# A comprehensive course in number theory by Alan Baker

By Alan Baker

Constructed from the author's renowned textual content, A Concise advent to the speculation of Numbers, this ebook presents a accomplished initiation to the entire significant branches of quantity idea. starting with the rudiments of the topic, the writer proceeds to extra complex issues, together with components of cryptography and primality checking out, an account of quantity fields within the classical vein together with houses in their devices, beliefs and excellent periods, features of analytic quantity thought together with reviews of the Riemann zeta-function, the prime-number theorem and primes in arithmetical progressions, an outline of the Hardy-Littlewood and sieve tools from respectively additive and multiplicative quantity concept and an exposition of the mathematics of elliptic curves. The e-book comprises many labored examples, routines and extra studying. Its wider assurance and flexibility make this e-book appropriate for classes extending from the basic to starting graduate stories.

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Extra resources for A comprehensive course in number theory

Example text

Our proof will be based on the identity (x 2 + y 2 + z 2 + w 2 )(x 2 + y 2 + z 2 + w 2 ) = (x x + yy + zz + ww )2 + (x y − yx + wz − zw )2 + (x z − zx + yw − wy )2 + (xw − wx + zy − yz )2 , which is related to the theory of quaternions. In view of the identity and the trivial representation 2 = 12 + 12 + 02 + 02 , it will suﬃce to prove the theorem for odd primes p. Now the numbers x 2 with 0 ≤ x ≤ 12 ( p − 1) are mutually incongruent (mod p), and the same holds for the numbers −1 − y 2 with 0 ≤ y ≤ 12 ( p − 1).

For another illustration of the use of the Jacobi symbol consider the equations 275 2 21 = = = −1. 275 21 21 Now if an = −1 then ap = − 1 for some prime factor p of n and, since x 2 ≡ a (mod n) implies x 2 ≡ a (mod p), it follows that a is a quadratic non-residue of n; hence 21 is a quadratic non-residue of 275. But the converse is not true. For 3 = − 23 = 1, we cannot conclude that 3 is a quadratic instance, though 275 residue of 275; indeed 35 = −1 and so 3 is a quadratic non-residue of 275. 6 Further reading The theories here date back to the Disquisitiones Arithmeticae of Gauss, and they are covered by numerous texts.

The notation Z p , which was used in the Concise Introduction, also commonly occurs but it is open to objection since it clashes with notation customarily adopted in the context of p-adic numbers. 4 Wilson’s theorem 21 Note that, when (a, n) divides b, an explicit solution to the congruence ax ≡ b (mod n) can always be obtained from Euclid’s algorithm although, as in the examples above, a simple observation often suﬃces. 3 The theorems of Fermat and Euler First we introduce the concept of a reduced set of residues (mod n).