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Restriction map of spaces of orderings of fields. (English. Russian original) Zbl 0612.12017

Sib. Math. J. 27, 181-187 (1986); translation from Sib. Mat. Zh. 27, No. 2(156), 47-54 (1986).
Let \(X_F\) denote the space of orderings of a formally real field \(F\). For every field extension \(F_1/F_0\), one can define a restriction map \(\varepsilon_{F_1/F_0}: X_{F_1}\to X_{F_0}\), assuming \(\varepsilon_{F_1/F_0}(L):= L\cap F_0\) for every positive cone \(L\) in \(F_1\). R. Elman, T. Y. Lam and A. R. Wadsworth [J. Reine Angew. Math. 306, 7–27 (1979; Zbl 0398.12019)] proved that \(\varepsilon_{F_1/F_0}\) is a continuous and open mapping, provided \(F_1/F_0\) is finitely generated.
Using the method of elimination of quantifiers the author proves that for every finitely generated field extension \(F_1/F_0\) the restriction map \(\varepsilon_{F_1/F_0}\) has a continuous section (i.e., there is a continuous mapping \(\sigma\colon Y\to X_{F_1}\) such that \(\varepsilon_{F_1/F_0}\circ \sigma = \operatorname{id}_Y\), where \(Y := \varepsilon_{F_1/F_0}(X_{F_1}))\).
This result is applied to solve some problems concerning the RRC-fields. Moreover, the general problem of existence of continuous sections of mappings of Boolean spaces is discussed.

MSC:

12D15 Fields related with sums of squares (formally real fields, Pythagorean fields, etc.)
12J15 Ordered fields
12L12 Model theory of fields

Citations:

Zbl 0398.12019
Full Text: DOI

References:

[1] T.-Y. Lam, ?The theory of ordered fields,? in: Ring Theory and Algebra. III, Dekker, New York-Basel (1980).
[2] R. Elman, T.-Y. Lam, and A. Wadsworth, ?Orderings under field extensions,? J. Reine Angew. Math.,306 (1979). · Zbl 0398.12019
[3] Yu. L. Ershov, ?Regularly r-closed fields,? Algebra Logika,22, No. 4, 382-402 (1983).
[4] Yu. L. Ershov, Realizable i-Groups. Some Problems of Analysis and Algebra [in Russian], Novosibirsk State Univ. (1985), pp. 46-60.
[5] T. Craven, ?The Boolean space of orderings of a field,? Trans. Am. Math. Soc.,209, 225-235 (1975). · Zbl 0315.12106 · doi:10.1090/S0002-9947-1975-0379448-X
[6] Yu. L. Ershov, ?Algorithmic problems in the theory of fields (positive aspects),? in: Reference Book for Mathematical Logic [in Russian], Part 3, Nauka, Moscow (1982), pp. 269-353.
[7] A. Prestel, ?Pseudo real closed fields,? Lect. Notes Math.,872, 127-156 (1981). · doi:10.1007/BFb0098621
[8] Yu. L. Ershov, ?On the number of linear orderings on a field,? Mat. Zametki,6, No. 2, 201-211 (1969).
[9] Yu. L. Ershov, ?On the Galois groups of maximal 2-extensions,? Mat. Zametki,36, No. 6, 931-941 (1984).
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