 = (5 2 ) 4 = (mod 11) = 81 4(mod 11).. Thus mod 11 = 4. The problem of determining whether a given integer is a prime may be very difficult. This fact is both interesting mathematically and useful in coding theory. Fermat s little theorem provides some help in working with prime numbers and provides the basis for many probabilistic primality tests. We will not give a proof of Fermat s theorem, since it involves concepts from the theory of groups that would take us too far afield. An elementary proof can be found in Introduction to Modern Algebra, Fourth Edition, by McCoy and Janusz (Allyn and Bacon, 1987). The converse of Fermat s little theorem is not true. In particular, there are composite numbers n such that 2 n 1 1(mod n). These are called pseudoprimes. They are very rare, but 341 is a pseudoprime. Fermat s little theorem can be used to reduce the problem of finding the remainder of a large power modulo a prime. In Example 3.8.1, we use the fact that 5 15 and 11 are relatively prime and Fermat s little theorem to get (5 15 ) 10 1(mod 11), thereby reducing to a smaller power of 5 modulo 11. One clearly has to be comfortable with the laws of exponents to carry out an exercise such as this.

7 3. APPLICATIONS OF NUMBER THEORY RSA System. The RSA system is a public key cryptosystem based on modular exponentiation modulo the product of two large primes. This system, named after the researchers who introduced it: Rivest, Shamir, and Adleman, is a public key cryptosystem. RSA Code (1) Find p and q, large primes. (2) Choose e so that e < pq and GCD(e, (p 1)(q 1)) = 1. e must be odd, but not necessarily prime. (3) Find d such that de 1(mod (p 1)(q 1)). (4) Encryption function f(t) = t e (mod pq). (5) Decryption function f 1 (c) = c d (mod pq). The public keys are (p, q, e) and the private key is d. Example Here is the routine, using p = 61, q = 53, e = 17, and d = The first prime number (destroy after computing e and d): p = The second prime number (destroy after computing e and d): q = Modulus (give this to others): pq = Public exponent (give this to others): e = Private exponent (keep this secret): d = Your public key is (pq, e) = (3233, 17). 7. Your private key is d = The encryption function is f(t) = (t 17 )(mod 3233). 9. The decryption function is : f 1 (c) = (c 2753 )(mod 3233). To encrypt the plaintext value 123, do this: f(123) = ( )(mod 3233) = (mod 3233) = 855 To decrypt the ciphertext value 855, do this: f 1 (855) = ( )(mod 3233) = (an incredibly huge number goes here) (mod 3233) = 123 The large exponential expressions can be reduced modulo 3233 in a piecemeal fashion, however, so that you don t actually have to calculate these large numbers.

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