algebraic closure

English

Noun

algebraic closure (plural algebraic closures)

  1. (algebra, field theory, of a field F) A field G such that every polynomial over F splits completely over G (i.e., every element of G is a root of some polynomial over F and every root of every polynomial over F is an element of G).
    • 1999, Shreeram S. Abhyankar, “Galois Theory of Semilinear Transformations”, in Helmut Voelklein, David Harbater, J. G. Thompson, Peter Müller, editors, Aspects of Galois Theory, Cambridge University Press, page 1:
      The calculation of these various Galois groups leads to a determination of the algebraic closures of the ground fields in the splitting fields of the corresponding vectorial polynomials.
    • 2000, Alain M. Robert, A Course in p-adic Analysis, Springer, page 127:
      It turns out that the algebraic closure is not complete, so we shall consider its completion : This field turns out to be algebraically closed and is a natural domain for the study of "analytic functions."
    • 2004, John Swallow, Exploratory Galois Theory, Cambridge University Press, page 179:
      While contains an algebraic closure of , it is by no means the only algebraically closed field containing an algebraic closure of . We denote by the algebraic closure of in ; this field is simply the subfield of consisting of algebraic numbers. The field is isomorphic, then, to any algebraic closure of , but even knowing that it is unique up to isomorphism very likely leaves us no more familiar with than we were.

Usage notes

  • Notations for the algebraic closure of a field include and .
  • Using Zorn's lemma (or the weaker ultrafilter lemma), it can be shown that every field has an algebraic closure, and that the algebraic closure of a field K is unique up to an isomorphism that fixes every member of K. Consequently, authors often speak of the (rather than an) algebraic closure of K. (See Algebraic closure on Wikipedia.Wikipedia )
  • The field of complex numbers, , is the algebraic closure of the field of real numbers, .
  • The algebraic closure of the field of p-adic numbers, , is denoted or . (Unlike , and indeed unlike , is not metrically complete: its metric completion, which is algebraically closed, is denoted or .)

Translations

References

  • Frédérique Oggier (2010) “Introduction to Algebraic Number Theory”, in ntu.edu.sg/~frederique/Teaching, archived from the original on 23 October 2014

Further reading

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