Download PDF by J. W. S. Cassels, A. Frohlich: Algebraic Number Theory: Proceedings of an Instructional

By J. W. S. Cassels, A. Frohlich

This publication offers a brisk, thorough remedy of the principles of algebraic quantity conception on which it builds to introduce extra complicated themes. all through, the authors emphasize the systematic improvement of innovations for the specific calculation of the elemental invariants corresponding to earrings of integers, type teams, and devices, combining at each one level thought with particular computations.

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Additional resources for Algebraic Number Theory: Proceedings of an Instructional Conference Organized by the London Mathematical Society (A Nato Advanced Study Institute W)

Example text

As kL 3 kL, =3 k,, and as (kL : kLO) is a power of p it follows that f(L,/L,) = 1. Also e(L1/LO) is prime to p. Thus Li is tamely ramified over L,. In the sequel we shall use repeatedly the fact that, for any tower E 3 F 13 K of fields, EJK is tamely ramified if and only if E/F and F/K are tamely ramified. This follows from the definition of tame ramification. As a first consequence we conclude the L, and its subfields are tamely ramified over K. Let L’ be a subfield of L, tamely ramified over K.

Now write a H a for the residue class map S + S/‘Q LEMMA 2. t&a) Proof. Remark: (4) = et,,,,(a). By Lemma 1, with A = S/pS, N = $3/p% Analogously one can show that N&a) = Nk,&V. PROPOSITION 4. uL(z)) 2 e-l. Proof. Write again A = SJpS, N = p/pS and denote by I the image in A of an element x of S, Choose a k-basis (al) of A, so that for 1 5 i I (e- 1)f the a, form a k-basis of N. We can lift (ai} back to an R-basis {xl} of S, so that jY, = a,. e. e. 4 2 (e - l>f, LOCAL 21 FIELDS L is said to be non-ramified over K, if (i) e(L/K) = 1.

UK(bi) > 0 for all i. COROLLARY. K has a totally ramified extension of prescribed degree e. Proof. Let uK(c) = 1 and E(X) = xp--cx-c. It is worth mentioning some results, which follow on from Proposition 1, but which we shall not be able to give in this course. (See Serre, Ch. e. F z k). Here the characteristics of k and K coincide. Conversely if this is the case then K is (to within value isomorphism) the field of formal power series over k. There remains the case when x = p # 0 but the characteristic of K is zero.

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