an:03843940
Zbl 0532.16007
Brookes, Christopher J. B.
Ideals in group rings of soluble groups of finite rank
EN
Math. Proc. Camb. Philos. Soc. 97, 27-49 (1985).
00149087
1985
j
16S34 20C07 16D25 20F16 16D60
primitive group rings; gamma-ideals of group algebras; standardisers of ideals; minimal primes; faithful almost standardised primes; orbital elements; maximum condition on ideals; plinths; valuation sphere; soluble groups of finite rank; annihilator-free prime ideals
The bulk of this paper is a study of \(\Gamma\)-ideals of the group algebra \(kA\) where \(\Gamma\) is a group acting on a torsion-free abelian group \(A\) of finite rank. Define the standardiser of an ideal \(I\) of \(kA\) to be the subgroup of \(\Gamma\) of elements \(\gamma\) such that \(I\cap kA_ 1=I^{\gamma}\cap kA_ 1\) for some finitely generated (f.g.) subgroup \(A_ 1\) (dependent on \(\gamma\)) of rank \(r(A)\). Every \(\Gamma\)-ideal has at least one minimal prime over it almost standardised by \(\Gamma\). This is significant because faithful almost standardised primes of \(kA\) are controlled by \(\Delta_{\Gamma}(A)\), the subgroup of \(\Gamma\)-orbital elements of \(A\) (Theorem A). The proof is based upon that of Theorem D of \textit{J. E. Roseblade} [Proc. Lond. Math. Soc. (3) 36, 385--447 (1978; Zbl 0391.16008)], the analogous result for f.g. \(A\). This was a generalisation of a theorem of \textit{G. M. Bergman} [Trans. Am. Math. Soc. 157, 459--470 (1971; Zbl 0197.17102)] which itself may be extended to the finite rank case. \(A\) is said to be a plinth if it is rationally irreducible as a \({\mathbb{Z}}\Gamma_ 1\)-module for all \(\Gamma_ 1\) of finite index in \(\Gamma\). Part of Theorem C states: Let \(A\) be a plinth which has the maximum condition on \(\Gamma\)-invariant subgroups. Then \(kA\) has the maximum condition on \(\Gamma\)-ideals and a non-zero \(\Gamma\)-ideal \(I\) satisfies \(\dim_ k(kA/I)<\infty\). Furthermore if \(\Gamma\) is f.g. nilpotent and \(G\) is any group containing \(A\) as a normal subgroup with \(G/A\cong\Gamma\) then \(kG\) has the maximum condition on ideals. This is significant because it provides numerous examples on non-Noetherian rings with the maximum condition on ideals. Moreover there are groups \(G\) with \(kG\), but not \({\mathbb{Z}}G\), having that property.
On the other hand the results about \(\Gamma\)-ideals do not carry over from the f.g. to the finite rank case. For fields of characteristic zero faithful primes need not be controlled by \(\Delta_{\Gamma}(A)\) even when \(A\) is the direct product of two plinths. For f.g. abelian \(\Gamma\) a condition is obtained in terms of the valuation sphere introduced by \textit{R. Bieri} and \textit{R. Strebel} [Proc. Lond. Math. Soc. (3) 41, 439--464 (1980; Zbl 0448.20029)], sufficient for the existence of this departure from polycyclic behaviour. This condition also proves to be a necessary one when \(\Gamma\) is infinite cyclic. This effectively provides a characterisation of the actions of an infinite cyclic group \(\Gamma\) for which all faithful \(\Gamma\)-primes are annihilator-free.
Two applications of this study of \(\Gamma\)-ideals are:
Corollary E. Let \(G\) be a soluble group of finite rank with no non-trivial finite normal subgroups and \(k\) be a field. Then all annihilator-free prime ideals of \(kG\) are controlled by \(\Delta(G)\).
Theorem F. Let \(G\) be a soluble group of finite rank and \(k\) be a non-absolute field. Then \(kG\) is primitive if \(\Delta(G)=1\).
C. J. B. Brookes
Zbl 0391.16008; Zbl 0197.17102; Zbl 0448.20029