Valued Fields (Springer Monographs in Mathematics) by Alexander Prestel, Antonio J. Engler

By Alexander Prestel, Antonio J. Engler

Absolute values and their completions – corresponding to the p-adic quantity fields – play an enormous position in quantity thought. Krull's generalization of absolute values to valuations made attainable purposes in different branches of arithmetic. In valuation thought, the concept finishing touch needs to be changed by way of that of "Henselization". This booklet develops the speculation of valuations in addition to of Henselizations, in accordance with the talents of a regular graduate direction in algebra.

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2 Ordered Fields Next we deal with orderings on a field K. An ordering ≥ on K is a binary relation that makes the additive group of K an ordered abelian group, and in addition satisfies the following: for all x, y ∈ K, 0 ≤ x, y =⇒ 0 ≤ xy . A field K is called real if it admits some ordering ≤. The set P≤ := { x ∈ K | 0 ≤ x } is called the positive cone of ≤. For a subring R of K, we define the ≤-convex hull of R in K by OR (≤) := { x ∈ K | x, −x ≤ a for some a ∈ R } . One easily sees that OR (≤) is a subring of K containing R.

The lexicographic product Z × Z, or more generally Z × · · · × Z, n-times, is of rank 2, respectively n. We may give, however, the direct product Z × Z another ordering that makes it a rank 1 ordered√abelian group. Indeed, we may just identify Z × Z with the subgroup Z + Z 2 of the additive reals and take the ordering induced from R. , an ordered abelian group that admits a minimal positive element, with respect to the ordering induced from R, Z × Z is densely ordered. Now, after this short excursion into ordered abelian groups, let us define valuations.

The next corollary gives an interesting characterization of fields that admit a non-archimedean ordering. 6. The field K admits a non-archimedean ordering if and only if K carries a non-trivial valuation with a real residue class field K. Proof. Assume that ≤ is a non-archimedean ordering of K. Then the valuation ring O(≤) defined above is non-trivial and ≤-convex. 2 Constructions of Valuations 39 the positive cone P of ≤ corresponds to the pair (ηP , P ). In particular, K admits an ordering, and thus is real.

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