Group Rings and Class Groups by Klaus W. Roggenkamp

By Klaus W. Roggenkamp

The 1st a part of the ebook facilities round the isomorphism challenge for finite teams; i.e. which houses of the finite team G could be decided by means of the critical staff ring ZZG ? The authors have attempted to give the implications roughly selfcontained and in as a lot generality as attainable about the ring of coefficients. within the first part, the category sum correspondence and a few similar effects are derived. This half is the facts of the subgroup pressure theorem (Scott - Roggenkamp; Weiss) which says finite subgroup of the p-adic necessary team ring of a finite p-group is conjugate to a subgroup of the finite workforce. A counterexample to the conjecture of Zassenhaus that workforce foundation are rationally conjugate, is gifted within the semilocal scenario (Scott - Roggenkamp). To this finish, a longer model of Clifford concept for p-adic vital team jewelry is gifted. furthermore, a number of examples are given to illustrate the complexity of the isomorphism challenge. the second one a part of the booklet is worried with a variety of facets of the constitution of jewelry of integers as Galois modules. It starts with a short review of significant ends up in the realm; thereafter the vast majority of the textual content makes a speciality of using the idea of Hopf algebras. It starts off with a radical and certain remedy of the mandatory foundational fabric and concludes with new and engaging purposes to cyclotomic thought and to elliptic curves with complicated multiplication. Examples are used all through either for motivation, and likewise to demonstrate new rules.

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N = la(g) . tr(7r(O'(g)). Because 19 = la(g) and because S is an entire ring we can cancel, if 19 is different from o. 1 and get immediately (c). From (c) it follows that 9 and O'(g) have the same order. The augmentation map shows that C(g) and C(O'(g)) have the same length. Hence (b) is established. 6 prove (d). More on the class sum correspondence §2 31 Character tables Next we consider properties of a finite group G reflected by its ordinary character table CT( G). Brauer CT( G) is an h x h - matrix M = (mij) of algebraic integers, where the columns are labelled by the conjugacy classes Cj of G, the rows are labelled by the ordinary irreducible characters Xi, and mij = Xi( C j ).

1. Theorem (Subgroup rigidity). Let V(RG) be the units in RG of augmentation one, and Gap-group. If V is a finite subgroup of V(RG) , then u· v· u- 1 C G for some unit u in RG . 2. Theorem. Let M be an RG-Iattice, and let N be a normal subgroup of G. ) M / IR(N) . M is a permutation module for G / N . Then M is an RG-permutation module. 2, though it is also of interest for its own sake. The notation is the one introduced above. 3. Theorem. ) Assume that K is a splitting field for G and all of its subgroups.

To do so, we shall be using induction on i: i = 0 : We recall, that 'TJo = eo is the trivial idempotent (equations (36) and (37)), and so M1 . 'TJo = Rr~(C and M 2 · 'TJo = Rr~~c are genuine permutation modules for G / C ; moreover, since c acts trivially on 'TJo, the reduction modulo (c - (0+1) is just reduction modulo (1 - (), and so This proves the case i = O. i > 0 : Assume that the modules Mj . 'TJd (( c - (i+1) . M j . 'TJi) are isomorphic for j = 1,2. 3, that (note i < p - 1), and c acts as (i+1 on these modules.

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