Computing Generators of Free Modules over Orders in Group Algebras

نویسندگان

  • WERNER BLEY
  • HENRI JOHNSTON
چکیده

Let E be a number field and G be a finite group. Let A be any OE-order of full rank in the group algebra E[G] and X be a (left) A-lattice. We give a necessary and sufficient condition for X to be free of given rank d over A. In the case that the Wedderburn decomposition E[G] ∼= ⊕χMχ is explicitly computable and each Mχ is in fact a matrix ring over a field, this leads to an algorithm that either gives elements α1, . . . , αd ∈ X such that X = Aα1 ⊕ . . .⊕Aαd or determines that no such elements exist. Let L/K be a finite Galois extension of number fields with Galois group G such that E is a subfield of K and put d = [K : E]. The algorithm can be applied to certain Galois modules that arise naturally in this situation. For example, one can take X to be OL, the ring of algebraic integers of L, and A to be the associated order A(E[G];OL) ⊆ E[G]. The application of the algorithm to this special situation is implemented in Magma under certain extra hypotheses when K = E = Q.

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تاریخ انتشار 2007