# Algebraic Number Theory by Serge Lang

By Serge Lang

This is a moment version of Lang's famous textbook. It covers the entire uncomplicated fabric of classical algebraic quantity conception, giving the coed the heritage beneficial for the learn of extra issues in algebraic quantity idea, akin to cyclotomic fields, or modular forms.

"Lang's books are constantly of significant price for the graduate pupil and the learn mathematician. This up-to-date variation of Algebraic quantity thought is not any exception."―-MATHEMATICAL REVIEWS

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Additional info for Algebraic Number Theory

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Let L be an additive subgroup of E. We define its complementary set (relative to the trace) to be the set of x E E such that Tr~(xL) CA, and denote it by L'. Then L' is an additive group. If AL = L, then AL' = L'. If L, Mare two additive subgroups of E, and L C M, then M' CL'. We also have the following properties. Proposition 1. 11 Wl, ... , Wn is a basis 01 E over K and then L = AWl + ... + Awn , L' = + ... + Aw~, Aw~ where {wH is the dual basis relative to the trace. Proof. Let a E L' and write with ai E K.

Let A be a Dedekind ring, K its quotient field, L a finite separable extension of K, and B the integral closure of A in L. If II is a prime ideal of A, then llB is an ideal of Band has a factorization into primes of B. j3 lies above p. 24 [1, §7] ALGEBRAIC INTEGERS If S is the complement of p in A, then multiplying the above factorization by S gives us the factorization of S-lp in S-lB. ßi remain distinct. ßilp). If we assume that A is a local ring, then p = (11") is principal (Proposition 15).

Then every unit 01 K is a norm 01 a unit in E. Prool. Let u be a unit in K. We identify the Galois group of E over K with the Galois group of the residue class field extension. It is a simple consequence of Hilbert's Theorem 90 (or anything else you can think of) that both the trace and norm from a finite extension of a finite field are surjective. Hence there exists a unit ao in E such that u == N~ao Then for some with Xl Cl E A. Let in BE. Then (mod p). [II, §5] TAMELY RAMIFIED EXTENSIONS 51 where Tr is the trace, and it is again an easy matter to verify that the trace is surjective in the residue class field extension.