Read e-book online A Concise Introduction to the Theory of Numbers PDF

By Alan Baker

Quantity thought has an extended and exceptional heritage and the options and difficulties in relation to the topic were instrumental within the origin of a lot of arithmetic. during this publication, Professor Baker describes the rudiments of quantity conception in a concise, basic and direct demeanour. notwithstanding many of the textual content is classical in content material, he contains many courses to additional research so one can stimulate the reader to delve into the nice wealth of literature dedicated to the topic. The publication relies on Professor Baker's lectures given on the collage of Cambridge and is meant for undergraduate scholars of arithmetic.

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R 1. Further, for some n r 1, we have - . . + . where pn/qn (n = 1,2,. ) denote the convergents to 8, and thus 8 satisfies the equation 2 Qnx +(&-I- P ~ ) x -~ n - I=O* Now the quadratic on the left has the value - p n - ~< 0 for x = 0, and it has the value pn + q, - ( p,-l + q,-J> 0 for x = -1. Hence the conjugate 8' of 8 satisfies -1 < 8'< 0, as required. As an immediate corollary we see that the continued fractions of Jd + [ J d ] and 1/(Jd -[Jd]) are purely periodic, where d is any positive integer, not a perfect square.

However, an irreducible element need not b e a prime. Consider, for example, the number 2 in the quadratic field Q(J(-5)). It is certainly irreducible, for if 2 = a/3then 4 = N(a)N(p); but N ( a ) and N ( p ) have the form x2+5y2 for some integers x, y, and, since the equation x2+ 5y2 = *2 has no integer solutions, it follows that either N ( a ) = *l or N(/3)= *l and thus either a or p is a unit. On the'other hand, 2 is not a prime in Q(J(-5)), for it divides but it does not divide either 1+ J(-5) or 1-4(-5); indeed, by taking norms, it is readily verified that each of the latter is irreducible.

T o establish the result it suffices to show that, when d > 0, there is a unit r) in a ( J d ) other than *1; for then r)' is a unit for all integers m, and, since the only roots of unity in Q ( J d ) are *l, we see that different m give distinct units. We shall use Dirichlet's theorem on Diophantine approximation (see 9 1 of Chapter 6); th'e theorem implies that, for any integer Q > 1, there exist rational integers p, q, with O < q < Q, such that la1 c 1/Q, where a = p - qdd. Now the conjugate ai = a eqJ d satisfies 1615 3 Q J d and thus we have I N ( a ) l 3Jd.

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