On Towers and Composita of Towers of Function Fields over Finite Fields

  • Authors:
  • Arnaldo Garcia;Henning Stichtenoth;Michael Thomas

  • Affiliations:
  • Instituto de Matemática Pura e Aplicada IMPA, Estrada Dona Castorina 110, 22460-320, Rio de Janeiro, RJ, Brazil;Mathematik und Informatik, Universität GH Essen, 1, D-45117, Essen, Germanyf1stichtenoth@uni-essen.def1;Mathematik und Informatik, Universität GH Essen, 1, D-45117, Essen, Germanyf1stichtenoth@uni-essen.def1

  • Venue:
  • Finite Fields and Their Applications
  • Year:
  • 1997

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Abstract

For a towerF"1@?F"2@? ... of algebraic function fieldsF"i/F"q, define @l = lim"i"-"~N(F"i)/g(F"i), whereN(F"i) is the number of rational places andg(F"i) is the genus ofF"i/F"q. The tower is said to be asymptotically good if @l 0. We give a very simple explicit example of an asymptotically good tower for all non-prime fields F"q. In this example, all stepsF"i"+"1/F"iare tamely ramified Kummer extensions. We then show that any function fieldF/F"qhaving at least one rational place can be embedded into an asymptotically good tower, and we study the behaviour of @l in the compositum of a towerF"1@?F"2@? ... with an extensionE/F"1.