A comprehensive course in number theory by Alan Baker

By Alan Baker

Constructed from the author's renowned textual content, A Concise creation to the speculation of Numbers, this booklet offers a finished initiation to the entire significant branches of quantity idea. starting with the rudiments of the topic, the writer proceeds to extra complex themes, together with parts of cryptography and primality checking out, an account of quantity fields within the classical vein together with houses in their devices, beliefs and perfect sessions, elements of analytic quantity thought together with reports of the Riemann zeta-function, the prime-number theorem and primes in arithmetical progressions, an outline of the Hardy-Littlewood and sieve equipment from respectively additive and multiplicative quantity concept and an exposition of the mathematics of elliptic curves. The e-book contains many labored examples, routines and extra examining. Its wider insurance and flexibility make this publication compatible for classes extending from the ordinary to starting graduate experiences.

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D d )μ(n/d) . 18 Arithmetical functions (vi) Show that n≤x μ(n)[x/n] = 1. Hence prove that | n≤x μ(n)/n | ≤ 1. (vii) Let m, n be positive integers and let d run through all divisors of (m, n). Prove that dμ(n/d) = μ(n/(m, n))φ(n)/φ(n/(m, n)). ) (viii) Prove that if n has k distinct prime factors then d|n |μ(d)| = 2k . (ix) Prove that (μ(d))2 /φ(d) = n/φ(n), d|n μ(d)φ(d) = 0. d|2n (x) Find all positive integers n such that (a) φ(n)|n, (b) φ(n) = 12 n, (c) φ(n) = φ(2n), (d) φ(n) = 12. n n 2 (xi) Prove that ∞ n = 1 φ(n)x /(1 − x ) = x/(1 − x) .

It is readily verified that 3 is a primitive root (mod 7) and we have 32 ≡ 2 (mod 7). Thus 5 ind x ≡ 2 (mod 6), which gives ind x = 4 and x ≡ 34 ≡ 4 (mod 7). Note that although there is no primitive root (mod 2 j ) for j > 2, the number 5 belongs to 2 j−2 (mod 2 j ) and every odd integer a is congruent (mod 2 j ) to just one integer of the form (−1)l 5m , where l = 0, 1 and m = 0, 1, . . , 2 j−2 − 1. The pair l, m has similar properties to the index defined above. 8 Further reading A good account of the elementary theory of congruences is given by T.

6 Primitive roots Let a, n be natural numbers with (a, n) = 1. The least natural number d such that a d ≡ 1 (mod n) is called the order of a (mod n), and a is said to belong to 24 Congruences d (mod n). By Euler’s theorem, the order d exists and it divides φ(n). In fact d divides every integer k such that a k ≡ 1 (mod n), for, by the division algorithm, k = dq + r with 0 ≤ r < d, whence a r ≡ 1 (mod n) and so r = 0. By a primitive root (mod n) we mean a number that belongs to φ(n) (mod n). Thus, for a prime p, a primitive root (mod p) is an integer g, not divisible by p, such that p − 1 is the smallest exponent with g p−1 ≡ 1 (mod p).

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